| Message ID | 20200320110321.1.I9df0264e151a740be292ad3ee3825f31b5997776@changeid (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | bdev: Reduce time holding bd_mutex in sync in blkdev_close() | expand |
On Fri, Mar 20, 2020 at 11:07:16AM -0700, Douglas Anderson wrote: > While trying to "dd" to the block device for a USB stick, I > encountered a hung task warning (blocked for > 120 seconds). I > managed to come up with an easy way to reproduce this on my system > (where /dev/sdb is the block device for my USB stick) with: > > while true; do dd if=/dev/zero of=/dev/sdb bs=4M; done > > With my reproduction here are the relevant bits from the hung task > detector: > > INFO: task udevd:294 blocked for more than 122 seconds. > ... > udevd D 0 294 1 0x00400008 > Call trace: > ... > mutex_lock_nested+0x40/0x50 > __blkdev_get+0x7c/0x3d4 > blkdev_get+0x118/0x138 > blkdev_open+0x94/0xa8 > do_dentry_open+0x268/0x3a0 > vfs_open+0x34/0x40 > path_openat+0x39c/0xdf4 > do_filp_open+0x90/0x10c > do_sys_open+0x150/0x3c8 > ... > > ... > Showing all locks held in the system: > ... > 1 lock held by dd/2798: > #0: ffffff814ac1a3b8 (&bdev->bd_mutex){+.+.}, at: __blkdev_put+0x50/0x204 > ... > dd D 0 2798 2764 0x00400208 > Call trace: > ... > schedule+0x8c/0xbc > io_schedule+0x1c/0x40 > wait_on_page_bit_common+0x238/0x338 > __lock_page+0x5c/0x68 > write_cache_pages+0x194/0x500 > generic_writepages+0x64/0xa4 > blkdev_writepages+0x24/0x30 > do_writepages+0x48/0xa8 > __filemap_fdatawrite_range+0xac/0xd8 > filemap_write_and_wait+0x30/0x84 > __blkdev_put+0x88/0x204 > blkdev_put+0xc4/0xe4 > blkdev_close+0x28/0x38 > __fput+0xe0/0x238 > ____fput+0x1c/0x28 > task_work_run+0xb0/0xe4 > do_notify_resume+0xfc0/0x14bc > work_pending+0x8/0x14 > > The problem appears related to the fact that my USB disk is terribly > slow and that I have a lot of RAM in my system to cache things. > Specifically my writes seem to be happening at ~15 MB/s and I've got > ~4 GB of RAM in my system that can be used for buffering. To write 4 > GB of buffer to disk thus takes ~4000 MB / ~15 MB/s = ~267 seconds. > > The 267 second number is a problem because in __blkdev_put() we call > sync_blockdev() while holding the bd_mutex. Any other callers who > want the bd_mutex will be blocked for the whole time. > > The problem is made worse because I believe blkdev_put() specifically > tells other tasks (namely udev) to go try to access the device at right > around the same time we're going to hold the mutex for a long time. > > Putting some traces around this (after disabling the hung task detector), > I could confirm: > dd: 437.608600: __blkdev_put() right before sync_blockdev() for sdb > udevd: 437.623901: blkdev_open() right before blkdev_get() for sdb > dd: 661.468451: __blkdev_put() right after sync_blockdev() for sdb > udevd: 663.820426: blkdev_open() right after blkdev_get() for sdb > > A simple fix for this is to realize that sync_blockdev() works fine if > you're not holding the mutex. Also, it's not the end of the world if > you sync a little early (though it can have performance impacts). > Thus we can make a guess that we're going to need to do the sync and > then do it without holding the mutex. We still do one last sync with > the mutex but it should be much, much faster. > > With this, my hung task warnings for my test case are gone. > > Signed-off-by: Douglas Anderson <dianders@chromium.org> > --- > I didn't put a "Fixes" annotation here because, as far as I can tell, > this issue has been here "forever" unless someone knows of something > else that changed that made this possible to hit. This could probably > get picked back to any stable tree that anyone is still maintaining. The idea ok, but I see no point in even taking the lock to check bd_openers given that it is going to be racy anyway.
diff --git a/fs/block_dev.c b/fs/block_dev.c index 69bf2fb6f7cd..e92c667c4031 100644 --- a/fs/block_dev.c +++ b/fs/block_dev.c @@ -1881,6 +1881,20 @@ static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part) struct block_device *victim = NULL; mutex_lock_nested(&bdev->bd_mutex, for_part); + if (bdev->bd_openers == 1) { + /* + * Sync early if it looks like we're the last one. If someone + * else opens the block device between now and the decrement + * of bd_openers then we did a sync that we didn't need to, + * but that's not the end of the world and we want to avoid + * long (could be several minute) syncs while holding the + * mutex. + */ + mutex_unlock(&bdev->bd_mutex); + sync_blockdev(bdev); + mutex_lock_nested(&bdev->bd_mutex, for_part); + } + if (for_part) bdev->bd_part_count--;
While trying to "dd" to the block device for a USB stick, I encountered a hung task warning (blocked for > 120 seconds). I managed to come up with an easy way to reproduce this on my system (where /dev/sdb is the block device for my USB stick) with: while true; do dd if=/dev/zero of=/dev/sdb bs=4M; done With my reproduction here are the relevant bits from the hung task detector: INFO: task udevd:294 blocked for more than 122 seconds. ... udevd D 0 294 1 0x00400008 Call trace: ... mutex_lock_nested+0x40/0x50 __blkdev_get+0x7c/0x3d4 blkdev_get+0x118/0x138 blkdev_open+0x94/0xa8 do_dentry_open+0x268/0x3a0 vfs_open+0x34/0x40 path_openat+0x39c/0xdf4 do_filp_open+0x90/0x10c do_sys_open+0x150/0x3c8 ... ... Showing all locks held in the system: ... 1 lock held by dd/2798: #0: ffffff814ac1a3b8 (&bdev->bd_mutex){+.+.}, at: __blkdev_put+0x50/0x204 ... dd D 0 2798 2764 0x00400208 Call trace: ... schedule+0x8c/0xbc io_schedule+0x1c/0x40 wait_on_page_bit_common+0x238/0x338 __lock_page+0x5c/0x68 write_cache_pages+0x194/0x500 generic_writepages+0x64/0xa4 blkdev_writepages+0x24/0x30 do_writepages+0x48/0xa8 __filemap_fdatawrite_range+0xac/0xd8 filemap_write_and_wait+0x30/0x84 __blkdev_put+0x88/0x204 blkdev_put+0xc4/0xe4 blkdev_close+0x28/0x38 __fput+0xe0/0x238 ____fput+0x1c/0x28 task_work_run+0xb0/0xe4 do_notify_resume+0xfc0/0x14bc work_pending+0x8/0x14 The problem appears related to the fact that my USB disk is terribly slow and that I have a lot of RAM in my system to cache things. Specifically my writes seem to be happening at ~15 MB/s and I've got ~4 GB of RAM in my system that can be used for buffering. To write 4 GB of buffer to disk thus takes ~4000 MB / ~15 MB/s = ~267 seconds. The 267 second number is a problem because in __blkdev_put() we call sync_blockdev() while holding the bd_mutex. Any other callers who want the bd_mutex will be blocked for the whole time. The problem is made worse because I believe blkdev_put() specifically tells other tasks (namely udev) to go try to access the device at right around the same time we're going to hold the mutex for a long time. Putting some traces around this (after disabling the hung task detector), I could confirm: dd: 437.608600: __blkdev_put() right before sync_blockdev() for sdb udevd: 437.623901: blkdev_open() right before blkdev_get() for sdb dd: 661.468451: __blkdev_put() right after sync_blockdev() for sdb udevd: 663.820426: blkdev_open() right after blkdev_get() for sdb A simple fix for this is to realize that sync_blockdev() works fine if you're not holding the mutex. Also, it's not the end of the world if you sync a little early (though it can have performance impacts). Thus we can make a guess that we're going to need to do the sync and then do it without holding the mutex. We still do one last sync with the mutex but it should be much, much faster. With this, my hung task warnings for my test case are gone. Signed-off-by: Douglas Anderson <dianders@chromium.org> --- I didn't put a "Fixes" annotation here because, as far as I can tell, this issue has been here "forever" unless someone knows of something else that changed that made this possible to hit. This could probably get picked back to any stable tree that anyone is still maintaining. fs/block_dev.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+)