[lustre-devel] [PATCH 16/28] lustre: statahead: missing barrier before wake_up

[NeilBrown]

On Sun, Oct 21 2018, James Simmons wrote:

>> On Sun, Oct 14 2018, James Simmons wrote:
>> 
>> > From: Lai Siyao <lai.siyao at whamcloud.com>
>> >
>> > A barrier is missing before wake_up() in ll_statahead_interpret(),
>> > which may cause 'ls' hang. Under the right conditions a basic 'ls'
>> > can fail. The debug logs show:
>> >
>> > statahead.c:683:ll_statahead_interpret()) sa_entry software rc -13
>> > statahead.c:1666:ll_statahead()) revalidate statahead software: -11.
>> >
>> > Obviously statahead failure didn't notify 'ls' process in time.
>> > The mi_cbdata can be stale so add a barrier before calling
>> > wake_up().
>> >
>> > Signed-off-by: Lai Siyao <lai.siyao at whamcloud.com>
>> > Signed-off-by: Bob Glossman <bob.glossman at intel.com>
>> > WC-bug-id: https://jira.whamcloud.com/browse/LU-9210
>> > Reviewed-on: https://review.whamcloud.com/27330
>> > Reviewed-by: Nathaniel Clark <nclark at whamcloud.com>
>> > Reviewed-by: Oleg Drokin <green at whamcloud.com>
>> > Signed-off-by: James Simmons <jsimmons at infradead.org>
>> > ---
>> >  drivers/staging/lustre/lustre/llite/statahead.c | 8 +++++++-
>> >  1 file changed, 7 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/drivers/staging/lustre/lustre/llite/statahead.c b/drivers/staging/lustre/lustre/llite/statahead.c
>> > index 1ad308c..0174a4c 100644
>> > --- a/drivers/staging/lustre/lustre/llite/statahead.c
>> > +++ b/drivers/staging/lustre/lustre/llite/statahead.c
>> > @@ -680,8 +680,14 @@ static int ll_statahead_interpret(struct ptlrpc_request *req,
>> >  
>> >  	spin_lock(&lli->lli_sa_lock);
>> >  	if (rc) {
>> > -		if (__sa_make_ready(sai, entry, rc))
>> > +		if (__sa_make_ready(sai, entry, rc)) {
>> > +			/* LU-9210 : Under the right conditions even 'ls'
>> > +			 * can cause the statahead to fail. Using a memory
>> > +			 * barrier resolves this issue.
>> > +			 */
>> > +			smp_mb();
>> >  			wake_up(&sai->sai_waitq);
>> > +		}
>> >  	} else {
>> >  		int first = 0;
>> >  		entry->se_minfo = minfo;
>> > -- 
>> > 1.8.3.1
>> 
>> Again, this is a fairly lame comment to justify the smp_mb().
>> It appears to me that the issue is most likely the value of
>> entry->se_state.
>> __sa_make_ready() sets this and revalidate_statahead_dentry tests it
>> after waiting on sai_waitq.
>> So I think it would be best if we changed __sa_make_ready() to
>> 
>> 	smp_store_release(&entry->se_state, ret < 0 ? SA_ENTRY_INVA : SA_ENTRY_SUCC)
>> 
>> and in ll_statahead_interpret() have
>> 
>> 	if (smp_load_acquire(&entry->se_state) == SA_ENTRY_SUCC &&
>>             entry->se_inode) {
>> 
>> This would make it obvious which variable was important, and would show
>> the paired synchronization points.
>
> If you think this is lame you should be the JIRA ticket and the original 
> patch. It had zero info so I attempted to extract what I could out of the
> ticket. Hopefully Lai can fill in the details. I have no problems fixing 
> this another way. I don't see a way in the ticket to easily reproduce this
> problem to see the new approach would fix it :-(

I have imposed the version that I think is correct.  See below.

Thanks,
NeilBrown

--- a/drivers/staging/lustre/lustre/llite/statahead.c
+++ b/drivers/staging/lustre/lustre/llite/statahead.c
@@ -322,7 +322,11 @@ __sa_make_ready(struct ll_statahead_info *sai, struct sa_entry *entry, int ret)
 		}
 	}
 	list_add(&entry->se_list, pos);
-	entry->se_state = ret < 0 ? SA_ENTRY_INVA : SA_ENTRY_SUCC;
+	/*
+	 * LU-9210: ll_statahead_interpet must be able to see this before
+	 * we wake it up
+	 */
+	smp_store_release(&entry->se_state, ret < 0 ? SA_ENTRY_INVA : SA_ENTRY_SUCC);
 
 	return (index == sai->sai_index_wait);
 }
@@ -1390,7 +1394,12 @@ static int revalidate_statahead_dentry(struct inode *dir,
 		}
 	}
 
-	if (entry->se_state == SA_ENTRY_SUCC && entry->se_inode) {
+	/*
+	 * We need to see the value that was set immediately before we
+	 * were woken up.
+	 */
+	if (smp_load_acquire(&entry->se_state) == SA_ENTRY_SUCC &&
+	    entry->se_inode) {
 		struct inode *inode = entry->se_inode;
 		struct lookup_intent it = { .it_op = IT_GETATTR,
 					    .it_lock_handle = entry->se_handle };
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