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Rework locking code in GetMultiXactIdMembers

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After commit 53c2a97a9266, the code flow around the "retry" goto label
in GetMultiXactIdMembers was confused about what was possible: we never
return there with a held lock, so there's no point in testing for one.
This realization lets us simplify the code a bit.  While at it, make the
scope of a couple of local variables in the same function a bit tighter.

Per Coverity.
This commit is contained in:
Alvaro Herrera 2024-03-04 17:48:01 +01:00
parent f9baaf96d3
commit a0b808baef
No known key found for this signature in database
GPG Key ID: 1C20ACB9D5C564AE
1 changed files with 22 additions and 31 deletions
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53
src/backend/access/transam/multixact.c
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@ -1247,14 +1247,12 @@ GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
MultiXactOffset offset;
int length;
int truelength;
int i;
MultiXactId oldestMXact;
MultiXactId nextMXact;
MultiXactId tmpMXact;
MultiXactOffset nextOffset;
MultiXactMember *ptr;
LWLock *lock;
LWLock *prevlock = NULL;
debug_elog3(DEBUG2, "GetMembers: asked for %u", multi);
@ -1361,18 +1359,9 @@ retry:
pageno = MultiXactIdToOffsetPage(multi);
entryno = MultiXactIdToOffsetEntry(multi);
/*
* If this page falls under a different bank, release the old bank's lock
* and acquire the lock of the new bank.
*/
/* Acquire the bank lock for the page we need. */
lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);
if (lock != prevlock)
{
if (prevlock != NULL)
LWLockRelease(prevlock);
LWLockAcquire(lock, LW_EXCLUSIVE);
prevlock = lock;
}
LWLockAcquire(lock, LW_EXCLUSIVE);
slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, multi);
offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
@ -1407,17 +1396,19 @@ retry:
if (pageno != prev_pageno)
{
LWLock *newlock;
/*
* Since we're going to access a different SLRU page, if this page
* falls under a different bank, release the old bank's lock and
* acquire the lock of the new bank.
*/
lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);
if (prevlock != lock)
newlock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);
if (newlock != lock)
{
LWLockRelease(prevlock);
LWLockAcquire(lock, LW_EXCLUSIVE);
prevlock = lock;
LWLockRelease(lock);
LWLockAcquire(newlock, LW_EXCLUSIVE);
lock = newlock;
}
slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, tmpMXact);
}
@ -1429,8 +1420,7 @@ retry:
if (nextMXOffset == 0)
{
/* Corner case 2: next multixact is still being filled in */
LWLockRelease(prevlock);
prevlock = NULL;
LWLockRelease(lock);
CHECK_FOR_INTERRUPTS();
pg_usleep(1000L);
goto retry;
@ -1439,14 +1429,14 @@ retry:
length = nextMXOffset - offset;
}
LWLockRelease(prevlock);
prevlock = NULL;
LWLockRelease(lock);
lock = NULL;
ptr = (MultiXactMember *) palloc(length * sizeof(MultiXactMember));
truelength = 0;
prev_pageno = -1;
for (i = 0; i < length; i++, offset++)
for (int i = 0; i < length; i++, offset++)
{
TransactionId *xactptr;
uint32 *flagsptr;
@ -1459,18 +1449,20 @@ retry:
if (pageno != prev_pageno)
{
LWLock *newlock;
/*
* Since we're going to access a different SLRU page, if this page
* falls under a different bank, release the old bank's lock and
* acquire the lock of the new bank.
*/
lock = SimpleLruGetBankLock(MultiXactMemberCtl, pageno);
if (lock != prevlock)
newlock = SimpleLruGetBankLock(MultiXactMemberCtl, pageno);
if (newlock != lock)
{
if (prevlock)
LWLockRelease(prevlock);
LWLockAcquire(lock, LW_EXCLUSIVE);
prevlock = lock;
if (lock)
LWLockRelease(lock);
LWLockAcquire(newlock, LW_EXCLUSIVE);
lock = newlock;
}
slotno = SimpleLruReadPage(MultiXactMemberCtl, pageno, true, multi);
@ -1496,8 +1488,7 @@ retry:
truelength++;
}
if (prevlock)
LWLockRelease(prevlock);
LWLockRelease(lock);
/* A multixid with zero members should not happen */
Assert(truelength > 0);