On Fri, Apr 28, 2023 at 12:29:47PM +1000, Dave Chinner wrote:
> 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.
Correct.
> ....
>
> > @@ -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.
Done. "Caller must hold sb->s_umount to coordinate changes in the
inodegc_enabled state."
> 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?
Yes, the exclusive freeze mechanism takes all the same locks and makes
all the same state changes as regular freeze.
>
> > + 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?
I'll paste that right in!
--D
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx
