From 39e605e3e224e14108bea363df86049c96b66f1d Mon Sep 17 00:00:00 2001 From: Andres Freund Date: Mon, 22 Sep 2025 14:44:52 -0400 Subject: [PATCH v5 3/3] 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(-) diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c index 42e1f50c7c8..3528a355eff 100644 --- a/src/backend/storage/buffer/bufmgr.c +++ b/src/backend/storage/buffer/bufmgr.c @@ -3272,7 +3272,6 @@ UnpinBufferNoOwner(BufferDesc *buf) ref->refcount--; if (ref->refcount == 0) { - uint32 buf_state; uint32 old_buf_state; /* @@ -3290,33 +3289,11 @@ 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); -- 2.48.1.76.g4e746b1a31.dirty