Re: Race condition in HEAD, possibly due to PGPROC splitup

On Thu, Dec 15, 2011 at 8:19 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> Comments?

I think there is a small bug here:

+ TransactionId xid = pgxact->xid;
+
+ /*
+ * Don't record locks for transactions if we know they have already
+ * issued their WAL record for commit but not yet released lock.
+ * It is still possible that we see locks held by already complete
+ * transactions, if they haven't yet zeroed their xids.
+ */
+ if (!TransactionIdIsValid(xid))
+ continue;

accessExclusiveLocks[index].xid = pgxact->xid;
accessExclusiveLocks[index].dbOid = lock->tag.locktag_field1;

...because you're fetching pgxact->xid, testing whether it's valid,
and then fetching it again. It could change in the meantime. You
probably want to change the assignment to read:

accessExclusiveLocks[index].xid = xid;

Also, we should probably change this pointer to be declared volatile:

PGXACT *pgxact = &ProcGlobal->allPgXact[proc->pgprocno];

Otherwise, I think the compiler might get cute and decide to fetch the
xid twice anyway, effectively undoing our attempt to pull it into a
local variable.

I think this comment could be clarified in some way, to make it more
clear that we had a bug at one point, and it was fixed - the "from a
time when they were possible" language wouldn't be entirely clear to
me after the fact:

+ * Zero xids should no longer be possible, but we may be replaying WAL
+ * from a time when they were possible.

It would probably make sense to break out of this loop if a match is found:

! for (i = 0; i < nxids; i++)
! {
! if (lock->xid == xids[i])
! found = true;
! }

I'm not sure I fully understand the rest of this, but those are the
only things I noticed on a read-through.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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