[lustre-devel] [PATCH] lustre: lu_object: fix possible hang waiting for LCS_LEAVING
NeilBrown
neilb at suse.com
Tue Oct 23 15:34:17 PDT 2018
As lu_context_key_quiesce() spins waiting for LCS_LEAVING to
change, it is important the we set and then clear in within a
non-preemptible region. If the thread that spins pre-empty the
thread that sets-and-clears the state while the state is LCS_LEAVING,
then it can spin indefinitely, particularly on a single-CPU machine.
Also update the comment to explain this dependency.
Fixes: ac3f8fd6e61b ("staging: lustre: remove locking from lu_context_exit()")
---
This is the cause of the "something" that went wrong in my recent
testing that I mentioned. I wonder if preempt_enable() has recently
been enhanced to encourage a preempt, to make this sort of bug easier to
see.
drivers/staging/lustre/lustre/obdclass/lu_object.c | 15 +++++++++------
1 file changed, 9 insertions(+), 6 deletions(-)
diff --git a/drivers/staging/lustre/lustre/obdclass/lu_object.c b/drivers/staging/lustre/lustre/obdclass/lu_object.c
index cb57abf03644..51497c144dd6 100644
--- a/drivers/staging/lustre/lustre/obdclass/lu_object.c
+++ b/drivers/staging/lustre/lustre/obdclass/lu_object.c
@@ -1654,17 +1654,20 @@ void lu_context_exit(struct lu_context *ctx)
unsigned int i;
LINVRNT(ctx->lc_state == LCS_ENTERED);
- /*
- * Ensure lu_context_key_quiesce() sees LCS_LEAVING
- * or we see LCT_QUIESCENT
- */
- smp_store_mb(ctx->lc_state, LCS_LEAVING);
/*
* Disable preempt to ensure we get a warning if
* any lct_exit ever tries to sleep. That would hurt
* lu_context_key_quiesce() which spins waiting for us.
+ * This also ensure we aren't preempted while the state
+ * is LCS_LEAVING, as that too would cause problems for
+ * lu_context_key_quiesce().
*/
preempt_disable();
+ /*
+ * Ensure lu_context_key_quiesce() sees LCS_LEAVING
+ * or we see LCT_QUIESCENT
+ */
+ smp_store_mb(ctx->lc_state, LCS_LEAVING);
if (ctx->lc_tags & LCT_HAS_EXIT && ctx->lc_value) {
for (i = 0; i < ARRAY_SIZE(lu_keys); ++i) {
struct lu_context_key *key;
@@ -1677,8 +1680,8 @@ void lu_context_exit(struct lu_context *ctx)
}
}
- preempt_enable();
smp_store_release(&ctx->lc_state, LCS_LEFT);
+ preempt_enable();
}
EXPORT_SYMBOL(lu_context_exit);
--
2.14.0.rc0.dirty
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