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.
Hence, it is correct to replace the drain_workqueue call with a loop
that drains the inodegc llists.
Fixes: 6191cf3ad59f ("xfs: flush inodegc workqueue tasks before cancel")
Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
---
fs/xfs/xfs_icache.c | 30 +++++++++++++++++++++++++-----
1 file changed, 25 insertions(+), 5 deletions(-)
diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
index 4b63c065ef19..14cb660d7f55 100644
--- a/fs/xfs/xfs_icache.c
+++ b/fs/xfs/xfs_icache.c
@@ -435,18 +435,23 @@ xfs_iget_check_free_state(
}
/* Make all pending inactivation work start immediately. */
-static void
+static bool
xfs_inodegc_queue_all(
struct xfs_mount *mp)
{
struct xfs_inodegc *gc;
int cpu;
+ bool ret = false;
for_each_online_cpu(cpu) {
gc = per_cpu_ptr(mp->m_inodegc, cpu);
- if (!llist_empty(&gc->list))
+ if (!llist_empty(&gc->list)) {
mod_delayed_work_on(cpu, mp->m_inodegc_wq, &gc->work, 0);
+ ret = true;
+ }
}
+
+ return ret;
}
/*
@@ -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)
{
+ 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);
trace_xfs_inodegc_stop(mp, __return_address);
}
/*
* Enable the inode inactivation background workers and schedule deferred inode
- * inactivation work if there is any.
+ * inactivation work if there is any. Do not call this while xfs_inodegc_stop
+ * is running.
*/
void
xfs_inodegc_start(
