linux-kernel - Re: bsg locking patches update

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Message-Id: <200805281351.m4SDpTIl030554@mbox.iij4u.or.jp>
Date:	Wed, 28 May 2008 22:51:29 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	pw@....edu
Cc:	fujita.tomonori@....ntt.co.jp
Subject: Re: bsg locking patches update

From: FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Subject: Re: bsg locking patches update
Date: Wed, 28 May 2008 21:00:56 +0900

> On Mon, 26 May 2008 12:53:18 -0400
> Pete Wyckoff <pw@....edu> wrote:
> 
> > I finally got around to testing the set of lifetime management
> > fixes you applied.  This is 2.6.26-rc3 with some varlen, bidi,
> > iser patches, and iovec on bsg, but nothing that should affect
> > the locking.
> > 
> > I can confirm that the first two of these three old bugs are
> > no longer reproducable:
> > 
> >     http://marc.info/?l=linux-scsi&m=120508166505141&w=2
> >     http://marc.info/?l=linux-scsi&m=120508177905365&w=2
> >     http://marc.info/?l=linux-scsi&m=120508178005376&w=2
> > 
> > Thanks!  The third, however, is a hang that still can happen.  But
> > it is very obscure and requires a bit of timing to get right.  As a
> > reminder, here's the setup, and updated traces.
> 
> Ah, sorry about it. I didn't understand the third correctly.
> 
> 
> > Maybe it is necessary to split up that bsg_mutex to use multiple
> > finer-grained locks.
> 
> We could but we use bsg_mutex to protect bsg_device_list and idr. So I
> think that we don't need hold bsg_mutex during
> bsg_complete_all_commands. How about this?
> 
> 
> diff --git a/block/bsg.c b/block/bsg.c
> index f0b7cd3..d81104e 100644
> --- a/block/bsg.c
> +++ b/block/bsg.c

On second thoughts, I realized that the previous patch leads to a race
between bsg_put_device and __bsg_get_device (__bsg_get_device possibly
finds a device that is being removed). Here's new one.


diff --git a/block/bsg.c b/block/bsg.c
index f0b7cd3..7cdec32 100644
--- a/block/bsg.c
+++ b/block/bsg.c
@@ -724,8 +724,13 @@ static int bsg_put_device(struct bsg_device *bd)
 	mutex_lock(&bsg_mutex);
 
 	do_free = atomic_dec_and_test(&bd->ref_count);
-	if (!do_free)
+	if (!do_free) {
+		mutex_unlock(&bsg_mutex);
 		goto out;
+	}
+
+	hlist_del(&bd->dev_list);
+	mutex_unlock(&bsg_mutex);
 
 	dprintk("%s: tearing down\n", bd->name);
 
@@ -741,10 +746,8 @@ static int bsg_put_device(struct bsg_device *bd)
 	 */
 	ret = bsg_complete_all_commands(bd);
 
-	hlist_del(&bd->dev_list);
 	kfree(bd);
 out:
-	mutex_unlock(&bsg_mutex);
 	kref_put(&q->bsg_dev.ref, bsg_kref_release_function);
 	if (do_free)
 		blk_put_queue(q);
--
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