From 94cd2fbf37b5f0b824e0f9a9bc23f762a8bb19b5 Mon Sep 17 00:00:00 2001 From: Michail Nikolaev Date: Sun, 21 Nov 2021 21:23:02 +0300 Subject: [PATCH v3 1/2] memory barrier instead of spinlock --- src/backend/storage/ipc/procarray.c | 42 +++++++---------------------- 1 file changed, 9 insertions(+), 33 deletions(-) diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c index a9945c80eb..da0c4eaa00 100644 --- a/src/backend/storage/ipc/procarray.c +++ b/src/backend/storage/ipc/procarray.c @@ -60,7 +60,6 @@ #include "pgstat.h" #include "storage/proc.h" #include "storage/procarray.h" -#include "storage/spin.h" #include "utils/acl.h" #include "utils/builtins.h" #include "utils/rel.h" @@ -81,7 +80,6 @@ typedef struct ProcArrayStruct int numKnownAssignedXids; /* current # of valid entries */ int tailKnownAssignedXids; /* index of oldest valid element */ int headKnownAssignedXids; /* index of newest element, + 1 */ - slock_t known_assigned_xids_lck; /* protects head/tail pointers */ /* * Highest subxid that has been removed from KnownAssignedXids array to @@ -425,7 +423,6 @@ CreateSharedProcArray(void) procArray->numKnownAssignedXids = 0; procArray->tailKnownAssignedXids = 0; procArray->headKnownAssignedXids = 0; - SpinLockInit(&procArray->known_assigned_xids_lck); procArray->lastOverflowedXid = InvalidTransactionId; procArray->replication_slot_xmin = InvalidTransactionId; procArray->replication_slot_catalog_xmin = InvalidTransactionId; @@ -4553,10 +4550,9 @@ ExpireOldKnownAssignedTransactionIds(TransactionId xid) * pointer. This wouldn't require any lock at all, except that on machines * with weak memory ordering we need to be careful that other processors * see the array element changes before they see the head pointer change. - * We handle this by using a spinlock to protect reads and writes of the - * head/tail pointers. (We could dispense with the spinlock if we were to - * create suitable memory access barrier primitives and use those instead.) - * The spinlock must be taken to read or write the head/tail pointers unless + * We handle this by using a memory barrier to protect writes of the + * head pointer. + * The memory barrier is taken before write the head pointer unless * the caller holds ProcArrayLock exclusively. * * Algorithmic analysis: @@ -4600,7 +4596,6 @@ KnownAssignedXidsCompress(bool force) int compress_index; int i; - /* no spinlock required since we hold ProcArrayLock exclusively */ head = pArray->headKnownAssignedXids; tail = pArray->tailKnownAssignedXids; @@ -4686,7 +4681,7 @@ KnownAssignedXidsAdd(TransactionId from_xid, TransactionId to_xid, /* * Since only the startup process modifies the head/tail pointers, we - * don't need a lock to read them here. + * are safe read them here. */ head = pArray->headKnownAssignedXids; tail = pArray->tailKnownAssignedXids; @@ -4744,21 +4739,20 @@ KnownAssignedXidsAdd(TransactionId from_xid, TransactionId to_xid, pArray->numKnownAssignedXids += nxids; /* - * Now update the head pointer. We use a spinlock to protect this + * Now update the head pointer. We use a memory barrier to protect this * pointer, not because the update is likely to be non-atomic, but to * ensure that other processors see the above array updates before they * see the head pointer change. * * If we're holding ProcArrayLock exclusively, there's no need to take the - * spinlock. + * barrier. */ if (exclusive_lock) pArray->headKnownAssignedXids = head; else { - SpinLockAcquire(&pArray->known_assigned_xids_lck); + pg_write_barrier(); pArray->headKnownAssignedXids = head; - SpinLockRelease(&pArray->known_assigned_xids_lck); } } @@ -4781,20 +4775,8 @@ KnownAssignedXidsSearch(TransactionId xid, bool remove) int tail; int result_index = -1; - if (remove) - { - /* we hold ProcArrayLock exclusively, so no need for spinlock */ - tail = pArray->tailKnownAssignedXids; - head = pArray->headKnownAssignedXids; - } - else - { - /* take spinlock to ensure we see up-to-date array contents */ - SpinLockAcquire(&pArray->known_assigned_xids_lck); - tail = pArray->tailKnownAssignedXids; - head = pArray->headKnownAssignedXids; - SpinLockRelease(&pArray->known_assigned_xids_lck); - } + tail = pArray->tailKnownAssignedXids; + head = pArray->headKnownAssignedXids; /* * Standard binary search. Note we can ignore the KnownAssignedXidsValid @@ -5032,13 +5014,9 @@ KnownAssignedXidsGetAndSetXmin(TransactionId *xarray, TransactionId *xmin, * cannot enter and then leave the array while we hold ProcArrayLock. We * might miss newly-added xids, but they should be >= xmax so irrelevant * anyway. - * - * Must take spinlock to ensure we see up-to-date array contents. */ - SpinLockAcquire(&procArray->known_assigned_xids_lck); tail = procArray->tailKnownAssignedXids; head = procArray->headKnownAssignedXids; - SpinLockRelease(&procArray->known_assigned_xids_lck); for (i = tail; i < head; i++) { @@ -5085,10 +5063,8 @@ KnownAssignedXidsGetOldestXmin(void) /* * Fetch head just once, since it may change while we loop. */ - SpinLockAcquire(&procArray->known_assigned_xids_lck); tail = procArray->tailKnownAssignedXids; head = procArray->headKnownAssignedXids; - SpinLockRelease(&procArray->known_assigned_xids_lck); for (i = tail; i < head; i++) { -- 2.25.1