linux-kernel - [PATCH] f2fs: Fix a race condition in next_free

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Date:	Fri, 12 Sep 2014 19:21:11 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	Jaegeuk Kim <jaegeuk@...nel.org>,
	Changman Lee <cm224.lee@...sung.com>
Cc:	linux-f2fs-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, Huang Ying <ying.huang@...el.com>,
	huang.ying.caritas@...il.com
Subject: [PATCH] f2fs: Fix a race condition in next_free_nid

The nm_i->fcnt checking is executed before spin_lock, so if another
thread delete the last free_nid from the list, the wrong nid may be
gotten.  So fix the race condition by moving the nm_i->fnct checking
into spin_lock.

Signed-off-by: Huang, Ying <ying.huang@...el.com>
---
 fs/f2fs/node.h |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/fs/f2fs/node.h
+++ b/fs/f2fs/node.h
@@ -115,9 +115,11 @@ static inline int next_free_nid(struct f
 	struct f2fs_nm_info *nm_i = NM_I(sbi);
 	struct free_nid *fnid;
 
-	if (nm_i->fcnt <= 0)
-		return -1;
 	spin_lock(&nm_i->free_nid_list_lock);
+	if (nm_i->fcnt <= 0) {
+		spin_unlock(&nm_i->free_nid_list_lock);
+		return -1;
+	}
 	fnid = list_entry(nm_i->free_nid_list.next, struct free_nid, list);
 	*nid = fnid->nid;
 	spin_unlock(&nm_i->free_nid_list_lock);
--
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