commit b8a9d66d upstream.

After commit 566c09c5 ("raid5: relieve lock contention in get_active_stripe()")
__find_stripe() is called under conf->hash_locks + hash.
But handle_stripe_clean_event() calls remove_hash() under
conf->device_lock.

Under some cirscumstances the hash chain can be circuited,
and we get an infinite loop with disabled interrupts and locked hash
lock in __find_stripe(). This leads to hard lockup on multiple CPUs
and following system crash.

I was able to reproduce this behavior on raid6 over 6 ssd disks.
The devices_handle_discard_safely option should be set to enable trim
support. The following script was used:

for i in `seq 1 32`; do
    dd if=/dev/zero of=large$i bs=10M count=100 &
done
Signed-off-by: Roman Gushchin <klamm@yandex-team.ru>
Fixes: 566c09c5 ("raid5: relieve lock contention in get_active_stripe()")
Signed-off-by: NeilBrown <neilb@suse.com>
Cc: Shaohua Li <shli@kernel.org>
[ luis: backported to 3.16: used Roman's backport to 3.14 ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>