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 <[email protected]>
---
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;
