On Thu, Apr 27, 2023 at 03:49:43PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@xxxxxxxxxx>
>
> syzbot reported this warning from the faux inodegc shrinker that tries
> to kick off inodegc work:
>
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 102 at kernel/workqueue.c:1445 __queue_work+0xd44/0x1120 kernel/workqueue.c:1444
> RIP: 0010:__queue_work+0xd44/0x1120 kernel/workqueue.c:1444
> Call Trace:
> __queue_delayed_work+0x1c8/0x270 kernel/workqueue.c:1672
> mod_delayed_work_on+0xe1/0x220 kernel/workqueue.c:1746
> xfs_inodegc_shrinker_scan fs/xfs/xfs_icache.c:2212 [inline]
> xfs_inodegc_shrinker_scan+0x250/0x4f0 fs/xfs/xfs_icache.c:2191
> do_shrink_slab+0x428/0xaa0 mm/vmscan.c:853
> shrink_slab+0x175/0x660 mm/vmscan.c:1013
> shrink_one+0x502/0x810 mm/vmscan.c:5343
> shrink_many mm/vmscan.c:5394 [inline]
> lru_gen_shrink_node mm/vmscan.c:5511 [inline]
> shrink_node+0x2064/0x35f0 mm/vmscan.c:6459
> kswapd_shrink_node mm/vmscan.c:7262 [inline]
> balance_pgdat+0xa02/0x1ac0 mm/vmscan.c:7452
> kswapd+0x677/0xd60 mm/vmscan.c:7712
> kthread+0x2e8/0x3a0 kernel/kthread.c:376
> ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:308
>
> This warning corresponds to this code in __queue_work:
>
> /*
> * For a draining wq, only works from the same workqueue are
> * allowed. The __WQ_DESTROYING helps to spot the issue that
> * queues a new work item to a wq after destroy_workqueue(wq).
> */
> if (unlikely(wq->flags & (__WQ_DESTROYING | __WQ_DRAINING) &&
> WARN_ON_ONCE(!is_chained_work(wq))))
> return;
>
> For this to trip, we must have a thread draining the inodedgc workqueue
> and a second thread trying to queue inodegc work to that workqueue.
> This can happen if freezing or a ro remount race with reclaim poking our
> faux inodegc shrinker and another thread dropping an unlinked O_RDONLY
> file:
>
> Thread 0 Thread 1 Thread 2
>
> xfs_inodegc_stop
>
> xfs_inodegc_shrinker_scan
> xfs_is_inodegc_enabled
> <yes, will continue>
>
> xfs_clear_inodegc_enabled
> xfs_inodegc_queue_all
> <list empty, do not queue inodegc worker>
>
> xfs_inodegc_queue
> <add to list>
> xfs_is_inodegc_enabled
> <no, returns>
>
> drain_workqueue
> <set WQ_DRAINING>
>
> llist_empty
> <no, will queue list>
> mod_delayed_work_on(..., 0)
> __queue_work
> <sees WQ_DRAINING, kaboom>
>
> In other words, everything between the access to inodegc_enabled state
> and the decision to poke the inodegc workqueue requires some kind of
> coordination to avoid the WQ_DRAINING state. We could perhaps introduce
> a lock here, but we could also try to eliminate WQ_DRAINING from the
> picture.
>
> We could replace the drain_workqueue call with a loop that flushes the
> workqueue and queues workers as long as there is at least one inode
> present in the per-cpu inodegc llists. We've disabled inodegc at this
> point, so we know that the number of queued inodes will eventually hit
> zero as long as xfs_inodegc_start cannot reactivate the workers.
>
> There are four callers of xfs_inodegc_start. Three of them come from the
> VFS with s_umount held: filesystem thawing, failed filesystem freezing,
> and the rw remount transition. The fourth caller is mounting rw (no
> remount or freezing possible).
>
> There are three callers ofs xfs_inodegc_stop. One is unmounting (no
> remount or thaw possible). Two of them come from the VFS with s_umount
> held: fs freezing and ro remount transition.
Ok, so effectively we are using s_umount to serialise inodegc
start/stop transitions.
....
> @@ -1911,24 +1916,39 @@ xfs_inodegc_flush(
>
> /*
> * Flush all the pending work and then disable the inode inactivation background
> - * workers and wait for them to stop.
> + * workers and wait for them to stop. Do not call xfs_inodegc_start until this
> + * finishes.
> */
> void
> xfs_inodegc_stop(
> struct xfs_mount *mp)
> {
I'd prefer to document that these two functions should not be called
without holding the sb->s_umount lock rather than leaving the
serialisation mechanism undocumented. When we add the exclusive
freeze code for the fscounter scrub sync, that's also going to hold
the sb->s_umount lock while inodegc is stopped and started, right?
> + bool rerun;
> +
> if (!xfs_clear_inodegc_enabled(mp))
> return;
>
> + /*
> + * Drain all pending inodegc work, including inodes that could be
> + * queued by racing xfs_inodegc_queue or xfs_inodegc_shrinker_scan
> + * threads that sample the inodegc state just prior to us clearing it.
> + * The inodegc flag state prevents new threads from queuing more
> + * inodes, so we queue pending work items and flush the workqueue until
> + * all inodegc lists are empty.
> + */
> xfs_inodegc_queue_all(mp);
> - drain_workqueue(mp->m_inodegc_wq);
> + do {
> + flush_workqueue(mp->m_inodegc_wq);
> + rerun = xfs_inodegc_queue_all(mp);
> + } while (rerun);
Would it be worth noting that we don't use drain_workqueue() because
it doesn't allow other unserialised mechanisms to reschedule inodegc
work while the draining is in progress?
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
