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 <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>