** Changed in: zfs-linux (Ubuntu Bionic)
Status: New => In Progress
** Changed in: zfs-linux (Ubuntu Xenial)
Status: New => In Progress
** Changed in: zfs-linux (Ubuntu Focal)
Status: New => In Progress
** Changed in: zfs-linux (Ubuntu Groovy)
Status: New => In Progress
** Changed in: zfs-linux (Ubuntu Hirsute)
Status: Confirmed => In Progress
** Bug watch added: Debian Bug tracker #983331
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=983331
** Also affects: zfs-linux (Debian) via
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=983331
Importance: Unknown
Status: Unknown
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https://bugs.launchpad.net/bugs/1916486
Title:
zfs_zrele_async can cause txg sync deadlocks
Status in zfs-linux package in Ubuntu:
In Progress
Status in zfs-linux source package in Xenial:
In Progress
Status in zfs-linux source package in Bionic:
In Progress
Status in zfs-linux source package in Focal:
In Progress
Status in zfs-linux source package in Groovy:
In Progress
Status in zfs-linux source package in Hirsute:
In Progress
Status in zfs-linux package in Debian:
Unknown
Bug description:
[Impact]
TXG sync stalls, causing ZFS workloads to hang indefinitely
[Description]
For certain ZFS workloads, we can see hung task timeouts in the kernel logs
due to a transaction group deadlock. Userspace process will hang and display
stack traces similar to the one below:
[49181.619711] clnt_server D 0 21699 28868 0x00000320
[49181.619715] Call Trace:
[49181.619725] __schedule+0x24e/0x880
[49181.619730] schedule+0x2c/0x80
[49181.619750] cv_wait_common+0x11e/0x140 [spl]
[49181.619763] ? wait_woken+0x80/0x80
[49181.619775] __cv_wait+0x15/0x20 [spl]
[49181.619872] zil_commit.part.14+0x80/0x8c0 [zfs]
[49181.619884] ? _cond_resched+0x19/0x40
[49181.619887] ? mutex_lock+0x12/0x40
[49181.619959] zil_commit+0x17/0x20 [zfs]
[49181.620026] zfs_fsync+0x77/0xe0 [zfs]
[49181.620093] zpl_fsync+0x68/0xa0 [zfs]
[49181.620100] vfs_fsync_range+0x51/0xb0
[49181.620105] do_fsync+0x3d/0x70
[49181.620109] SyS_fsync+0x10/0x20
[49181.620114] do_syscall_64+0x73/0x130
[49181.620119] entry_SYSCALL_64_after_hwframe+0x41/0xa6
We also might see a kworker thread blocking in the zfs writeback/evict path:
[49181.881570] kworker/u17:3 D 0 4915 2 0x80000000
[49181.881576] Workqueue: writeback wb_workfn (flush-zfs-10)
[49181.881577] Call Trace:
[49181.881580] __schedule+0x24e/0x880
[49181.881582] ? atomic_t_wait+0x60/0x60
[49181.881584] schedule+0x2c/0x80
[49181.881588] bit_wait+0x11/0x60
[49181.881592] __wait_on_bit+0x4c/0x90
[49181.881596] ? atomic_t_wait+0x60/0x60
[49181.881599] __inode_wait_for_writeback+0xb9/0xf0
[49181.881601] ? bit_waitqueue+0x40/0x40
[49181.881605] inode_wait_for_writeback+0x26/0x40
[49181.881609] evict+0xb5/0x1a0
[49181.881611] iput+0x19c/0x230
[49181.881648] zfs_iput_async+0x1d/0x80 [zfs]
[49181.881682] zfs_get_data+0x1d4/0x2a0 [zfs]
[49181.881718] zil_commit.part.14+0x640/0x8c0 [zfs]
[49181.881752] zil_commit+0x17/0x20 [zfs]
[49181.881784] zpl_writepages+0xd5/0x160 [zfs]
[49181.881787] do_writepages+0x4b/0xe0
[49181.881790] __writeback_single_inode+0x45/0x350
[49181.881792] ? __writeback_single_inode+0x45/0x350
[49181.881794] writeback_sb_inodes+0x1d7/0x530
[49181.881796] wb_writeback+0xfb/0x300
[49181.881799] wb_workfn+0xad/0x400
[49181.881800] ? wb_workfn+0xad/0x400
[49181.881803] ? __switch_to_asm+0x35/0x70
[49181.881809] process_one_work+0x1de/0x420
[49181.881811] worker_thread+0x32/0x410
[49181.881813] kthread+0x121/0x140
[49181.881815] ? process_one_work+0x420/0x420
[49181.881817] ? kthread_create_worker_on_cpu+0x70/0x70
[49181.881819] ret_from_fork+0x35/0x40
This is caused by a race between ZFS writeback and evict threads,
usually during a transaction group sync operation. It's possible to
have two iput() threads racing for the same inode: one of them
scheduled async and the other executed synchronously as part of the
writeback path. If the writeback thread tries to evict the inode while
the async thread is running, it might re-enter the block layer for the
same inode due to ZFS counters being in an inconsistent state. This
then causes the kworker thread to stall the writeback, which in turn
prevents the transaction group sync to complete and locks other ZFS
threads.
This is fixed by the upstream commit:
- Fix zrele race in zrele_async that can cause hang (2921ad6cba54) [0]
[Test Case]
Being a race condition, this issue has been hard to reproduce consistently.
This has been reported on heavy I/O workloads, mixing file creation and
deletion. We have some reports both from upstream and from Ubuntu users that
this is usually reproducible on e.g. heavy SQL workloads or on complex
ccache-enabled builds [1].
[0] https://github.com/openzfs/zfs/pull/11530
[1] https://github.com/openzfs/zfs/issues/11527
[Regression Potential]
The patch has been tested in the ZFS test suite and in production
environments, so the potential for further regressions should be fairly
controlled. Potential regressions might arise in the ZFS writeback path, so we
should monitor heavy I/O workloads that put a lot of stress in the sync and
evict paths.
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