mirror/postgresql
2
0
Fork 0
mirror of https://git.postgresql.org/git/postgresql.git synced 2024-12-27 08:39:28 +08:00
Code Issues Packages Projects Releases Wiki Activity

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

Browse Source 
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
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
src/backend/storage/buffer/bufmgr.c
View File

@ -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)