| Message ID | 160494581071.772573.10466314698408344068.stgit@magnolia (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | vfs: remove lockdep fs freeze weirdness | expand |
On Mon 09-11-20 10:16:50, Darrick J. Wong wrote: > From: Darrick J. Wong <darrick.wong@oracle.com> > > __sb_start_write has some weird looking lockdep code that claims to > exist to handle nested freeze locking requests from xfs. The code as > written seems broken -- if we think we hold a read lock on any of the > higher freeze levels (e.g. we hold SB_FREEZE_WRITE and are trying to > lock SB_FREEZE_PAGEFAULT), it converts a blocking lock attempt into a > trylock. > > However, it's not correct to downgrade a blocking lock attempt to a > trylock unless the downgrading code or the callers are prepared to deal > with that situation. Neither __sb_start_write nor its callers handle > this at all. For example: > > sb_start_pagefault ignores the return value completely, with the result > that if xfs_filemap_fault loses a race with a different thread trying to > fsfreeze, it will proceed without pagefault freeze protection (thereby > breaking locking rules) and then unlocks the pagefault freeze lock that > it doesn't own on its way out (thereby corrupting the lock state), which > leads to a system hang shortly afterwards. > > Normally, this won't happen because our ownership of a read lock on a > higher freeze protection level blocks fsfreeze from grabbing a write > lock on that higher level. *However*, if lockdep is offline, > lock_is_held_type unconditionally returns 1, which means that > percpu_rwsem_is_held returns 1, which means that __sb_start_write > unconditionally converts blocking freeze lock attempts into trylocks, > even when we *don't* hold anything that would block a fsfreeze. > > Apparently this all held together until 5.10-rc1, when bugs in lockdep > caused lockdep to shut itself off early in an fstests run, and once > fstests gets to the "race writes with freezer" tests, kaboom. This > might explain the long trail of vanishingly infrequent livelocks in > fstests after lockdep goes offline that I've never been able to > diagnose. > > We could fix it by spinning on the trylock if wait==true, but AFAICT the > locking works fine if lockdep is not built at all (and I didn't see any > complaints running fstests overnight), so remove this snippet entirely. > > NOTE: Commit f4b554af9931 in 2015 created the current weird logic (which > used to exist in a different form in commit 5accdf82ba25c from 2012) in > __sb_start_write. XFS solved this whole problem in the late 2.6 era by > creating a variant of transactions (XFS_TRANS_NO_WRITECOUNT) that don't > grab intwrite freeze protection, thus making lockdep's solution > unnecessary. The commit claims that Dave Chinner explained that the > trylock hack + comment could be removed, but nobody ever did. > > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> > Reviewed-by: Christoph Hellwig <hch@lst.de> Thanks for cleaning this up. You can add: Reviewed-by: Jan Kara <jack@suse.cz> Honza > --- > fs/super.c | 33 ++++----------------------------- > 1 file changed, 4 insertions(+), 29 deletions(-) > > > diff --git a/fs/super.c b/fs/super.c > index a51c2083cd6b..e1fd667454d4 100644 > --- a/fs/super.c > +++ b/fs/super.c > @@ -1647,36 +1647,11 @@ EXPORT_SYMBOL(__sb_end_write); > */ > int __sb_start_write(struct super_block *sb, int level, bool wait) > { > - bool force_trylock = false; > - int ret = 1; > + if (!wait) > + return percpu_down_read_trylock(sb->s_writers.rw_sem + level-1); > > -#ifdef CONFIG_LOCKDEP > - /* > - * We want lockdep to tell us about possible deadlocks with freezing > - * but it's it bit tricky to properly instrument it. Getting a freeze > - * protection works as getting a read lock but there are subtle > - * problems. XFS for example gets freeze protection on internal level > - * twice in some cases, which is OK only because we already hold a > - * freeze protection also on higher level. Due to these cases we have > - * to use wait == F (trylock mode) which must not fail. > - */ > - if (wait) { > - int i; > - > - for (i = 0; i < level - 1; i++) > - if (percpu_rwsem_is_held(sb->s_writers.rw_sem + i)) { > - force_trylock = true; > - break; > - } > - } > -#endif > - if (wait && !force_trylock) > - percpu_down_read(sb->s_writers.rw_sem + level-1); > - else > - ret = percpu_down_read_trylock(sb->s_writers.rw_sem + level-1); > - > - WARN_ON(force_trylock && !ret); > - return ret; > + percpu_down_read(sb->s_writers.rw_sem + level-1); > + return 1; > } > EXPORT_SYMBOL(__sb_start_write); > >
diff --git a/fs/super.c b/fs/super.c index a51c2083cd6b..e1fd667454d4 100644 --- a/fs/super.c +++ b/fs/super.c @@ -1647,36 +1647,11 @@ EXPORT_SYMBOL(__sb_end_write); */ int __sb_start_write(struct super_block *sb, int level, bool wait) { - bool force_trylock = false; - int ret = 1; + if (!wait) + return percpu_down_read_trylock(sb->s_writers.rw_sem + level-1); -#ifdef CONFIG_LOCKDEP - /* - * We want lockdep to tell us about possible deadlocks with freezing - * but it's it bit tricky to properly instrument it. Getting a freeze - * protection works as getting a read lock but there are subtle - * problems. XFS for example gets freeze protection on internal level - * twice in some cases, which is OK only because we already hold a - * freeze protection also on higher level. Due to these cases we have - * to use wait == F (trylock mode) which must not fail. - */ - if (wait) { - int i; - - for (i = 0; i < level - 1; i++) - if (percpu_rwsem_is_held(sb->s_writers.rw_sem + i)) { - force_trylock = true; - break; - } - } -#endif - if (wait && !force_trylock) - percpu_down_read(sb->s_writers.rw_sem + level-1); - else - ret = percpu_down_read_trylock(sb->s_writers.rw_sem + level-1); - - WARN_ON(force_trylock && !ret); - return ret; + percpu_down_read(sb->s_writers.rw_sem + level-1); + return 1; } EXPORT_SYMBOL(__sb_start_write);