xfs: xfs_do_force_shutdown needs to block racing shutdowns

When we call xfs_forced_shutdown(), the caller often expects the
filesystem to be completely shut down when it returns. However,
if we have racing xfs_forced_shutdown() calls, the first caller sets
the mount shutdown flag then goes to shutdown the log. The second
caller sees the mount shutdown flag and returns immediately - it
does not wait for the log to be shut down.

Unfortunately, xfs_forced_shutdown() is used in some places that
expect it to completely shut down the filesystem before it returns
(e.g. xfs_trans_log_inode()). As such, returning before the log has
been shut down leaves us in a place where the transaction failed to
complete correctly but we still call xfs_trans_commit(). This
situation arises because xfs_trans_log_inode() does not return an
error and instead calls xfs_force_shutdown() to ensure that the
transaction being committed is aborted.

Unfortunately, we have a race condition where xfs_trans_commit()
needs to check xlog_is_shutdown() because it can't abort log items
before the log is shut down, but it needs to use xfs_is_shutdown()
because xfs_forced_shutdown() does not block waiting for the log to
shut down.

To fix this conundrum, first we make all calls to
xfs_forced_shutdown() block until the log is also shut down. This
means we can then safely use xfs_forced_shutdown() as a mechanism
that ensures the currently running transaction will be aborted by
xfs_trans_commit() regardless of the shutdown check it uses.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

fs/xfs/xfs_fsops.c

@@ -17,6 +17,7 @@
 #include "xfs_fsops.h"
 #include "xfs_trans_space.h"
 #include "xfs_log.h"
+#include "xfs_log_priv.h"
 #include "xfs_ag.h"
 #include "xfs_ag_resv.h"
 #include "xfs_trace.h"
@@ -518,8 +519,11 @@ xfs_do_force_shutdown(
 	int		tag;
 	const char	*why;
 
-	if (test_and_set_bit(XFS_OPSTATE_SHUTDOWN, &mp->m_opstate))
+
+	if (test_and_set_bit(XFS_OPSTATE_SHUTDOWN, &mp->m_opstate)) {
+		xlog_shutdown_wait(mp->m_log);
 		return;
+	}
 	if (mp->m_sb_bp)
 		mp->m_sb_bp->b_flags |= XBF_DONE;
 

fs/xfs/xfs_log.c

@@ -3923,6 +3923,7 @@ xlog_force_shutdown(
 	xlog_state_shutdown_callbacks(log);
 	spin_unlock(&log->l_icloglock);
 
+	wake_up_var(&log->l_opstate);
 	return log_error;
 }
 

fs/xfs/xfs_log_priv.h

@@ -484,6 +484,17 @@ xlog_is_shutdown(struct xlog *log)
 	return test_bit(XLOG_IO_ERROR, &log->l_opstate);
 }
 
+/*
+ * Wait until the xlog_force_shutdown() has marked the log as shut down
+ * so xlog_is_shutdown() will always return true.
+ */
+static inline void
+xlog_shutdown_wait(
+	struct xlog	*log)
+{
+	wait_var_event(&log->l_opstate, xlog_is_shutdown(log));
+}
+
 /* common routines */
 extern int
 xlog_recover(