Ensure that all direct uses of spinlock-protected data structures use

'volatile' pointers to access those structures, so that optimizing
compilers will not decide to move the structure accesses outside of the
spinlock-acquire-to-spinlock-release sequence.  There are no known bugs
in these uses at present, but based on bad experience with lwlock.c,
it seems prudent to ensure that we protect these other uses too.
Per pghackers discussion around 12-Dec.  (Note: it should not be
necessary to worry about structures protected by LWLocks, since the
LWLock acquire and release operations are not inline macros.)

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index b1ea722..46e3470 100644 (file)
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Header: /cvsroot/pgsql/src/backend/access/transam/xlog.c,v 1.84 2001/12/23 07:25:39 tgl Exp $
+ * $Header: /cvsroot/pgsql/src/backend/access/transam/xlog.c,v 1.85 2001/12/28 18:16:41 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -617,10 +617,15 @@ begin:;
        START_CRIT_SECTION();
 
        /* update LogwrtResult before doing cache fill check */
-       SpinLockAcquire_NoHoldoff(&XLogCtl->info_lck);
-       LogwrtRqst = XLogCtl->LogwrtRqst;
-       LogwrtResult = XLogCtl->LogwrtResult;
-       SpinLockRelease_NoHoldoff(&XLogCtl->info_lck);
+       {
+               /* use volatile pointer to prevent code rearrangement */
+               volatile XLogCtlData *xlogctl = XLogCtl;
+
+               SpinLockAcquire_NoHoldoff(&xlogctl->info_lck);
+               LogwrtRqst = xlogctl->LogwrtRqst;
+               LogwrtResult = xlogctl->LogwrtResult;
+               SpinLockRelease_NoHoldoff(&xlogctl->info_lck);
+       }
 
        /*
         * If cache is half filled then try to acquire write lock and do
@@ -838,16 +843,20 @@ begin:;
 
        if (updrqst)
        {
-               SpinLockAcquire_NoHoldoff(&XLogCtl->info_lck);
+               /* use volatile pointer to prevent code rearrangement */
+               volatile XLogCtlData *xlogctl = XLogCtl;
+
+               SpinLockAcquire_NoHoldoff(&xlogctl->info_lck);
                /* advance global request to include new block(s) */
-               if (XLByteLT(XLogCtl->LogwrtRqst.Write, WriteRqst))
-                       XLogCtl->LogwrtRqst.Write = WriteRqst;
+               if (XLByteLT(xlogctl->LogwrtRqst.Write, WriteRqst))
+                       xlogctl->LogwrtRqst.Write = WriteRqst;
                /* update local result copy while I have the chance */
-               LogwrtResult = XLogCtl->LogwrtResult;
-               SpinLockRelease_NoHoldoff(&XLogCtl->info_lck);
+               LogwrtResult = xlogctl->LogwrtResult;
+               SpinLockRelease_NoHoldoff(&xlogctl->info_lck);
        }
 
        END_CRIT_SECTION();
+
        return (RecPtr);
 }
 
@@ -892,11 +901,16 @@ AdvanceXLInsertBuffer(void)
                FinishedPageRqstPtr = XLogCtl->xlblocks[Insert->curridx];
 
                /* Before waiting, get info_lck and update LogwrtResult */
-               SpinLockAcquire_NoHoldoff(&XLogCtl->info_lck);
-               if (XLByteLT(XLogCtl->LogwrtRqst.Write, FinishedPageRqstPtr))
-                       XLogCtl->LogwrtRqst.Write = FinishedPageRqstPtr;
-               LogwrtResult = XLogCtl->LogwrtResult;
-               SpinLockRelease_NoHoldoff(&XLogCtl->info_lck);
+               {
+                       /* use volatile pointer to prevent code rearrangement */
+                       volatile XLogCtlData *xlogctl = XLogCtl;
+
+                       SpinLockAcquire_NoHoldoff(&xlogctl->info_lck);
+                       if (XLByteLT(xlogctl->LogwrtRqst.Write, FinishedPageRqstPtr))
+                               xlogctl->LogwrtRqst.Write = FinishedPageRqstPtr;
+                       LogwrtResult = xlogctl->LogwrtResult;
+                       SpinLockRelease_NoHoldoff(&xlogctl->info_lck);
+               }
 
                update_needed = false;  /* Did the shared-request update */
 
@@ -1149,13 +1163,18 @@ XLogWrite(XLogwrtRqst WriteRqst)
         * 'result' values.  This is not absolutely essential, but it saves
         * some code in a couple of places.
         */
-       SpinLockAcquire_NoHoldoff(&XLogCtl->info_lck);
-       XLogCtl->LogwrtResult = LogwrtResult;
-       if (XLByteLT(XLogCtl->LogwrtRqst.Write, LogwrtResult.Write))
-               XLogCtl->LogwrtRqst.Write = LogwrtResult.Write;
-       if (XLByteLT(XLogCtl->LogwrtRqst.Flush, LogwrtResult.Flush))
-               XLogCtl->LogwrtRqst.Flush = LogwrtResult.Flush;
-       SpinLockRelease_NoHoldoff(&XLogCtl->info_lck);
+       {
+               /* use volatile pointer to prevent code rearrangement */
+               volatile XLogCtlData *xlogctl = XLogCtl;
+
+               SpinLockAcquire_NoHoldoff(&xlogctl->info_lck);
+               xlogctl->LogwrtResult = LogwrtResult;
+               if (XLByteLT(xlogctl->LogwrtRqst.Write, LogwrtResult.Write))
+                       xlogctl->LogwrtRqst.Write = LogwrtResult.Write;
+               if (XLByteLT(xlogctl->LogwrtRqst.Flush, LogwrtResult.Flush))
+                       xlogctl->LogwrtRqst.Flush = LogwrtResult.Flush;
+               SpinLockRelease_NoHoldoff(&xlogctl->info_lck);
+       }
 
        Write->LogwrtResult = LogwrtResult;
 }
@@ -1206,11 +1225,16 @@ XLogFlush(XLogRecPtr record)
        WriteRqstPtr = record;
 
        /* read LogwrtResult and update local state */
-       SpinLockAcquire_NoHoldoff(&XLogCtl->info_lck);
-       if (XLByteLT(WriteRqstPtr, XLogCtl->LogwrtRqst.Write))
-               WriteRqstPtr = XLogCtl->LogwrtRqst.Write;
-       LogwrtResult = XLogCtl->LogwrtResult;
-       SpinLockRelease_NoHoldoff(&XLogCtl->info_lck);
+       {
+               /* use volatile pointer to prevent code rearrangement */
+               volatile XLogCtlData *xlogctl = XLogCtl;
+
+               SpinLockAcquire_NoHoldoff(&xlogctl->info_lck);
+               if (XLByteLT(WriteRqstPtr, xlogctl->LogwrtRqst.Write))
+                       WriteRqstPtr = xlogctl->LogwrtRqst.Write;
+               LogwrtResult = xlogctl->LogwrtResult;
+               SpinLockRelease_NoHoldoff(&xlogctl->info_lck);
+       }
 
        /* done already? */
        if (!XLByteLE(record, LogwrtResult.Flush))

diff --git a/src/backend/storage/ipc/shmem.c b/src/backend/storage/ipc/shmem.c
index 3304ec6..2b0eee5 100644 (file)
--- a/src/backend/storage/ipc/shmem.c
+++ b/src/backend/storage/ipc/shmem.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *       $Header: /cvsroot/pgsql/src/backend/storage/ipc/shmem.c,v 1.62 2001/10/25 05:49:42 momjian Exp $
+ *       $Header: /cvsroot/pgsql/src/backend/storage/ipc/shmem.c,v 1.63 2001/12/28 18:16:43 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -132,21 +132,23 @@ ShmemAlloc(Size size)
 {
        uint32          newFree;
        void       *newSpace;
+       /* use volatile pointer to prevent code rearrangement */
+       volatile PGShmemHeader *shmemseghdr = ShmemSegHdr;
 
        /*
         * ensure all space is adequately aligned.
         */
        size = MAXALIGN(size);
 
-       Assert(ShmemSegHdr != NULL);
+       Assert(shmemseghdr != NULL);
 
        SpinLockAcquire(ShmemLock);
 
-       newFree = ShmemSegHdr->freeoffset + size;
-       if (newFree <= ShmemSegHdr->totalsize)
+       newFree = shmemseghdr->freeoffset + size;
+       if (newFree <= shmemseghdr->totalsize)
        {
-               newSpace = (void *) MAKE_PTR(ShmemSegHdr->freeoffset);
-               ShmemSegHdr->freeoffset = newFree;
+               newSpace = (void *) MAKE_PTR(shmemseghdr->freeoffset);
+               shmemseghdr->freeoffset = newFree;
        }
        else
                newSpace = NULL;

diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 5fa8f13..b1be68a 100644 (file)
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *       $Header: /cvsroot/pgsql/src/backend/storage/lmgr/proc.c,v 1.116 2001/11/08 20:37:52 momjian Exp $
+ *       $Header: /cvsroot/pgsql/src/backend/storage/lmgr/proc.c,v 1.117 2001/12/28 18:16:43 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -203,12 +203,14 @@ void
 InitProcess(void)
 {
        SHMEM_OFFSET myOffset;
+       /* use volatile pointer to prevent code rearrangement */
+       volatile PROC_HDR *procglobal = ProcGlobal;
 
        /*
         * ProcGlobal should be set by a previous call to InitProcGlobal (if
         * we are a backend, we inherit this by fork() from the postmaster).
         */
-       if (ProcGlobal == NULL)
+       if (procglobal == NULL)
                elog(STOP, "InitProcess: Proc Header uninitialized");
 
        if (MyProc != NULL)
@@ -219,12 +221,12 @@ InitProcess(void)
         */
        SpinLockAcquire(ProcStructLock);
 
-       myOffset = ProcGlobal->freeProcs;
+       myOffset = procglobal->freeProcs;
 
        if (myOffset != INVALID_OFFSET)
        {
                MyProc = (PROC *) MAKE_PTR(myOffset);
-               ProcGlobal->freeProcs = MyProc->links.next;
+               procglobal->freeProcs = MyProc->links.next;
                SpinLockRelease(ProcStructLock);
        }
        else
@@ -437,6 +439,9 @@ ProcReleaseLocks(bool isCommit)
 static void
 ProcKill(void)
 {
+       /* use volatile pointer to prevent code rearrangement */
+       volatile PROC_HDR *procglobal = ProcGlobal;
+
        Assert(MyProc != NULL);
 
        /* Release any LW locks I am holding */
@@ -463,8 +468,8 @@ ProcKill(void)
                ProcFreeSem(MyProc->sem.semId, MyProc->sem.semNum);
 
        /* Add PROC struct to freelist so space can be recycled in future */
-       MyProc->links.next = ProcGlobal->freeProcs;
-       ProcGlobal->freeProcs = MAKE_OFFSET(MyProc);
+       MyProc->links.next = procglobal->freeProcs;
+       procglobal->freeProcs = MAKE_OFFSET(MyProc);
 
        /* PROC struct isn't mine anymore */
        MyProc = NULL;
@@ -1044,10 +1049,12 @@ disable_sigalrm_interrupt(void)
 static void
 ProcGetNewSemIdAndNum(IpcSemaphoreId *semId, int *semNum)
 {
-       int                     i;
-       int                     semMapEntries = ProcGlobal->semMapEntries;
-       SEM_MAP_ENTRY *procSemMap = ProcGlobal->procSemMap;
+       /* use volatile pointer to prevent code rearrangement */
+       volatile PROC_HDR *procglobal = ProcGlobal;
+       int                     semMapEntries = procglobal->semMapEntries;
+       volatile SEM_MAP_ENTRY *procSemMap = procglobal->procSemMap;
        int32           fullmask = (1 << PROC_NSEMS_PER_SET) - 1;
+       int                     i;
 
        SpinLockAcquire(ProcStructLock);