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On shutdown when quotas are enabled, the shutdown can deadlock trying to unpin the dquot buffer buf_log_item like so: [ 3319.483590] task:kworker/20:0H state:D stack:14360 pid:1962230 tgid:1962230 ppid:2 task_flags:0x4208060 flags:0x00004000 [ 3319.493966] Workqueue: xfs-log/dm-6 xlog_ioend_work [ 3319.498458] Call Trace: [ 3319.500800] <TASK> [ 3319.502809] __schedule+0x699/0xb70 [ 3319.512672] schedule+0x64/0xd0 [ 3319.515573] schedule_timeout+0x30/0xf0 [ 3319.528125] __down_common+0xc3/0x200 [ 3319.531488] __down+0x1d/0x30 [ 3319.534186] down+0x48/0x50 [ 3319.540501] xfs_buf_lock+0x3d/0xe0 [ 3319.543609] xfs_buf_item_unpin+0x85/0x1b0 [ 3319.547248] xlog_cil_committed+0x289/0x570 [ 3319.571411] xlog_cil_process_committed+0x6d/0x90 [ 3319.575590] xlog_state_shutdown_callbacks+0x52/0x110 [ 3319.580017] xlog_force_shutdown+0x169/0x1a0 [ 3319.583780] xlog_ioend_work+0x7c/0xb0 [ 3319.587049] process_scheduled_works+0x1d6/0x400 [ 3319.591127] worker_thread+0x202/0x2e0 [ 3319.594452] kthread+0x20c/0x240 The CIL push has seen the deadlock, so it has aborted the push and is running CIL checkpoint completion to abort all the items in the checkpoint. This calls ->iop_unpin(remove = true) to clean up the log items in the checkpoint. When a buffer log item is unpined like this, it needs to lock the buffer to run io completion to correctly fail the buffer and run all the required completions to fail attached log items as well. In this case, the attempt to lock the buffer on unpin is hanging because the buffer is already locked. I suspected a leaked XFS_BLI_HOLD state because of XFS_BLI_STALE handling changes I was testing, so I went looking for pin events on HOLD buffers and unpin events on locked buffer. That isolated this one buffer with these two events: xfs_buf_item_pin: dev 251:6 daddr 0xa910 bbcount 0x2 hold 2 pincount 0 lock 0 flags DONE|KMEM recur 0 refcount 1 bliflags HOLD|DIRTY|LOGGED liflags DIRTY .... xfs_buf_item_unpin: dev 251:6 daddr 0xa910 bbcount 0x2 hold 4 pincount 1 lock 0 flags DONE|KMEM recur 0 refcount 1 bliflags DIRTY liflags ABORTED Firstly, bbcount = 0x2, which means it is not a single sector structure. That rules out every xfs_trans_bhold() case except one: dquot buffers. Then hung task dumping gave this trace: [ 3197.312078] task:fsync-tester state:D stack:12080 pid:2051125 tgid:2051125 ppid:1643233 task_flags:0x400000 flags:0x00004002 [ 3197.323007] Call Trace: [ 3197.325581] <TASK> [ 3197.327727] __schedule+0x699/0xb70 [ 3197.334582] schedule+0x64/0xd0 [ 3197.337672] schedule_timeout+0x30/0xf0 [ 3197.350139] wait_for_completion+0xbd/0x180 [ 3197.354235] __flush_workqueue+0xef/0x4e0 [ 3197.362229] xlog_cil_force_seq+0xa0/0x300 [ 3197.374447] xfs_log_force+0x77/0x230 [ 3197.378015] xfs_qm_dqunpin_wait+0x49/0xf0 [ 3197.382010] xfs_qm_dqflush+0x55/0x460 [ 3197.385663] xfs_qm_dquot_isolate+0x29e/0x4d0 [ 3197.389977] __list_lru_walk_one+0x141/0x220 [ 3197.398867] list_lru_walk_one+0x10/0x20 [ 3197.402713] xfs_qm_shrink_scan+0x6a/0x100 [ 3197.406699] do_shrink_slab+0x18a/0x350 [ 3197.410512] shrink_slab+0xf7/0x430 [ 3197.413967] drop_slab+0x97/0xf0 [ 3197.417121] drop_caches_sysctl_handler+0x59/0xc0 [ 3197.421654] proc_sys_call_handler+0x18b/0x280 [ 3197.426050] proc_sys_write+0x13/0x20 [ 3197.429750] vfs_write+0x2b8/0x3e0 [ 3197.438532] ksys_write+0x7e/0xf0 [ 3197.441742] __x64_sys_write+0x1b/0x30 [ 3197.445363] x64_sys_call+0x2c72/0x2f60 [ 3197.449044] do_syscall_64+0x6c/0x140 [ 3197.456341] entry_SYSCALL_64_after_hwframe+0x76/0x7e Yup, another test run by check-parallel is running drop_caches concurrently and the dquot shrinker for the hung filesystem is running. That's trying to flush a dirty dquot from reclaim context, and it waiting on a log force to complete. xfs_qm_dqflush is called with the dquot buffer held locked, and so we've called xfs_log_force() with that buffer locked. Now the log force is waiting for a workqueue flush to complete, and that workqueue flush is waiting of CIL checkpoint processing to finish. The CIL checkpoint processing is aborting all the log items it has, and that requires locking aborted buffers to cancel them. Now, normally this isn't a problem if we are issuing a log force to unpin an object, because the ->iop_unpin() method wakes pin waiters first. That results in the pin waiter finishing off whatever it was doing, dropping the lock and then xfs_buf_item_unpin() can lock the buffer and fail it. However, xfs_qm_dqflush() is waiting on the -dquot- unpin event, not the dquot buffer unpin event, and so it never gets woken and so does not drop the buffer lock. Inodes do not have this problem, as they can only be written from one spot (->iop_push) whilst dquots can be written from multiple places (memory reclaim, ->iop_push, xfs_dq_dqpurge, and quotacheck). The reason that the dquot buffer has an attached buffer log item is that it has been recently allocated. Initialisation of the dquot buffer logs the buffer directly, thereby pinning it in memory. We then modify the dquot in a separate operation, and have memory reclaim racing with a shutdown and we trigger this deadlock. check-parallel reproduces this reliably on 1kB FSB filesystems with quota enabled because it does all of these things concurrently without having to explicitly write tests to exercise these corner case conditions. xfs_qm_dquot_logitem_push() doesn't have this deadlock because it checks if the dquot is pinned before locking the dquot buffer and skipping it if it is pinned. This means the xfs_qm_dqunpin_wait() log force in xfs_qm_dqflush() never triggers and we unlock the buffer safely allowing a concurrent shutdown to fail the buffer appropriately. xfs_qm_dqpurge() could have this problem as it is called from quotacheck and we might have allocated dquot buffers when recording the quota updates. This can be fixed by calling xfs_qm_dqunpin_wait() before we lock the dquot buffer. Because we hold the dquot locked, nothing will be able to add to the pin count between the unpin_wait and the dqflush callout, so this now makes xfs_qm_dqpurge() safe against this race. xfs_qm_dquot_isolate() can also be fixed this same way but, quite frankly, we shouldn't be doing IO in memory reclaim context. If the dquot is pinned or dirty, simply rotate it and let memory reclaim come back to it later, same as we do for inodes. This then gets rid of the nasty issue in xfs_qm_flush_one() where quotacheck writeback races with memory reclaim flushing the dquots. We can lift xfs_qm_dqunpin_wait() up into this code, then get rid of the "can't get the dqflush lock" buffer write to cycle the dqlfush lock and enable it to be flushed again. checking if the dquot is pinned and returning -EAGAIN so that the dquot walk will revisit the dquot again later. Finally, with xfs_qm_dqunpin_wait() lifted into all the callers, we can remove it from the xfs_qm_dqflush() code. Signed-off-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com> Signed-off-by: Carlos Maiolino <cem@kernel.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.