linux-kernel - Re: [PATCH] block: fix mq request allocation

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Message-ID: <x49ob4z700n.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 02 Dec 2013 10:20:08 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] block: fix mq request allocation

Ming Lei <tom.leiming@...il.com> writes:

> blk_mq_alloc_request_pinned() may return NULL request in case of
> !__GFP_WAIT, so cause its callers to derefence NULL pointer for
> releasing current context.
>
> This patch introduces two flags to address the issue.

Hi, Ming,


Good catch, but your patch seems overly complicated.  How about
something like the following (compile-tested only), instead?  Note that
I did not touch blk_make_request, as the put_ctx there seems to
correlate to a get_ctx earlier in the function (not a leaked reference
from __blk_mq_alloc_request).

-Jeff

p.s. Jens, every time I see GFP_ATOMIC|__GFP_WAIT, my head explodes.  Just sayin'.

Signed-off-by: Jeff Moyer <jmoyer@...hat.com>


diff --git a/block/blk-mq.c b/block/blk-mq.c
index cdc629c..70fd6f9 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -202,10 +202,12 @@ static struct request *blk_mq_alloc_request_pinned(struct request_queue *q,
 		if (rq) {
 			blk_mq_rq_ctx_init(q, ctx, rq, rw);
 			break;
-		} else if (!(gfp & __GFP_WAIT))
-			break;
+		}
 
 		blk_mq_put_ctx(ctx);
+		if (!(gfp & __GFP_WAIT))
+			break;
+
 		__blk_mq_run_hw_queue(hctx);
 		blk_mq_wait_for_tags(hctx->tags);
 	} while (1);
@@ -222,7 +224,8 @@ struct request *blk_mq_alloc_request(struct request_queue *q, int rw,
 		return NULL;
 
 	rq = blk_mq_alloc_request_pinned(q, rw, gfp, reserved);
-	blk_mq_put_ctx(rq->mq_ctx);
+	if (rq)
+		blk_mq_put_ctx(rq->mq_ctx);
 	return rq;
 }
 
@@ -235,7 +238,8 @@ struct request *blk_mq_alloc_reserved_request(struct request_queue *q, int rw,
 		return NULL;
 
 	rq = blk_mq_alloc_request_pinned(q, rw, gfp, true);
-	blk_mq_put_ctx(rq->mq_ctx);
+	if (rq)
+		blk_mq_put_ctx(rq->mq_ctx);
 	return rq;
 }
 EXPORT_SYMBOL(blk_mq_alloc_reserved_request);
--
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