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Relax lock level for setting PGPROC->statusFlags

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We don't actually need a lock to set PGPROC->statusFlags itself; what we
do need is a shared lock on either XidGenLock or ProcArrayLock in order to
ensure MyProc->pgxactoff keeps still while we modify the mirror array in
ProcGlobal->statusFlags.  Some places were using an exclusive lock for
that, which is excessive.  Relax those to use shared lock only.

procarray.c has a couple of places with somewhat brittle assumptions
about PGPROC changes: ProcArrayEndTransaction uses only shared lock, so
it's permissible to change MyProc only.  On the other hand,
ProcArrayEndTransactionInternal also changes other procs, so it must
hold exclusive lock.  Add asserts to ensure those assumptions continue
to hold.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/20201117155501.GA13805@alvherre.pgsql
This commit is contained in:
Alvaro Herrera
2020-11-18 13:24:22 -03:00
parent 2cccb627f1
commit 27838981be
6 changed files with 16 additions and 5 deletions
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2
src/backend/commands/vacuum.c
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@ -1741,7 +1741,7 @@ vacuum_rel(Oid relid, RangeVar *relation, VacuumParams *params)
* MyProc->xid/xmin, otherwise GetOldestNonRemovableTransactionId()
* might appear to go backwards, which is probably Not Good.
*/
LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
LWLockAcquire(ProcArrayLock, LW_SHARED);
MyProc->statusFlags |= PROC_IN_VACUUM;
if (params->is_wraparound)
MyProc->statusFlags |= PROC_VACUUM_FOR_WRAPAROUND;