Fix reference counting race on log buffers

49641f1a · Dave Chinner <[email protected]> · 2008-07-11 17:43:55 +10:00

When we release the iclog, we do an atomic_dec_and_lock to determine if
we are the last reference and need to trigger update of log headers and
writeout.  However, in xlog_state_get_iclog_space() we also need to
check if we have the last reference count there.  If we do, we release
the log buffer, otherwise we decrement the reference count.

But the compare and decrement in xlog_state_get_iclog_space() is not
atomic, so both places can see a reference count of 2 and neither will
release the iclog.  That leads to a filesystem hang.

Close the race by replacing the atomic_read() and atomic_dec() pair with
atomic_add_unless() to ensure that they are executed atomically.

Signed-off-by: Dave Chinner <[email protected]>
Reviewed-by: Tim Shimmin <[email protected]>
Tested-by: Eric Sandeen <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Commit: 49641f1acfdfd437ed9b0a70b86bf36626c02afe
Parents: 61ca9daa

Changes

1 files changed · +11 −4

File	Status	Add	Del	Content
fs/xfs/xfs_log.c	modified	+11	−4	View file

fs/xfs/xfs_log.c

View file

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index afaee30..ad3d26d 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -2427,13 +2427,20 @@ restart:
 	if (iclog->ic_size - iclog->ic_offset < 2*sizeof(xlog_op_header_t)) {
 		xlog_state_switch_iclogs(log, iclog, iclog->ic_size);
 
-		/* If I'm the only one writing to this iclog, sync it to disk */
-		if (atomic_read(&iclog->ic_refcnt) == 1) {
+		/*
+		 * If I'm the only one writing to this iclog, sync it to disk.
+		 * We need to do an atomic compare and decrement here to avoid
+		 * racing with concurrent atomic_dec_and_lock() calls in
+		 * xlog_state_release_iclog() when there is more than one
+		 * reference to the iclog.
+		 */
+		if (!atomic_add_unless(&iclog->ic_refcnt, -1, 1)) {
+			/* we are the only one */
 			spin_unlock(&log->l_icloglock);
-			if ((error = xlog_state_release_iclog(log, iclog)))
+			error = xlog_state_release_iclog(log, iclog);
+			if (error)
 				return error;
 		} else {
-			atomic_dec(&iclog->ic_refcnt);
 			spin_unlock(&log->l_icloglock);
 		}
 		goto restart;