From 0ca1a8331c0fa5e57844e003a5d667a15b1e002c Mon Sep 17 00:00:00 2001 From: "Darrick J. Wong" Date: Mon, 2 Mar 2026 09:31:58 -0800 Subject: [PATCH] xfs: fix race between healthmon unmount and read_iter xfs/1879 on one of my test VMs got stuck due to the xfs_io healthmon subcommand sleeping in wait_event_interruptible at: xfs_healthmon_read_iter+0x558/0x5f8 [xfs] vfs_read+0x248/0x320 ksys_read+0x78/0x120 Looking at xfs_healthmon_read_iter, in !O_NONBLOCK mode it will sleep until the mount cookie == DETACHED_MOUNT_COOKIE, there are events waiting to be formatted, or there are formatted events in the read buffer that could be copied to userspace. Poking into the running kernel, I see that there are zero events in the list, the read buffer is empty, and the mount cookie is indeed in DETACHED state. IOWs, xfs_healthmon_has_eventdata should have returned true, but instead we're asleep waiting for a wakeup. I think what happened here is that xfs_healthmon_read_iter and xfs_healthmon_unmount were racing with each other, and _read_iter lost the race. _unmount queued an unmount event, which woke up _read_iter. It found, formatted, and copied the event out to userspace. That cleared out the pending event list and emptied the read buffer. xfs_io then called read() again, so _has_eventdata decided that we should sleep on the empty event queue. Next, _unmount called xfs_healthmon_detach, which set the mount cookie to DETACHED. Unfortunately, it didn't call wake_up_all on the hm, so the wait_event_interruptible in the _read_iter thread remains asleep. That's why the test stalled. Fix this by moving the wake_up_all call to xfs_healthmon_detach. Fixes: b3a289a2a9397b ("xfs: create event queuing, formatting, and discovery infrastructure") Signed-off-by: Darrick J. Wong Reviewed-by: Christoph Hellwig Signed-off-by: Carlos Maiolino --- fs/xfs/xfs_healthmon.c | 17 ++++++++++------- 1 file changed, 10 insertions(+), 7 deletions(-) diff --git a/fs/xfs/xfs_healthmon.c b/fs/xfs/xfs_healthmon.c index 4a06d6632f65..26c325d34bd1 100644 --- a/fs/xfs/xfs_healthmon.c +++ b/fs/xfs/xfs_healthmon.c @@ -141,6 +141,16 @@ xfs_healthmon_detach( hm->mount_cookie = DETACHED_MOUNT_COOKIE; spin_unlock(&xfs_healthmon_lock); + /* + * Wake up any readers that might remain. This can happen if unmount + * races with the healthmon fd owner entering ->read_iter, having + * already emptied the event queue. + * + * In the ->release case there shouldn't be any readers because the + * only users of the waiter are read and poll. + */ + wake_up_all(&hm->wait); + trace_xfs_healthmon_detach(hm); xfs_healthmon_put(hm); } @@ -1027,13 +1037,6 @@ xfs_healthmon_release( * process can create another health monitor file. */ xfs_healthmon_detach(hm); - - /* - * Wake up any readers that might be left. There shouldn't be any - * because the only users of the waiter are read and poll. - */ - wake_up_all(&hm->wait); - xfs_healthmon_put(hm); return 0; }