| Message ID | 20230626131542.3711391-1-yangerkun@huaweicloud.com (mailing list archive) |
|---|---|
| State | Deferred, archived |
| Headers | show |
| Series | xfs: fix deadlock when set label online | expand |
On Mon, Jun 26, 2023 at 09:15:42PM +0800, yangerkun wrote: > From: yangerkun <yangerkun@huawei.com> > > Combine use of xfs_trans_hold and xfs_trans_set_sync in xfs_sync_sb_buf > can trigger a deadlock once shutdown happened concurrently. xlog_ioend_work > will first unpin the sb(which stuck with xfs_buf_lock), then wakeup > xfs_sync_sb_buf. However, xfs_sync_sb_buf never get the chance to unlock > sb until been wakeup by xlog_ioend_work. > > xfs_sync_sb_buf > xfs_trans_getsb // lock sb buf > xfs_trans_bhold // sb buf keep lock until success commit > xfs_trans_commit > ... > xfs_log_force_seq > xlog_force_lsn > xlog_wait_on_iclog > xlog_wait(&iclog->ic_force_wait... // shutdown happened > xfs_buf_relse // unlock sb buf > > xlog_ioend_work > xlog_force_shutdown > xlog_state_shutdown_callbacks > xlog_cil_process_committed > xlog_cil_committed > ... > xfs_buf_item_unpin > xfs_buf_lock // deadlock > wake_up_all(&iclog->ic_force_wait) > > xfs_ioc_setlabel use xfs_sync_sb_buf to make sure userspace will see the > change for sb immediately. We can simply call xfs_ail_push_all_sync to > do this and sametime fix the deadlock. Why is this deadlock specific to the superblock buffer? Can't any buffer that is held locked over a synchronous transaction commit deadlock during a shutdown like this? Cheers, Dave.
在 2023/6/27 5:45, Dave Chinner 写道: > On Mon, Jun 26, 2023 at 09:15:42PM +0800, yangerkun wrote: >> From: yangerkun <yangerkun@huawei.com> >> >> Combine use of xfs_trans_hold and xfs_trans_set_sync in xfs_sync_sb_buf >> can trigger a deadlock once shutdown happened concurrently. xlog_ioend_work >> will first unpin the sb(which stuck with xfs_buf_lock), then wakeup >> xfs_sync_sb_buf. However, xfs_sync_sb_buf never get the chance to unlock >> sb until been wakeup by xlog_ioend_work. >> >> xfs_sync_sb_buf >> xfs_trans_getsb // lock sb buf >> xfs_trans_bhold // sb buf keep lock until success commit >> xfs_trans_commit >> ... >> xfs_log_force_seq >> xlog_force_lsn >> xlog_wait_on_iclog >> xlog_wait(&iclog->ic_force_wait... // shutdown happened >> xfs_buf_relse // unlock sb buf >> >> xlog_ioend_work >> xlog_force_shutdown >> xlog_state_shutdown_callbacks >> xlog_cil_process_committed >> xlog_cil_committed >> ... >> xfs_buf_item_unpin >> xfs_buf_lock // deadlock >> wake_up_all(&iclog->ic_force_wait) >> >> xfs_ioc_setlabel use xfs_sync_sb_buf to make sure userspace will see the >> change for sb immediately. We can simply call xfs_ail_push_all_sync to >> do this and sametime fix the deadlock. > > Why is this deadlock specific to the superblock buffer? Hi Dave, Thanks a lot for your revirew! We find this problem when do some code reading(which can help us to fix another growfs bug). And then reproduce it easily when we set label online frequently with IO error inject at the sametime. > > Can't any buffer that is held locked over a synchronous transaction > commit deadlock during a shutdown like this? After check all place use xfs_buf_bhold, it seems xfs_sync_sb_buf is the only convict that combine use xfs_trans_hold and xfs_trans_set_sync(I'm not familiar with xfs yet, so I may have some problems with my code check)... Thanks, Yang Erkun. > > Cheers, > > Dave.
On Tue, Jun 27, 2023 at 04:42:41PM +0800, yangerkun wrote: > > > 在 2023/6/27 5:45, Dave Chinner 写道: > > On Mon, Jun 26, 2023 at 09:15:42PM +0800, yangerkun wrote: > > > From: yangerkun <yangerkun@huawei.com> > > > > > > Combine use of xfs_trans_hold and xfs_trans_set_sync in xfs_sync_sb_buf > > > can trigger a deadlock once shutdown happened concurrently. xlog_ioend_work > > > will first unpin the sb(which stuck with xfs_buf_lock), then wakeup > > > xfs_sync_sb_buf. However, xfs_sync_sb_buf never get the chance to unlock > > > sb until been wakeup by xlog_ioend_work. > > > > > > xfs_sync_sb_buf > > > xfs_trans_getsb // lock sb buf > > > xfs_trans_bhold // sb buf keep lock until success commit > > > xfs_trans_commit > > > ... > > > xfs_log_force_seq > > > xlog_force_lsn > > > xlog_wait_on_iclog > > > xlog_wait(&iclog->ic_force_wait... // shutdown happened > > > xfs_buf_relse // unlock sb buf > > > > > > xlog_ioend_work > > > xlog_force_shutdown > > > xlog_state_shutdown_callbacks > > > xlog_cil_process_committed > > > xlog_cil_committed > > > ... > > > xfs_buf_item_unpin > > > xfs_buf_lock // deadlock > > > wake_up_all(&iclog->ic_force_wait) > > > > > > xfs_ioc_setlabel use xfs_sync_sb_buf to make sure userspace will see the > > > change for sb immediately. We can simply call xfs_ail_push_all_sync to > > > do this and sametime fix the deadlock. > > > > Why is this deadlock specific to the superblock buffer? > > Hi Dave, > > Thanks a lot for your revirew! We find this problem when do some code > reading(which can help us to fix another growfs bug). And then reproduce it > easily when we set label online frequently with IO error inject at the > sametime. Right, I know how it can be triggered; that's not actually my concern... > > Can't any buffer that is held locked over a synchronous transaction > > commit deadlock during a shutdown like this? > > After check all place use xfs_buf_bhold, it seems xfs_sync_sb_buf is the > only convict that combine use xfs_trans_hold and xfs_trans_set_sync(I'm not > familiar with xfs yet, so I may have some problems with my code check)... Yes, I can also see that. But my concern is that this change only addresses the symptom, but leaves the underlying deadlock unsolved. Indeed, this isn't xfs_trans_commit() I'm worried about here; it's the call to xfs_log_force(mp, XFS_LOG_SYNC) or xfs_log_force_seq(XFS_LOG_SYNC) with a buffer held locked that I'm worried about. i.e. We have a buffer in the CIL (from a previous transaction) that we currently hold locked while we call xfs_log_force(XFS_LOG_SYNC). If a shutdown occurs while we are waiting for journal IO completion to occur, then xlog_ioend_work() will attempt to lock the buffer and deadlock, right? e.g. I'm thinking of things like busy extent flushing (hold AGF + AGFL + AG btree blocks locked when we call xfs_log_force()) could also be vulnerable to the same deadlock... If that's true, how do we avoid the shutdown from causing a deadlock in these situations? Cheers, Dave.
在 2023/6/29 7:10, Dave Chinner 写道: > On Tue, Jun 27, 2023 at 04:42:41PM +0800, yangerkun wrote: >> >> >> 在 2023/6/27 5:45, Dave Chinner 写道: >>> On Mon, Jun 26, 2023 at 09:15:42PM +0800, yangerkun wrote: >>>> From: yangerkun <yangerkun@huawei.com> >>>> >>>> Combine use of xfs_trans_hold and xfs_trans_set_sync in xfs_sync_sb_buf >>>> can trigger a deadlock once shutdown happened concurrently. xlog_ioend_work >>>> will first unpin the sb(which stuck with xfs_buf_lock), then wakeup >>>> xfs_sync_sb_buf. However, xfs_sync_sb_buf never get the chance to unlock >>>> sb until been wakeup by xlog_ioend_work. >>>> >>>> xfs_sync_sb_buf >>>> xfs_trans_getsb // lock sb buf >>>> xfs_trans_bhold // sb buf keep lock until success commit >>>> xfs_trans_commit >>>> ... >>>> xfs_log_force_seq >>>> xlog_force_lsn >>>> xlog_wait_on_iclog >>>> xlog_wait(&iclog->ic_force_wait... // shutdown happened >>>> xfs_buf_relse // unlock sb buf >>>> >>>> xlog_ioend_work >>>> xlog_force_shutdown >>>> xlog_state_shutdown_callbacks >>>> xlog_cil_process_committed >>>> xlog_cil_committed >>>> ... >>>> xfs_buf_item_unpin >>>> xfs_buf_lock // deadlock >>>> wake_up_all(&iclog->ic_force_wait) >>>> >>>> xfs_ioc_setlabel use xfs_sync_sb_buf to make sure userspace will see the >>>> change for sb immediately. We can simply call xfs_ail_push_all_sync to >>>> do this and sametime fix the deadlock. >>> >>> Why is this deadlock specific to the superblock buffer? >> >> Hi Dave, >> >> Thanks a lot for your revirew! We find this problem when do some code >> reading(which can help us to fix another growfs bug). And then reproduce it >> easily when we set label online frequently with IO error inject at the >> sametime. > > Right, I know how it can be triggered; that's not actually my > concern... > >>> Can't any buffer that is held locked over a synchronous transaction >>> commit deadlock during a shutdown like this? >> >> After check all place use xfs_buf_bhold, it seems xfs_sync_sb_buf is the >> only convict that combine use xfs_trans_hold and xfs_trans_set_sync(I'm not >> familiar with xfs yet, so I may have some problems with my code check)... > > Yes, I can also see that. But my concern is that this change only > addresses the symptom, but leaves the underlying deadlock unsolved. > > Indeed, this isn't xfs_trans_commit() I'm worried about here; it's > the call to xfs_log_force(mp, XFS_LOG_SYNC) or > xfs_log_force_seq(XFS_LOG_SYNC) with a buffer held locked that I'm > worried about. > > i.e. We have a buffer in the CIL (from a previous transaction) that > we currently hold locked while we call xfs_log_force(XFS_LOG_SYNC). > If a shutdown occurs while we are waiting for journal IO completion > to occur, then xlog_ioend_work() will attempt to lock the buffer and > deadlock, right? > > e.g. I'm thinking of things like busy extent flushing (hold AGF + > AGFL + AG btree blocks locked when we call xfs_log_force()) could > also be vulnerable to the same deadlock... You mean something like xfs_allocbt_alloc_block(call xfs_log_force to flush busy extent which keep agf locked sametime)? We call xfs_log_force(mp, XFS_LOG_SYNC) after lock agf and before xfs_trans_commit. It seems ok since xfs_buf_item_unpin will not call xfs_buf_lock because bli_refcount still keep active(once we hold locked agf, the bli_refcount will inc in _xfs_trans_bjoin, and keep it until xfs_trans_commit success(clean agf item) or .iop_unpin(dirty agf item, call from xlog_ioend_work) which can be called after xfs_trans_commit too)... > > If that's true, how do we avoid the shutdown from causing a deadlock > in these situations? > > Cheers, > > Dave.
On Thu, Jun 29, 2023 at 07:55:10PM +0800, yangerkun wrote: > 在 2023/6/29 7:10, Dave Chinner 写道: > > On Tue, Jun 27, 2023 at 04:42:41PM +0800, yangerkun wrote: > > > 在 2023/6/27 5:45, Dave Chinner 写道: > > > > On Mon, Jun 26, 2023 at 09:15:42PM +0800, yangerkun wrote: > > > > > From: yangerkun <yangerkun@huawei.com> > > > > > > > > > > Combine use of xfs_trans_hold and xfs_trans_set_sync in xfs_sync_sb_buf > > > > > can trigger a deadlock once shutdown happened concurrently. xlog_ioend_work > > > > > will first unpin the sb(which stuck with xfs_buf_lock), then wakeup > > > > > xfs_sync_sb_buf. However, xfs_sync_sb_buf never get the chance to unlock > > > > > sb until been wakeup by xlog_ioend_work. > > > > > > > > > > xfs_sync_sb_buf > > > > > xfs_trans_getsb // lock sb buf > > > > > xfs_trans_bhold // sb buf keep lock until success commit > > > > > xfs_trans_commit > > > > > ... > > > > > xfs_log_force_seq > > > > > xlog_force_lsn > > > > > xlog_wait_on_iclog > > > > > xlog_wait(&iclog->ic_force_wait... // shutdown happened > > > > > xfs_buf_relse // unlock sb buf > > > > > > > > > > xlog_ioend_work > > > > > xlog_force_shutdown > > > > > xlog_state_shutdown_callbacks > > > > > xlog_cil_process_committed > > > > > xlog_cil_committed > > > > > ... > > > > > xfs_buf_item_unpin > > > > > xfs_buf_lock // deadlock > > > > > wake_up_all(&iclog->ic_force_wait) > > > > > > > > > > xfs_ioc_setlabel use xfs_sync_sb_buf to make sure userspace will see the > > > > > change for sb immediately. We can simply call xfs_ail_push_all_sync to > > > > > do this and sametime fix the deadlock. > > > > > > > > Why is this deadlock specific to the superblock buffer? > > > > > > Hi Dave, > > > > > > Thanks a lot for your revirew! We find this problem when do some code > > > reading(which can help us to fix another growfs bug). And then reproduce it > > > easily when we set label online frequently with IO error inject at the > > > sametime. > > > > Right, I know how it can be triggered; that's not actually my > > concern... > > > > > > Can't any buffer that is held locked over a synchronous transaction > > > > commit deadlock during a shutdown like this? > > > > > > After check all place use xfs_buf_bhold, it seems xfs_sync_sb_buf is the > > > only convict that combine use xfs_trans_hold and xfs_trans_set_sync(I'm not > > > familiar with xfs yet, so I may have some problems with my code check)... > > > > Yes, I can also see that. But my concern is that this change only > > addresses the symptom, but leaves the underlying deadlock unsolved. > > > > Indeed, this isn't xfs_trans_commit() I'm worried about here; it's > > the call to xfs_log_force(mp, XFS_LOG_SYNC) or > > xfs_log_force_seq(XFS_LOG_SYNC) with a buffer held locked that I'm > > worried about. > > > > i.e. We have a buffer in the CIL (from a previous transaction) that > > we currently hold locked while we call xfs_log_force(XFS_LOG_SYNC). > > If a shutdown occurs while we are waiting for journal IO completion > > to occur, then xlog_ioend_work() will attempt to lock the buffer and > > deadlock, right? > > > > e.g. I'm thinking of things like busy extent flushing (hold AGF + > > AGFL + AG btree blocks locked when we call xfs_log_force()) could > > also be vulnerable to the same deadlock... > > You mean something like xfs_allocbt_alloc_block(call xfs_log_force to > flush busy extent which keep agf locked sametime)? > > We call xfs_log_force(mp, XFS_LOG_SYNC) after lock agf and before > xfs_trans_commit. It seems ok since xfs_buf_item_unpin will not call > xfs_buf_lock because bli_refcount still keep active(once we hold locked > agf, the bli_refcount will inc in _xfs_trans_bjoin, and keep it until > xfs_trans_commit success(clean agf item) or .iop_unpin(dirty agf item, > call from xlog_ioend_work) which can be called after xfs_trans_commit > too)... Again, I gave an example of the class of issue I'm worried about. Again, you chased the one example given through, but haven't mentioned a thing about all the other code paths that lead to xfs_log_force(SYNC) that might hold buffers locked that I didn't mention. I don't want to have to ask every person who proposes a fix about every possible code path the bug may manifest in -one at a time-. I use examples to point you in the right direction for further analysis of the rest of the code base, not because that's the only thing I want checked. Please use your initiative to look at all the callers of xfs_log_force(SYNC) and determine if they are all safe or whether there are landmines lurked or even more bugs of a similar sort. When we learn about a new issue, this is the sort of audit work that is necessary to determine the scope of the issue. We need to perform such audits because they direct the scope of the fix necessary. We are not interested in slapping a band-aid fix over the symptom that was reported - that only leads to more band-aid fixes as the same issue appears in other places. Now we know there is a lock ordering problem in this code, so before we attempt to fix it we need to know how widespread it is, what the impact is, how different code paths avoid it, etc. That requires a code audit to determine, and that requires looking at all the paths into xfs_log_force(XFS_LOG_SYNC) to determine if they are safe or not and documenting that. Yes, it's more work *right now* than slapping a quick band-aid fix over it, but it's much less work in the long run for us and we don't have to keep playing whack-a-mole because we fixed it the right way the first time. -Dave.
在 2023/6/30 6:24, Dave Chinner 写道: > On Thu, Jun 29, 2023 at 07:55:10PM +0800, yangerkun wrote: >> 在 2023/6/29 7:10, Dave Chinner 写道: >>> On Tue, Jun 27, 2023 at 04:42:41PM +0800, yangerkun wrote: >>>> 在 2023/6/27 5:45, Dave Chinner 写道: >>>>> On Mon, Jun 26, 2023 at 09:15:42PM +0800, yangerkun wrote: >>>>>> From: yangerkun <yangerkun@huawei.com> >>>>>> >>>>>> Combine use of xfs_trans_hold and xfs_trans_set_sync in xfs_sync_sb_buf >>>>>> can trigger a deadlock once shutdown happened concurrently. xlog_ioend_work >>>>>> will first unpin the sb(which stuck with xfs_buf_lock), then wakeup >>>>>> xfs_sync_sb_buf. However, xfs_sync_sb_buf never get the chance to unlock >>>>>> sb until been wakeup by xlog_ioend_work. >>>>>> >>>>>> xfs_sync_sb_buf >>>>>> xfs_trans_getsb // lock sb buf >>>>>> xfs_trans_bhold // sb buf keep lock until success commit >>>>>> xfs_trans_commit >>>>>> ... >>>>>> xfs_log_force_seq >>>>>> xlog_force_lsn >>>>>> xlog_wait_on_iclog >>>>>> xlog_wait(&iclog->ic_force_wait... // shutdown happened >>>>>> xfs_buf_relse // unlock sb buf >>>>>> >>>>>> xlog_ioend_work >>>>>> xlog_force_shutdown >>>>>> xlog_state_shutdown_callbacks >>>>>> xlog_cil_process_committed >>>>>> xlog_cil_committed >>>>>> ... >>>>>> xfs_buf_item_unpin >>>>>> xfs_buf_lock // deadlock >>>>>> wake_up_all(&iclog->ic_force_wait) >>>>>> >>>>>> xfs_ioc_setlabel use xfs_sync_sb_buf to make sure userspace will see the >>>>>> change for sb immediately. We can simply call xfs_ail_push_all_sync to >>>>>> do this and sametime fix the deadlock. >>>>> >>>>> Why is this deadlock specific to the superblock buffer? >>>> >>>> Hi Dave, >>>> >>>> Thanks a lot for your revirew! We find this problem when do some code >>>> reading(which can help us to fix another growfs bug). And then reproduce it >>>> easily when we set label online frequently with IO error inject at the >>>> sametime. >>> >>> Right, I know how it can be triggered; that's not actually my >>> concern... >>> >>>>> Can't any buffer that is held locked over a synchronous transaction >>>>> commit deadlock during a shutdown like this? >>>> >>>> After check all place use xfs_buf_bhold, it seems xfs_sync_sb_buf is the >>>> only convict that combine use xfs_trans_hold and xfs_trans_set_sync(I'm not >>>> familiar with xfs yet, so I may have some problems with my code check)... >>> >>> Yes, I can also see that. But my concern is that this change only >>> addresses the symptom, but leaves the underlying deadlock unsolved. >>> >>> Indeed, this isn't xfs_trans_commit() I'm worried about here; it's >>> the call to xfs_log_force(mp, XFS_LOG_SYNC) or >>> xfs_log_force_seq(XFS_LOG_SYNC) with a buffer held locked that I'm >>> worried about. >>> >>> i.e. We have a buffer in the CIL (from a previous transaction) that >>> we currently hold locked while we call xfs_log_force(XFS_LOG_SYNC). >>> If a shutdown occurs while we are waiting for journal IO completion >>> to occur, then xlog_ioend_work() will attempt to lock the buffer and >>> deadlock, right? >>> >>> e.g. I'm thinking of things like busy extent flushing (hold AGF + >>> AGFL + AG btree blocks locked when we call xfs_log_force()) could >>> also be vulnerable to the same deadlock... >> >> You mean something like xfs_allocbt_alloc_block(call xfs_log_force to >> flush busy extent which keep agf locked sametime)? >> >> We call xfs_log_force(mp, XFS_LOG_SYNC) after lock agf and before >> xfs_trans_commit. It seems ok since xfs_buf_item_unpin will not call >> xfs_buf_lock because bli_refcount still keep active(once we hold locked >> agf, the bli_refcount will inc in _xfs_trans_bjoin, and keep it until >> xfs_trans_commit success(clean agf item) or .iop_unpin(dirty agf item, >> call from xlog_ioend_work) which can be called after xfs_trans_commit >> too)... > > Again, I gave an example of the class of issue I'm worried about. > Again, you chased the one example given through, but haven't > mentioned a thing about all the other code paths that lead to > xfs_log_force(SYNC) that might hold buffers locked that I didn't > mention. > > I don't want to have to ask every person who proposes a fix about > every possible code path the bug may manifest in -one at a time-. I > use examples to point you in the right direction for further > analysis of the rest of the code base, not because that's the only > thing I want checked. Please use your initiative to look at all the > callers of xfs_log_force(SYNC) and determine if they are all safe or > whether there are landmines lurked or even more bugs of a similar > sort. Hi Dave, Thank you very much for pointing this out! I'm so sorry for the lack of awareness of a comprehensive investigation does there any other place can trigger the bug too... > > When we learn about a new issue, this is the sort of audit work that > is necessary to determine the scope of the issue. We need to perform > such audits because they direct the scope of the fix necessary. We > are not interested in slapping a band-aid fix over the symptom that > was reported - that only leads to more band-aid fixes as the same > issue appears in other places. Yes, agree with you and thanks for your advise, it can really help me to forbid a band-aid fix however leads to more band-aid fixes, so can contribute better! > > Now we know there is a lock ordering problem in this code, so before > we attempt to fix it we need to know how widespread it is, what the > impact is, how different code paths avoid it, etc. That requires a > code audit to determine, and that requires looking at all the paths > into xfs_log_force(XFS_LOG_SYNC) to determine if they are safe or > not and documenting that. > > Yes, it's more work *right now* than slapping a quick band-aid fix > over it, but it's much less work in the long run for us and we don't > have to keep playing whack-a-mole because we fixed it the right way > the first time. > I will try to look all paths into xfs_log_force(XFS_LOG_SYNC) or xfs_log_force_seq(XFS_LOG_SYNC) to check if it's safe or not. Thanks again for your advise! Thanks, Yang Erkun. > -Dave.
在 2023/6/30 10:19, yangerkun 写道: > > > 在 2023/6/30 6:24, Dave Chinner 写道: >> On Thu, Jun 29, 2023 at 07:55:10PM +0800, yangerkun wrote: >>> 在 2023/6/29 7:10, Dave Chinner 写道: >>>> On Tue, Jun 27, 2023 at 04:42:41PM +0800, yangerkun wrote: >>>>> 在 2023/6/27 5:45, Dave Chinner 写道: >>>>>> On Mon, Jun 26, 2023 at 09:15:42PM +0800, yangerkun wrote: >>>>>>> From: yangerkun <yangerkun@huawei.com> >>>>>>> >>>>>>> Combine use of xfs_trans_hold and xfs_trans_set_sync in >>>>>>> xfs_sync_sb_buf >>>>>>> can trigger a deadlock once shutdown happened concurrently. >>>>>>> xlog_ioend_work >>>>>>> will first unpin the sb(which stuck with xfs_buf_lock), then wakeup >>>>>>> xfs_sync_sb_buf. However, xfs_sync_sb_buf never get the chance to >>>>>>> unlock >>>>>>> sb until been wakeup by xlog_ioend_work. >>>>>>> >>>>>>> xfs_sync_sb_buf >>>>>>> xfs_trans_getsb // lock sb buf >>>>>>> xfs_trans_bhold // sb buf keep lock until success commit >>>>>>> xfs_trans_commit >>>>>>> ... >>>>>>> xfs_log_force_seq >>>>>>> xlog_force_lsn >>>>>>> xlog_wait_on_iclog >>>>>>> xlog_wait(&iclog->ic_force_wait... // shutdown happened >>>>>>> xfs_buf_relse // unlock sb buf >>>>>>> >>>>>>> xlog_ioend_work >>>>>>> xlog_force_shutdown >>>>>>> xlog_state_shutdown_callbacks >>>>>>> xlog_cil_process_committed >>>>>>> xlog_cil_committed >>>>>>> ... >>>>>>> xfs_buf_item_unpin >>>>>>> xfs_buf_lock // deadlock >>>>>>> wake_up_all(&iclog->ic_force_wait) >>>>>>> >>>>>>> xfs_ioc_setlabel use xfs_sync_sb_buf to make sure userspace will >>>>>>> see the >>>>>>> change for sb immediately. We can simply call >>>>>>> xfs_ail_push_all_sync to >>>>>>> do this and sametime fix the deadlock. >>>>>> >>>>>> Why is this deadlock specific to the superblock buffer? >>>>> >>>>> Hi Dave, >>>>> >>>>> Thanks a lot for your revirew! We find this problem when do some code >>>>> reading(which can help us to fix another growfs bug). And then >>>>> reproduce it >>>>> easily when we set label online frequently with IO error inject at the >>>>> sametime. >>>> >>>> Right, I know how it can be triggered; that's not actually my >>>> concern... >>>> >>>>>> Can't any buffer that is held locked over a synchronous transaction >>>>>> commit deadlock during a shutdown like this? >>>>> >>>>> After check all place use xfs_buf_bhold, it seems xfs_sync_sb_buf >>>>> is the >>>>> only convict that combine use xfs_trans_hold and >>>>> xfs_trans_set_sync(I'm not >>>>> familiar with xfs yet, so I may have some problems with my code >>>>> check)... >>>> >>>> Yes, I can also see that. But my concern is that this change only >>>> addresses the symptom, but leaves the underlying deadlock unsolved. >>>> >>>> Indeed, this isn't xfs_trans_commit() I'm worried about here; it's >>>> the call to xfs_log_force(mp, XFS_LOG_SYNC) or >>>> xfs_log_force_seq(XFS_LOG_SYNC) with a buffer held locked that I'm >>>> worried about. >>>> >>>> i.e. We have a buffer in the CIL (from a previous transaction) that >>>> we currently hold locked while we call xfs_log_force(XFS_LOG_SYNC). >>>> If a shutdown occurs while we are waiting for journal IO completion >>>> to occur, then xlog_ioend_work() will attempt to lock the buffer and >>>> deadlock, right? >>>> >>>> e.g. I'm thinking of things like busy extent flushing (hold AGF + >>>> AGFL + AG btree blocks locked when we call xfs_log_force()) could >>>> also be vulnerable to the same deadlock... >>> >>> You mean something like xfs_allocbt_alloc_block(call xfs_log_force to >>> flush busy extent which keep agf locked sametime)? >>> >>> We call xfs_log_force(mp, XFS_LOG_SYNC) after lock agf and before >>> xfs_trans_commit. It seems ok since xfs_buf_item_unpin will not call >>> xfs_buf_lock because bli_refcount still keep active(once we hold locked >>> agf, the bli_refcount will inc in _xfs_trans_bjoin, and keep it until >>> xfs_trans_commit success(clean agf item) or .iop_unpin(dirty agf item, >>> call from xlog_ioend_work) which can be called after xfs_trans_commit >>> too)... >> >> Again, I gave an example of the class of issue I'm worried about. >> Again, you chased the one example given through, but haven't >> mentioned a thing about all the other code paths that lead to >> xfs_log_force(SYNC) that might hold buffers locked that I didn't >> mention. >> >> I don't want to have to ask every person who proposes a fix about >> every possible code path the bug may manifest in -one at a time-. I >> use examples to point you in the right direction for further >> analysis of the rest of the code base, not because that's the only >> thing I want checked. Please use your initiative to look at all the >> callers of xfs_log_force(SYNC) and determine if they are all safe or >> whether there are landmines lurked or even more bugs of a similar >> sort. > > Hi Dave, > > Thank you very much for pointing this out! I'm so sorry for the lack of > awareness of a comprehensive investigation does there any other place > can trigger the bug too... > >> >> When we learn about a new issue, this is the sort of audit work that >> is necessary to determine the scope of the issue. We need to perform >> such audits because they direct the scope of the fix necessary. We >> are not interested in slapping a band-aid fix over the symptom that >> was reported - that only leads to more band-aid fixes as the same >> issue appears in other places. > > Yes, agree with you and thanks for your advise, it can really help me to > forbid a band-aid fix however leads to more band-aid fixes, so can > contribute better! > >> >> Now we know there is a lock ordering problem in this code, so before >> we attempt to fix it we need to know how widespread it is, what the >> impact is, how different code paths avoid it, etc. That requires a >> code audit to determine, and that requires looking at all the paths >> into xfs_log_force(XFS_LOG_SYNC) to determine if they are safe or >> not and documenting that. >> >> Yes, it's more work *right now* than slapping a quick band-aid fix >> over it, but it's much less work in the long run for us and we don't >> have to keep playing whack-a-mole because we fixed it the right way >> the first time. >> > > I will try to look all paths into xfs_log_force(XFS_LOG_SYNC) or > xfs_log_force_seq(XFS_LOG_SYNC) to check if it's safe or not. Thanks > again for your advise! > > Thanks, > Yang Erkun. > >> -Dave. > Hi, Dave, Sorry for the late reply, I was quiet busy last month and it also took me long time to check does all the callers of xfs_log_force(SYNC)/xfs_log_force_seq(SYNC) was safe. I'm not familiar with xfs yet, so if there's anything wrong with the description below, please point it out! The logic I choose was to check will we call xfs_log_force(SYNC)/xfs_log_force_seq(SYNC) between xfs_buf_lock/xfs_buf_trylock and xfs_buf_unlock at the same thread context(I have check other item's .iop_unpin, it seems only xfs_buf item can trigger the problem since it will try to lock the buf in xfs_buf_item_unpin; besides, different thread context call for xfs_buf_lock&xfs_log_force(SYNC)/xfs_log_force_seq(SYNC) and xfs_buf_unlock is safe too since this unlock will not wait until xfs_log_force(SYNC)/xfs_log_force_seq(SYNC) success return), and once it happend, will we trigger the bug too? I divide the logic of calling xfs_buf_lock/xfs_buf_trylock into two categories: 1. Later the xfs_buf will join the tp(xfs_trans_bjoin will inc .bli_refcount) a. xfs_trans_bjoin will inc .bli_refcount b. xfs_buf_item_pin will inc .bli_refcount when the item of xfs_buf was dirty c. xfs_buf_item_committing will dec .bli_refcount no matter the item was dirty or not, and normally it will unlock the xfs_buf, or keep the xfs_buf locked when we see XFS_BLI_HOLD d. xfs_buf_item_unpin will dec .bli_refcount, and it won't call xfs_buf_lock when another .bli_refcount exist xfs_log_force(SYNC)/xfs_log_force_seq(SYNC) can happend before we commit the tp(like xfs_create, it will first read&lock agi buf, then call xfs_dir_createname, which may trigger agfl fixup, the block allocation may see the busy extent and call xfs_log_force(SYNC) to flush the busy extent journal). It won't trigger the problem since xfs_trans_bjoin will keep another .bli_refcount. xfs_log_force_seq(SYNC) can happend when we commit the tp since we see XFS_TRANS_SYNC(see __xfs_trans_commit), the only case we can trigger the deadlock was that we have combine called xfs_trans_bhold, or we will unlock xfs_buf in xfs_buf_item_committing. The case which this patch try to fix was the only case combine call for xfs_trans_bhold and set XFS_TRANS_SYNC. After commit tp, xfs_buf will only keep locked because of xfs_trans_bhold, and once there is a XFS_TRANS_PERM_LOG_RES tp, we may trigger another commit, then xfs_log_force(SYNC)/xfs_log_force_seq(SYNC) can happend too. But it is safe too since we will first rejoin xfs_buf to tp which help protect us. 2. The xfs_buf won't join the tp xfs_buf_readahead_map xfs_buf_read xfs_buf_get xfs_buf_incore xfs_buf_delwri_cancel xfs_buf_delwri_submit_buffers xfs_buf_delwri_pushbuf xfs_buf_item_unpin xfs_iflush_shutdown_abort xfs_log_quiesce xlog_do_recover xfs_freesb xfs_add_incompat_log_feature xfs_clear_incompat_log_features Most case above was io, for xfs_buf_unlock called from another thread context, it is safe; for xfs_buf_unlock called from same thread context, it is safe too since we won't trigger xfs_log_force(SYNC)/xfs_log_force_seq(SYNC) between xfs_buf_lock and xfs_buf_unlock. From above, it seems only xfs_sync_sb_buf can trigger this deadlock... And I prefer to add some comments to xfs_trans_bhold to notice that there is a bug when combine use xfs_trans_bhold and xfs_log_force(SYNC)/xfs_log_force_seq(SYNC)... Dave, sorry again for the late reply, and look forward to your reply! Thanks, Yang Erkun.
diff --git a/fs/xfs/libxfs/xfs_sb.c b/fs/xfs/libxfs/xfs_sb.c index ba0f17bc1dc0..8b89e65c5e1e 100644 --- a/fs/xfs/libxfs/xfs_sb.c +++ b/fs/xfs/libxfs/xfs_sb.c @@ -1089,38 +1089,6 @@ xfs_update_secondary_sbs( return saved_error ? saved_error : error; } -/* - * Same behavior as xfs_sync_sb, except that it is always synchronous and it - * also writes the superblock buffer to disk sector 0 immediately. - */ -int -xfs_sync_sb_buf( - struct xfs_mount *mp) -{ - struct xfs_trans *tp; - struct xfs_buf *bp; - int error; - - error = xfs_trans_alloc(mp, &M_RES(mp)->tr_sb, 0, 0, 0, &tp); - if (error) - return error; - - bp = xfs_trans_getsb(tp); - xfs_log_sb(tp); - xfs_trans_bhold(tp, bp); - xfs_trans_set_sync(tp); - error = xfs_trans_commit(tp); - if (error) - goto out; - /* - * write out the sb buffer to get the changes to disk - */ - error = xfs_bwrite(bp); -out: - xfs_buf_relse(bp); - return error; -} - void xfs_fs_geometry( struct xfs_mount *mp, diff --git a/fs/xfs/libxfs/xfs_sb.h b/fs/xfs/libxfs/xfs_sb.h index a5e14740ec9a..fb420312c476 100644 --- a/fs/xfs/libxfs/xfs_sb.h +++ b/fs/xfs/libxfs/xfs_sb.h @@ -15,7 +15,6 @@ struct xfs_perag; extern void xfs_log_sb(struct xfs_trans *tp); extern int xfs_sync_sb(struct xfs_mount *mp, bool wait); -extern int xfs_sync_sb_buf(struct xfs_mount *mp); extern void xfs_sb_mount_common(struct xfs_mount *mp, struct xfs_sb *sbp); extern void xfs_sb_from_disk(struct xfs_sb *to, struct xfs_dsb *from); extern void xfs_sb_to_disk(struct xfs_dsb *to, struct xfs_sb *from); diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c index 55bb01173cde..9bacc71d2c9e 100644 --- a/fs/xfs/xfs_ioctl.c +++ b/fs/xfs/xfs_ioctl.c @@ -27,6 +27,7 @@ #include "xfs_trace.h" #include "xfs_icache.h" #include "xfs_trans.h" +#include "xfs_trans_priv.h" #include "xfs_acl.h" #include "xfs_btree.h" #include <linux/fsmap.h> @@ -1798,9 +1799,11 @@ xfs_ioc_setlabel( * buffered reads from userspace (i.e. from blkid) are invalidated, * and userspace will see the newly-written label. */ - error = xfs_sync_sb_buf(mp); + error = xfs_sync_sb(mp, true); if (error) goto out; + + xfs_ail_push_all_sync(mp->m_ail); /* * growfs also updates backup supers so lock against that. */