[PATCH] sbitmap: Protect swap_lock from hardirq

[PATCH] sbitmap: Protect swap_lock from hardirq

[Ming Lei]

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

Re: [PATCH] sbitmap: Protect swap_lock from hardirq

[Jens Axboe]

On 1/14/19 8:59 PM, Ming Lei wrote:
> 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.

Thanks Ming, I'll queue this up for shipping this week.

-- 
Jens Axboe

Re: [PATCH] sbitmap: Protect swap_lock from hardirq

[Linus Torvalds]

On Tue, Jan 15, 2019 at 4:28 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> Thanks Ming, I'll queue this up for shipping this week.

Oops. I _just_ applied it to my tree as a follow-up to Steven's
softirq version. I just hadn't had time to build test and push out
yet.

               Linus

Re: [PATCH] sbitmap: Protect swap_lock from hardirq

[Jens Axboe]

On 1/14/19 9:31 PM, Linus Torvalds wrote:
> On Tue, Jan 15, 2019 at 4:28 PM Jens Axboe <axboe@kernel.dk> wrote:
>>
>> Thanks Ming, I'll queue this up for shipping this week.
> 
> Oops. I _just_ applied it to my tree as a follow-up to Steven's
> softirq version. I just hadn't had time to build test and push out
> yet.

No big deal, fwiw, this is what I queued up:

http://git.kernel.dk/cgit/linux-block/commit/?h=for-linus&id=8218a55b6b911d396565da4ed5ca8b18bf0d38fb

which has a spelling error fixed, and the indentation a bit nicer
for the locking scenario. But I can just drop it.

-- 
Jens Axboe