Use a separate spinlock to protect LWLockTranches

Previously we reused the shmem allocator's ShmemLock to also protect
lwlock.c's shared memory structures. Introduce a separate spinlock for
lwlock.c for the sake of modularity. Now that lwlock.c has its own
shared memory struct (LWLockTranches), this is easy to do.

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://www.postgresql.org/message-id/47aaf57e-1b7b-4e12-bda2-0316081ff50e@iki.fi

diff --git a/src/backend/storage/ipc/shmem.c b/src/backend/storage/ipc/shmem.c
index 0fb3bc5992930b1378766641f060949ba9aa0eaa..b3199082de25ce8bb74b837455261c3c0d69e7d8 100644 (file)
--- a/src/backend/storage/ipc/shmem.c
+++ b/src/backend/storage/ipc/shmem.c
@@ -105,7 +105,6 @@ static void *ShmemBase;                     /* start address of shared memory */
 static void *ShmemEnd;                 /* end+1 address of shared memory */
 
 static ShmemAllocatorData *ShmemAllocator;
-slock_t    *ShmemLock;                 /* points to ShmemAllocator->shmem_lock */
 static HTAB *ShmemIndex = NULL; /* primary index hashtable for shmem */
 
 /* To get reliable results for NUMA inquiry we need to "touch pages" once */
@@ -166,7 +165,6 @@ InitShmemAllocator(PGShmemHeader *seghdr)
                ShmemAllocator->free_offset = offset;
        }
 
-       ShmemLock = &ShmemAllocator->shmem_lock;
        ShmemSegHdr = seghdr;
        ShmemBase = seghdr;
        ShmemEnd = (char *) ShmemBase + seghdr->totalsize;
@@ -200,7 +198,7 @@ InitShmemAllocator(PGShmemHeader *seghdr)
  *
  * Throws error if request cannot be satisfied.
  *
- * Assumes ShmemLock and ShmemSegHdr are initialized.
+ * Assumes ShmemSegHdr is initialized.
  */
 void *
 ShmemAlloc(Size size)
@@ -259,7 +257,7 @@ ShmemAllocRaw(Size size, Size *allocated_size)
 
        Assert(ShmemSegHdr != NULL);
 
-       SpinLockAcquire(ShmemLock);
+       SpinLockAcquire(&ShmemAllocator->shmem_lock);
 
        newStart = ShmemAllocator->free_offset;
 
@@ -272,7 +270,7 @@ ShmemAllocRaw(Size size, Size *allocated_size)
        else
                newSpace = NULL;
 
-       SpinLockRelease(ShmemLock);
+       SpinLockRelease(&ShmemAllocator->shmem_lock);
 
        /* note this assert is okay with newSpace == NULL */
        Assert(newSpace == (void *) CACHELINEALIGN(newSpace));

diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index 55b200a5e7d2ddc1ef0024910713c99942262289..f9e368770b75c40ea8539900afae75dac3d34581 100644 (file)
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -191,6 +191,8 @@ typedef struct LWLockTrancheShmemData
        }                       user_defined[MAX_USER_DEFINED_TRANCHES];
 
        int                     num_user_defined;       /* 'user_defined' entries in use */
+
+       slock_t         lock;                   /* protects the above */
 } LWLockTrancheShmemData;
 
 LWLockTrancheShmemData *LWLockTranches;
@@ -435,6 +437,7 @@ CreateLWLocks(void)
                        ShmemAlloc(sizeof(LWLockTrancheShmemData));
 
                /* Initialize the dynamic-allocation counter for tranches */
+               SpinLockInit(&LWLockTranches->lock);
                LWLockTranches->num_user_defined = 0;
 
                /* Allocate and initialize the main array */
@@ -515,9 +518,9 @@ InitLWLockAccess(void)
 LWLockPadded *
 GetNamedLWLockTranche(const char *tranche_name)
 {
-       SpinLockAcquire(ShmemLock);
+       SpinLockAcquire(&LWLockTranches->lock);
        LocalNumUserDefinedTranches = LWLockTranches->num_user_defined;
-       SpinLockRelease(ShmemLock);
+       SpinLockRelease(&LWLockTranches->lock);
 
        /*
         * Obtain the position of base address of LWLock belonging to requested
@@ -568,15 +571,12 @@ LWLockNewTrancheId(const char *name)
                                 errdetail("LWLock tranche names must be no longer than %d bytes.",
                                                   NAMEDATALEN - 1)));
 
-       /*
-        * We use the ShmemLock spinlock to protect the counter and the tranche
-        * names.
-        */
-       SpinLockAcquire(ShmemLock);
+       /* The counter and the tranche names are protected by the spinlock */
+       SpinLockAcquire(&LWLockTranches->lock);
 
        if (LWLockTranches->num_user_defined >= MAX_USER_DEFINED_TRANCHES)
        {
-               SpinLockRelease(ShmemLock);
+               SpinLockRelease(&LWLockTranches->lock);
                ereport(ERROR,
                                (errmsg("maximum number of tranches already registered"),
                                 errdetail("No more than %d tranches may be registered.",
@@ -595,7 +595,7 @@ LWLockNewTrancheId(const char *name)
        /* the locks are not in the main array */
        LWLockTranches->user_defined[idx].main_array_idx = -1;
 
-       SpinLockRelease(ShmemLock);
+       SpinLockRelease(&LWLockTranches->lock);
 
        return LWTRANCHE_FIRST_USER_DEFINED + idx;
 }
@@ -705,14 +705,14 @@ GetLWTrancheName(uint16 trancheId)
         * lookups can avoid taking the spinlock as long as the backend-local
         * counter (LocalNumUserDefinedTranches) is greater than the requested
         * tranche ID.  Else, we need to first update the backend-local counter
-        * with ShmemLock held before attempting the lookup again.  In practice,
-        * the latter case is probably rare.
+        * with the spinlock held before attempting the lookup again.  In
+        * practice, the latter case is probably rare.
         */
        if (idx >= LocalNumUserDefinedTranches)
        {
-               SpinLockAcquire(ShmemLock);
+               SpinLockAcquire(&LWLockTranches->lock);
                LocalNumUserDefinedTranches = LWLockTranches->num_user_defined;
-               SpinLockRelease(ShmemLock);
+               SpinLockRelease(&LWLockTranches->lock);
 
                if (idx >= LocalNumUserDefinedTranches)
                        elog(ERROR, "tranche %d is not registered", trancheId);

diff --git a/src/include/storage/shmem.h b/src/include/storage/shmem.h
index 0de8a36429b0fbbd7e010cd97030fe34727307da..2a9e9becd260d7aee0bbd7cf8a0920a9276c55d1 100644 (file)
--- a/src/include/storage/shmem.h
+++ b/src/include/storage/shmem.h
@@ -26,7 +26,6 @@
 
 
 /* shmem.c */
-extern PGDLLIMPORT slock_t *ShmemLock;
 typedef struct PGShmemHeader PGShmemHeader; /* avoid including
                                                                                         * storage/pg_shmem.h here */
 extern void InitShmemAllocator(PGShmemHeader *seghdr);