linux-kernel - [PATCH] zcache: fix deadlock condition

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Date:	Wed, 25 Jan 2012 14:32:51 -0800 (PST)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Konrad Wilk <konrad.wilk@...cle.com>, linux-kernel@...r.kernel.org,
	devel@...verdev.osuosl.org, Greg Kroah-Hartman <gregkh@...e.de>
Cc:	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Nitin Gupta <ngupta@...are.org>
Subject: [PATCH] zcache: fix deadlock condition

I discovered this deadlock condition awhile ago working on RAMster
but it affects zcache as well.  The list spinlock must be
locked prior to the page spinlock and released after.  As
a result, the page copy must also be done while the locks are held.

Applies to 3.2.  Konrad, please push (via GregKH?)...
this is definitely a bug fix so need not be pushed during
a -rc0 window.

Signed-off-by: Dan Magenheimer <dan.magenheimer@...cle.com>

diff --git a/drivers/staging/zcache/zcache-main.c b/drivers/staging/zcache/zcache-main.c
index 56c1f9c..5b9f74e 100644
--- a/drivers/staging/zcache/zcache-main.c
+++ b/drivers/staging/zcache/zcache-main.c
@@ -358,8 +358,8 @@ static struct zbud_hdr *zbud_create(uint16_t client_id, uint16_t pool_id,
 	if (unlikely(zbpg == NULL))
 		goto out;
 	/* ok, have a page, now compress the data before taking locks */
-	spin_lock(&zbpg->lock);
 	spin_lock(&zbud_budlists_spinlock);
+	spin_lock(&zbpg->lock);
 	list_add_tail(&zbpg->bud_list, &zbud_unbuddied[nchunks].list);
 	zbud_unbuddied[nchunks].count++;
 	zh = &zbpg->buddy[0];
@@ -389,12 +389,11 @@ init_zh:
 	zh->oid = *oid;
 	zh->pool_id = pool_id;
 	zh->client_id = client_id;
-	/* can wait to copy the data until the list locks are dropped */
-	spin_unlock(&zbud_budlists_spinlock);
-
 	to = zbud_data(zh, size);
 	memcpy(to, cdata, size);
 	spin_unlock(&zbpg->lock);
+	spin_unlock(&zbud_budlists_spinlock);
+
 	zbud_cumul_chunk_counts[nchunks]++;
 	atomic_inc(&zcache_zbud_curr_zpages);
 	zcache_zbud_cumul_zpages++;
--
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