From 5310fac6e0fcb1c7fcefb3446767673f9107328c Mon Sep 17 00:00:00 2001 From: Andres Freund Date: Thu, 6 Nov 2025 16:43:16 -0500 Subject: bufmgr: Use atomic sub for unpinning buffers The prior commit made it legal to modify BufferDesc.state while the buffer header spinlock is held. This allows us to replace the CAS loop inUnpinBufferNoOwner() with an atomic sub. This improves scalability significantly. See the prior commits for more background. Reviewed-by: Matthias van de Meent Discussion: https://postgr.es/m/fvfmkr5kk4nyex56ejgxj3uzi63isfxovp2biecb4bspbjrze7@az2pljabhnff --- src/backend/storage/buffer/bufmgr.c | 29 +++-------------------------- 1 file changed, 3 insertions(+), 26 deletions(-) (limited to 'src') diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c index 4b9fd76294d..327ddb7adc8 100644 --- a/src/backend/storage/buffer/bufmgr.c +++ b/src/backend/storage/buffer/bufmgr.c @@ -3267,7 +3267,6 @@ UnpinBufferNoOwner(BufferDesc *buf) ref->refcount--; if (ref->refcount == 0) { - uint32 buf_state; uint32 old_buf_state; /* @@ -3285,33 +3284,11 @@ UnpinBufferNoOwner(BufferDesc *buf) */ Assert(!LWLockHeldByMe(BufferDescriptorGetContentLock(buf))); - /* - * Decrement the shared reference count. - * - * Since buffer spinlock holder can update status using just write, - * it's not safe to use atomic decrement here; thus use a CAS loop. - * - * TODO: The above requirement does not hold anymore, in a future - * commit this will be rewritten to release the pin in a single atomic - * operation. - */ - old_buf_state = pg_atomic_read_u32(&buf->state); - for (;;) - { - if (old_buf_state & BM_LOCKED) - old_buf_state = WaitBufHdrUnlocked(buf); - - buf_state = old_buf_state; - - buf_state -= BUF_REFCOUNT_ONE; - - if (pg_atomic_compare_exchange_u32(&buf->state, &old_buf_state, - buf_state)) - break; - } + /* decrement the shared reference count */ + old_buf_state = pg_atomic_fetch_sub_u32(&buf->state, BUF_REFCOUNT_ONE); /* Support LockBufferForCleanup() */ - if (buf_state & BM_PIN_COUNT_WAITER) + if (old_buf_state & BM_PIN_COUNT_WAITER) WakePinCountWaiter(buf); ForgetPrivateRefCountEntry(ref); -- cgit v1.2.3