[PATCH] sbitmap: Protect swap_lock from softirqs

[PATCH] sbitmap: Protect swap_lock from softirqs

[Steven Rostedt]

From: Steven Rostedt (VMware) <rostedt@goodmis.org>

The swap_lock used by sbitmap has a chain with locks taken from
softirq, but the swap_lock is not protected from being preempted by
softirqs.

A chain exists of:

 sbq->ws[i].wait -> dispatch_wait_lock -> swap_lock

Where the sbq->ws[i].wait lock can be taken from softirq context, which
means all locks below it in the chain must also be protected from
softirqs.

Cc: stable@vger.kernel.org
Reported-by: Clark Williams <williams@redhat.com>
Fixes: 58ab5e32e6fd ("sbitmap: silence bogus lockdep IRQ warning")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 65c2d06250a6..864354000e04 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -26,14 +26,9 @@
 static inline bool sbitmap_deferred_clear(struct sbitmap *sb, int index)
 {
 	unsigned long mask, val;
-	unsigned long __maybe_unused flags;
 	bool ret = false;
 
-	/* Silence bogus lockdep warning */
-#if defined(CONFIG_LOCKDEP)
-	local_irq_save(flags);
-#endif
-	spin_lock(&sb->map[index].swap_lock);
+	spin_lock_bh(&sb->map[index].swap_lock);
 
 	if (!sb->map[index].cleared)
 		goto out_unlock;
@@ -54,10 +49,7 @@ static inline bool sbitmap_deferred_clear(struct sbitmap *sb, int index)
 
 	ret = true;
 out_unlock:
-	spin_unlock(&sb->map[index].swap_lock);
-#if defined(CONFIG_LOCKDEP)
-	local_irq_restore(flags);
-#endif
+	spin_unlock_bh(&sb->map[index].swap_lock);
 	return ret;
 }
 

Re: [PATCH] sbitmap: Protect swap_lock from softirqs

[Linus Torvalds]

On Tue, Jan 15, 2019 at 5:25 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> The swap_lock used by sbitmap has a chain with locks taken from
> softirq, but the swap_lock is not protected from being preempted by
> softirqs.
>
> A chain exists of:
>
>  sbq->ws[i].wait -> dispatch_wait_lock -> swap_lock
>
> Where the sbq->ws[i].wait lock can be taken from softirq context, which
> means all locks below it in the chain must also be protected from
> softirqs.
>
> Cc: stable@vger.kernel.org

Ack and applied, except for the "stable" tag.

Both the original buggy code, and the incorrect "shut up lockdep" were
only merged after 4.20.

                 Linus