From 78253592e0ee2e5f36d62ea8789271bb7a28fea7 Mon Sep 17 00:00:00 2001 From: Andres Freund Date: Fri, 21 Feb 2025 12:59:56 -0500 Subject: [PATCH v2 2/4] Base LWLock limits directly on MAX_BACKENDS Jacob reported that comments for LW_SHARED_MASK referenced a MAX_BACKENDS limit of 2^23-1, but that MAX_BACKENDS is actually limited to 2^18-1. The limit was lowered in 48354581a49c, but the comment in lwlock.c wasn't updated. Instead of just fixing the comment, it seems better to directly base the lwlock defines on MAX_BACKENDS and add static assertions to ensure that there is enough space. That way there's no comment that can go out of sync in the future. As part of that change I noticed that for some reason the high bit wasn't used for flags, which seems somewhat odd. Redefine the flag values to start at the highest bit. Reported-by: Jacob Brazeal Reviewed-by: Jacob Brazeal Discussion: https://postgr.es/m/CA+COZaBO_s3LfALq=b+HcBHFSOEGiApVjrRacCe4VP9m7CJsNQ@mail.gmail.com --- src/backend/storage/lmgr/lwlock.c | 28 +++++++++++++++++++--------- 1 file changed, 19 insertions(+), 9 deletions(-) diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c index d08144e9c22..7c7bf3f300c 100644 --- a/src/backend/storage/lmgr/lwlock.c +++ b/src/backend/storage/lmgr/lwlock.c @@ -91,19 +91,29 @@ #endif -#define LW_FLAG_HAS_WAITERS ((uint32) 1 << 30) -#define LW_FLAG_RELEASE_OK ((uint32) 1 << 29) -#define LW_FLAG_LOCKED ((uint32) 1 << 28) +#define LW_FLAG_HAS_WAITERS ((uint32) 1 << 31) +#define LW_FLAG_RELEASE_OK ((uint32) 1 << 30) +#define LW_FLAG_LOCKED ((uint32) 1 << 29) +#define LW_FLAG_BITS 3 +#define LW_FLAG_MASK (((1< (uint32) MAX_BACKENDS, - "MAX_BACKENDS too big for lwlock.c"); + +StaticAssertDecl(((MAX_BACKENDS + 1) & MAX_BACKENDS) == 0, + "MAX_BACKENDS + 1 needs to be a power of 2"); + +StaticAssertDecl((MAX_BACKENDS & LW_FLAG_MASK) == 0, + "MAX_BACKENDS and LW_FLAG_MASK overlap"); + +StaticAssertDecl((LW_VAL_EXCLUSIVE & LW_FLAG_MASK) == 0, + "LW_VAL_EXCLUSIVE and LW_FLAG_MASK overlap"); /* * There are three sorts of LWLock "tranches": -- 2.48.1.76.g4e746b1a31.dirty