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Subject[PATCH] sbitmap: Protect swap_lock from hardirq
Date
The original report is actually one real deadlock:

[ 106.132865] Possible interrupt unsafe locking scenario:
[ 106.132865]
[ 106.133659] CPU0 CPU1
[ 106.134194] ---- ----
[ 106.134733] lock(&(&sb->map[i].swap_lock)->rlock);
[ 106.135318] local_irq_disable();
[ 106.136014] lock(&sbq->ws[i].wait);
[ 106.136747] lock(&(&hctx->dispatch_wait_lock)->rlock);
[ 106.137742] <Interrupt>
[ 106.138110] lock(&sbq->ws[i].wait);

Because we may call blk_mq_get_driver_tag() directly from
blk_mq_dispatch_rq_list() without holding any lock, then HARDIRQ may come
and the above DEADLOCK is triggered.

ab53dcfb3e7b ("sbitmap: Protect swap_lock from hardirq") tries to fix
this issue by using 'spin_lock_bh', which isn't enough because we complete
request from hardirq context direclty in case of multiqueue.

Cc: Clark Williams <williams@redhat.com>
Fixes: ab53dcfb3e7b ("sbitmap: Protect swap_lock from hardirq")
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
lib/sbitmap.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 864354000e04..5b382c1244ed 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -27,8 +27,9 @@ static inline bool sbitmap_deferred_clear(struct sbitmap *sb, int index)
{
unsigned long mask, val;
bool ret = false;
+ unsigned long flags;

- spin_lock_bh(&sb->map[index].swap_lock);
+ spin_lock_irqsave(&sb->map[index].swap_lock, flags);

if (!sb->map[index].cleared)
goto out_unlock;
@@ -49,7 +50,7 @@ static inline bool sbitmap_deferred_clear(struct sbitmap *sb, int index)

ret = true;
out_unlock:
- spin_unlock_bh(&sb->map[index].swap_lock);
+ spin_unlock_irqrestore(&sb->map[index].swap_lock, flags);
return ret;
}

--
2.14.4
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