linux-kernel - Re: __might_sleep() warnings on v3.19-rc6

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Date:	Mon, 2 Feb 2015 17:09:37 +1100
From:	NeilBrown <neilb@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Fengguang Wu <fengguang.wu@...el.com>,
	Peter Zijlstra <peterz@...radead.org>, LKP <lkp@...org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Shaohua Li <shli@...nel.org>,
	Dan Williams <dan.j.williams@...el.com>,
	"linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>
Subject: Re: __might_sleep() warnings on v3.19-rc6

On Sun, 1 Feb 2015 21:08:12 -0800 Linus Torvalds
<torvalds@...ux-foundation.org> wrote:

> On Sun, Feb 1, 2015 at 3:03 PM, NeilBrown <neilb@...e.de> wrote:
> >
> > I guess I could
> >       __set_current_state(TASK_RUNNING);
> > somewhere to defeat the warning, and add a comment explaining why.
> >
> > Would that be a good thing?
> 
> Use "sched_annotate_sleep()" instead, but yes, add a comment about why it's ok.
> 
>                         Linus
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OK - following patch is queued to appear in a pull request tomorrow  (I hope).

Thanks,
NeilBrown

From: NeilBrown <neilb@...e.de>
Date: Mon, 2 Feb 2015 17:08:03 +1100
Subject: [PATCH] md/bitmap: fix a might_sleep() warning.

commit 8eb23b9f35aae413140d3fda766a98092c21e9b0
    sched: Debug nested sleeps

causes false-positive warnings in RAID5 code.

This annotation removes them and adds a comment
explaining why there is no real problem.

Reported-by: Fengguang Wu <fengguang.wu@...el.com>
Signed-off-by: NeilBrown <neilb@...e.de>

diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
index da3604e73e8a..1695ee5f3ffc 100644
--- a/drivers/md/bitmap.c
+++ b/drivers/md/bitmap.c
@@ -72,6 +72,19 @@ __acquires(bitmap->lock)
 	/* this page has not been allocated yet */
 
 	spin_unlock_irq(&bitmap->lock);
+	/* It is possible that this is being called inside a
+	 * prepare_to_wait/finish_wait loop from raid5c:make_request().
+	 * In general it is not permitted to sleep in that context as it
+	 * can cause the loop to spin freely.
+	 * That doesn't apply here as we can only reach this point
+	 * once with any loop.
+	 * When this function completes, either bp[page].map or
+	 * bp[page].hijacked.  In either case, this function will
+	 * abort before getting to this point again.  So there is
+	 * no risk of a free-spin, and so it is safe to assert
+	 * that sleeping here is allowed.
+	 */
+	sched_annotate_sleep();
 	mappage = kzalloc(PAGE_SIZE, GFP_NOIO);
 	spin_lock_irq(&bitmap->lock);
 

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