linux-kernel - [PATCH 5/8] Unionfs: avoid a deadlock during branch-management on a pivot

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Date:	Fri,  2 Nov 2007 21:22:46 -0400
From:	Erez Zadok <ezk@...sunysb.edu>
To:	akpm@...ux-foundation.org
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	viro@....linux.org.uk, hch@...radead.org,
	Erez Zadok <ezk@...sunysb.edu>
Subject: [PATCH 5/8] Unionfs: avoid a deadlock during branch-management on a pivot_root'ed union

Signed-off-by: Erez Zadok <ezk@...sunysb.edu>
Acked-by: Herton Ronaldo Krzesinski <herton@...driva.com>
---
 fs/unionfs/union.h |   39 +++++++++++++++++++++++++++++++++------
 1 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/fs/unionfs/union.h b/fs/unionfs/union.h
index 6333488..0476f97 100644
--- a/fs/unionfs/union.h
+++ b/fs/unionfs/union.h
@@ -142,11 +142,18 @@ struct unionfs_sb_info {
 	 * This rwsem is used to make sure that a branch management
 	 * operation...
 	 *   1) will not begin before all currently in-flight operations
-	 *      complete
+	 *      complete.
 	 *   2) any new operations do not execute until the currently
-	 *      running branch management operation completes
+	 *      running branch management operation completes.
+	 *
+	 * The write_lock_owner records the PID of the task which grabbed
+	 * the rw_sem for writing.  If the same task also tries to grab the
+	 * read lock, we allow it.  This prevents a self-deadlock when
+	 * branch-management is used on a pivot_root'ed union, because we
+	 * have to ->lookup paths which belong to the same union.
 	 */
 	struct rw_semaphore rwsem;
+	pid_t write_lock_owner;	/* PID of rw_sem owner (write lock) */
 	int high_branch_id;	/* last unique branch ID given */
 	struct unionfs_data *data;
 };
@@ -234,10 +241,30 @@ static inline off_t rdstate2offset(struct unionfs_dir_state *buf)
 	return tmp;
 }
 
-#define unionfs_read_lock(sb)	 down_read(&UNIONFS_SB(sb)->rwsem)
-#define unionfs_read_unlock(sb)	 up_read(&UNIONFS_SB(sb)->rwsem)
-#define unionfs_write_lock(sb)	 down_write(&UNIONFS_SB(sb)->rwsem)
-#define unionfs_write_unlock(sb) up_write(&UNIONFS_SB(sb)->rwsem)
+static inline void unionfs_read_lock(struct super_block *sb)
+{
+	if (UNIONFS_SB(sb)->write_lock_owner &&
+	    UNIONFS_SB(sb)->write_lock_owner == current->pid)
+		return;
+	down_read(&UNIONFS_SB(sb)->rwsem);
+}
+static inline void unionfs_read_unlock(struct super_block *sb)
+{
+	if (UNIONFS_SB(sb)->write_lock_owner &&
+	    UNIONFS_SB(sb)->write_lock_owner == current->pid)
+		return;
+	up_read(&UNIONFS_SB(sb)->rwsem);
+}
+static inline void unionfs_write_lock(struct super_block *sb)
+{
+	down_write(&UNIONFS_SB(sb)->rwsem);
+	UNIONFS_SB(sb)->write_lock_owner = current->pid;
+}
+static inline void unionfs_write_unlock(struct super_block *sb)
+{
+	up_write(&UNIONFS_SB(sb)->rwsem);
+	UNIONFS_SB(sb)->write_lock_owner = 0;
+}
 
 static inline void unionfs_double_lock_dentry(struct dentry *d1,
 					      struct dentry *d2)
-- 
1.5.2.2

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