Replace CAS loop with single TAS in ProcArrayGroupClearXid()

Single pg_atomic_exchange_u32() is expected to be faster than loop of
pg_atomic_compare_exchange_u32().  Also, it would be consistent with
clog group update code.

Discussion: https://postgr.es/m/CAPpHfdtxLsC-bqfxFcHswZ91OxXcZVNDBBVfg9tAWU0jvn1tQA%40mail.gmail.com
Reviewed-by: Amit Kapila

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index bd20497d81aecfdac83f99757ef5a4faf605c178..bf2f4dbed2d373a542cc3c164e5ec03992351f2e 100644 (file)
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -542,14 +542,8 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
     * group XID clearing, saving a pointer to the head of the list.  Trying
     * to pop elements one at a time could lead to an ABA problem.
     */
-   while (true)
-   {
-       nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);
-       if (pg_atomic_compare_exchange_u32(&procglobal->procArrayGroupFirst,
-                                          &nextidx,
-                                          INVALID_PGPROCNO))
-           break;
-   }
+   nextidx = pg_atomic_exchange_u32(&procglobal->procArrayGroupFirst,
+                                    INVALID_PGPROCNO);
 
    /* Remember head of list so we can perform wakeups after dropping lock. */
    wakeidx = nextidx;