Re: Parallel bitmap heap scan

(On Tue, Feb 28, 2017 at 10:48 AM, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> 0001- same as previous with some changes for freeing the shared memory stuff.

+ if (--ptbase->refcount == 0)
+ dsa_free(dsa, istate->pagetable);
+
+ if (istate->spages)
+ {
+ ptpages = dsa_get_address(dsa, istate->spages);
+ if (--ptpages->refcount == 0)
+ dsa_free(dsa, istate->spages);
+ }
+ if (istate->schunks)
+ {
+ ptchunks = dsa_get_address(dsa, istate->schunks);
+ if (--ptchunks->refcount == 0)
+ dsa_free(dsa, istate->schunks);
+ }

This doesn't involve any locking, which I think will happen to work
with the current usage pattern but doesn't seem very robust in
general. I think you either need the refcounts to be protected by a
spinlock, or maybe better, use pg_atomic_uint32 for them. You want
something like if (pg_atomic_sub_fetch_u32(&refcount, 1) == 0) {
dsa_free(...) }

Otherwise, there's no guarantee it will get freed exactly once.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company