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Make partition-lock-release coding more transparent in BufferAlloc().

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Coverity complained that oldPartitionLock was possibly dereferenced after
having been set to NULL.  That actually can't happen, because we'd only use
it if (oldFlags & BM_TAG_VALID) is true.  But nonetheless Coverity is
justified in complaining, because at line 1275 we actually overwrite
oldFlags, and then still expect its BM_TAG_VALID bit to be a safe guide to
whether to release the oldPartitionLock.  Thus, the code would be incorrect
if someone else had changed the buffer's BM_TAG_VALID flag meanwhile.
That should not happen, since we hold pin on the buffer throughout this
sequence, but it's starting to look like a rather shaky chain of logic.
And there's no need for such assumptions, because we can simply replace
the (oldFlags & BM_TAG_VALID) tests with (oldPartitionLock != NULL),
which has identical results and makes it plain to all comers that we don't
dereference a null pointer.  A small side benefit is that the range of
liveness of oldFlags is greatly reduced, possibly allowing the compiler
to save a register.

This is just cleanup, not an actual bug fix, so there seems no need
for a back-patch.
This commit is contained in:
Tom Lane
2016-04-18 18:05:56 -04:00
parent 75c24d0f74
commit a0382e2d7e
1 changed files with 6 additions and 5 deletions
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11
src/backend/storage/buffer/bufmgr.c
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@@ -1198,9 +1198,10 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
{
/* if it wasn't valid, we need only the new partition */
LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
/* these just keep the compiler quiet about uninit variables */
/* remember we have no old-partition lock or tag */
oldPartitionLock = NULL;
/* this just keeps the compiler quiet about uninit variables */
oldHash = 0;
oldPartitionLock = 0;
}
/*
@@ -1223,7 +1224,7 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
UnpinBuffer(buf, true);
/* Can give up that buffer's mapping partition lock now */
if ((oldFlags & BM_TAG_VALID) &&
if (oldPartitionLock != NULL &&
oldPartitionLock != newPartitionLock)
LWLockRelease(oldPartitionLock);
@@ -1277,7 +1278,7 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
UnlockBufHdr(buf, buf_state);
BufTableDelete(&newTag, newHash);
if ((oldFlags & BM_TAG_VALID) &&
if (oldPartitionLock != NULL &&
oldPartitionLock != newPartitionLock)
LWLockRelease(oldPartitionLock);
LWLockRelease(newPartitionLock);
@@ -1303,7 +1304,7 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
UnlockBufHdr(buf, buf_state);
if (oldFlags & BM_TAG_VALID)
if (oldPartitionLock != NULL)
{
BufTableDelete(&oldTag, oldHash);
if (oldPartitionLock != newPartitionLock)