Fix incorrect comment in SetBufferCommitInfoNeedsSave().

Noah Misch spotted the fact that the old comment is in fact incorrect, due
to memory ordering hazards.

diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 613d7544c6356e0bdcf9978ac4cc41d97cef2262..ad5a97a3523ef9e128cf1ffda20cb194fc3a72e9 100644 (file)
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -2376,9 +2376,11 @@ SetBufferCommitInfoNeedsSave(Buffer buffer)
         * making the first scan after commit of an xact that added/deleted many
         * tuples.      So, be as quick as we can if the buffer is already dirty.  We
         * do this by not acquiring spinlock if it looks like the status bits are
-        * already OK.  (Note it is okay if someone else clears BM_JUST_DIRTIED
-        * immediately after we look, because the buffer content update is already
-        * done and will be reflected in the I/O.)
+        * already.  Since we make this test unlocked, there's a chance we might
+        * fail to notice that the flags have just been cleared, and failed to reset
+        * them, due to memory-ordering issues.  But since this function is only
+        * intended to be used in cases where failing to write out the data would
+        * be harmless anyway, it doesn't really matter.
         */
        if ((bufHdr->flags & (BM_DIRTY | BM_JUST_DIRTIED)) !=
                (BM_DIRTY | BM_JUST_DIRTIED))