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** Changed in: zfs-linux (Debian)
       Status: New => Fix Released

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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:
  Fix Released
Status in zfs-linux source package in Xenial:
  Fix Released
Status in zfs-linux source package in Bionic:
  Fix Released
Status in zfs-linux source package in Focal:
  Fix Released
Status in zfs-linux source package in Groovy:
  Fix Released
Status in zfs-linux source package in Hirsute:
  Fix Released
Status in zfs-linux package in Debian:
  Fix Released

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 (43eaef6de817) [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, causing write hangs and eventually stalling all ZFS-backed operations indefinitely. We should monitor heavy I/O workloads that put a lot of stress in the sync and evict paths to exercise the new changes.

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