[Cluster-devel] [GFS2 PATCH] gfs2: Close timing window with GLF_INVALIDATE_IN_PROGRESS

[Bob Peterson]

Hi,

This patch closes a timing window in which two processes compete
and overlap in the execution of do_xmote for the same glock:

             Process A                              Process B
   ------------------------------------   -----------------------------
1. Grabs gl_lockref and calls do_xmote
2.                                        Grabs gl_lockref but is blocked
3. Sets GLF_INVALIDATE_IN_PROGRESS
4. Unlocks gl_lockref
5.                                        Calls do_xmote
6. Call glops->go_sync
7. test_and_clear_bit GLF_DIRTY
8. Call gfs2_log_flush                    Call glops->go_sync
9. (slow IO, so it blocks a long time)    test_and_clear_bit GLF_DIRTY
                                          It's not dirty (step 7) returns
10.                                       Tests GLF_INVALIDATE_IN_PROGRESS
11.                                       Calls go_inval (rgrp_go_inval)
12.                                       gfs2_rgrp_relse does brelse
13.                                       truncate_inode_pages_range
14.                                       Calls lm_lock UN

In step 14 we've just told dlm to give the glock to another node
when, in fact, process A has not finished the IO and synced all
buffer_heads to disk and make sure their revokes are done.

This patch fixes the problem by changing the GLF_INVALIDATE_IN_PROGRESS
to use test_and_set_bit, and if the bit is already set, process B just
ignores it and trusts that process A will do the do_xmote in the proper
order.

Signed-off-by: Bob Peterson <rpeterso at redhat.com>
---
 fs/gfs2/glock.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
index faa88bd594e2..4a4a390ffd00 100644
--- a/fs/gfs2/glock.c
+++ b/fs/gfs2/glock.c
@@ -558,7 +558,19 @@ __acquires(&gl->gl_lockref.lock)
 	GLOCK_BUG_ON(gl, gl->gl_state == gl->gl_target);
 	if ((target == LM_ST_UNLOCKED || target == LM_ST_DEFERRED) &&
 	    glops->go_inval) {
-		set_bit(GLF_INVALIDATE_IN_PROGRESS, &gl->gl_flags);
+		/*
+		 * If another process is already doing the invalidate we should
+		 * not interfere. If we call go_sync and it finds the glock is
+		 * not dirty, we might call go_inval prematurely before the
+		 * other go_sync has finished with its revokes. If we then call
+		 * lm_lock prematurely, we've really screwed up: we cannot tell
+		 * dlm to give the glock away until we're synced and
+		 * invalidated. Best thing is to return and trust the other
+		 * process will finish do_xmote tasks in their proper order.
+		 */
+		if (test_and_set_bit(GLF_INVALIDATE_IN_PROGRESS,
+				     &gl->gl_flags))
+			return;
 		do_error(gl, 0); /* Fail queued try locks */
 	}
 	gl->gl_req = target;