linux-kernel - [PATCH] sbitmap: Protect swap

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Date:   Tue, 15 Jan 2019 11:59:52 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, Ming Lei <ming.lei@...hat.com>,
        Clark Williams <williams@...hat.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] sbitmap: Protect swap_lock from hardirq

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@...hat.com>
Fixes: ab53dcfb3e7b ("sbitmap: Protect swap_lock from hardirq")
Cc: Jens Axboe <axboe@...nel.dk>
Cc: Ming Lei <ming.lei@...hat.com>
Cc: Guenter Roeck <linux@...ck-us.net>
Cc: Steven Rostedt (VMware) <rostedt@...dmis.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org
Signed-off-by: Ming Lei <ming.lei@...hat.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