Centralize decision-making about where to get a backend's PGPROC.

This code was originally written as part of parallel query effort, but
it seems to have independent value, because if we make one decision
about where to get a PGPROC when we allocate and then put it back on a
different list at backend-exit time, bad things happen.  This isn't
just a theoretical risk; we fixed an actual problem of this type in
commit e280c630a87e1b8325770c6073097d109d79a00f.

diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 455ad2663402f9726880bcb0c5682150ca7433d7..324347b2c5879349f92cbbeaf126467eeb64c403 100644 (file)
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -242,18 +242,21 @@ InitProcGlobal(void)
            /* PGPROC for normal backend, add to freeProcs list */
            procs[i].links.next = (SHM_QUEUE *) ProcGlobal->freeProcs;
            ProcGlobal->freeProcs = &procs[i];
+           procs[i].procgloballist = &ProcGlobal->freeProcs;
        }
        else if (i < MaxConnections + autovacuum_max_workers + 1)
        {
            /* PGPROC for AV launcher/worker, add to autovacFreeProcs list */
            procs[i].links.next = (SHM_QUEUE *) ProcGlobal->autovacFreeProcs;
            ProcGlobal->autovacFreeProcs = &procs[i];
+           procs[i].procgloballist = &ProcGlobal->autovacFreeProcs;
        }
        else if (i < MaxBackends)
        {
            /* PGPROC for bgworker, add to bgworkerFreeProcs list */
            procs[i].links.next = (SHM_QUEUE *) ProcGlobal->bgworkerFreeProcs;
            ProcGlobal->bgworkerFreeProcs = &procs[i];
+           procs[i].procgloballist = &ProcGlobal->bgworkerFreeProcs;
        }
 
        /* Initialize myProcLocks[] shared memory queues. */
@@ -281,6 +284,7 @@ InitProcess(void)
 {
    /* use volatile pointer to prevent code rearrangement */
    volatile PROC_HDR *procglobal = ProcGlobal;
+   PGPROC * volatile * procgloballist;
 
    /*
     * ProcGlobal should be set up already (if we are a backend, we inherit
@@ -292,9 +296,17 @@ InitProcess(void)
    if (MyProc != NULL)
        elog(ERROR, "you already exist");
 
+   /* Decide which list should supply our PGPROC. */
+   if (IsAnyAutoVacuumProcess())
+       procgloballist = &procglobal->autovacFreeProcs;
+   else if (IsBackgroundWorker)
+       procgloballist = &procglobal->bgworkerFreeProcs;
+   else
+       procgloballist = &procglobal->freeProcs;
+
    /*
-    * Try to get a proc struct from the free list.  If this fails, we must be
-    * out of PGPROC structures (not to mention semaphores).
+    * Try to get a proc struct from the appropriate free list.  If this
+    * fails, we must be out of PGPROC structures (not to mention semaphores).
     *
     * While we are holding the ProcStructLock, also copy the current shared
     * estimate of spins_per_delay to local storage.
@@ -303,21 +315,11 @@ InitProcess(void)
 
    set_spins_per_delay(procglobal->spins_per_delay);
 
-   if (IsAnyAutoVacuumProcess())
-       MyProc = procglobal->autovacFreeProcs;
-   else if (IsBackgroundWorker)
-       MyProc = procglobal->bgworkerFreeProcs;
-   else
-       MyProc = procglobal->freeProcs;
+   MyProc = *procgloballist;
 
    if (MyProc != NULL)
    {
-       if (IsAnyAutoVacuumProcess())
-           procglobal->autovacFreeProcs = (PGPROC *) MyProc->links.next;
-       else if (IsBackgroundWorker)
-           procglobal->bgworkerFreeProcs = (PGPROC *) MyProc->links.next;
-       else
-           procglobal->freeProcs = (PGPROC *) MyProc->links.next;
+       *procgloballist = (PGPROC *) MyProc->links.next;
        SpinLockRelease(ProcStructLock);
    }
    else
@@ -335,6 +337,12 @@ InitProcess(void)
    }
    MyPgXact = &ProcGlobal->allPgXact[MyProc->pgprocno];
 
+   /*
+    * Cross-check that the PGPROC is of the type we expect; if this were
+    * not the case, it would get returned to the wrong list.
+    */
+   Assert(MyProc->procgloballist == procgloballist);
+
    /*
     * Now that we have a PGPROC, mark ourselves as an active postmaster
     * child; this is so that the postmaster can detect it if we exit without
@@ -761,6 +769,7 @@ ProcKill(int code, Datum arg)
    /* use volatile pointer to prevent code rearrangement */
    volatile PROC_HDR *procglobal = ProcGlobal;
    PGPROC     *proc;
+   PGPROC * volatile * procgloballist;
 
    Assert(MyProc != NULL);
 
@@ -799,24 +808,12 @@ ProcKill(int code, Datum arg)
    MyProc = NULL;
    DisownLatch(&proc->procLatch);
 
+   procgloballist = proc->procgloballist;
    SpinLockAcquire(ProcStructLock);
 
    /* Return PGPROC structure (and semaphore) to appropriate freelist */
-   if (IsAnyAutoVacuumProcess())
-   {
-       proc->links.next = (SHM_QUEUE *) procglobal->autovacFreeProcs;
-       procglobal->autovacFreeProcs = proc;
-   }
-   else if (IsBackgroundWorker)
-   {
-       proc->links.next = (SHM_QUEUE *) procglobal->bgworkerFreeProcs;
-       procglobal->bgworkerFreeProcs = proc;
-   }
-   else
-   {
-       proc->links.next = (SHM_QUEUE *) procglobal->freeProcs;
-       procglobal->freeProcs = proc;
-   }
+   proc->links.next = (SHM_QUEUE *) *procgloballist;
+   *procgloballist = proc;
 
    /* Update shared estimate of spins_per_delay */
    procglobal->spins_per_delay = update_spins_per_delay(procglobal->spins_per_delay);

diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index e807a2e020da4bc1f6fcf6898b480edf403578d8..202a672bca5286da305103581340930c38ce65ac 100644 (file)
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -78,6 +78,7 @@ struct PGPROC
 {
    /* proc->links MUST BE FIRST IN STRUCT (see ProcSleep,ProcWakeup,etc) */
    SHM_QUEUE   links;          /* list link if process is in a list */
+   PGPROC    **procgloballist; /* procglobal list that owns this PGPROC */
 
    PGSemaphoreData sem;        /* ONE semaphore to sleep on */
    int         waitStatus;     /* STATUS_WAITING, STATUS_OK or STATUS_ERROR */