On Tue, Feb 16, 2021 at 12:43:42PM +0900, Fujii Masao wrote: > On 2021/02/16 6:28, Andres Freund wrote: >> So what? It's just about free to initialize a spinlock, whether it's >> using the fallback implementation or not. Initializing upon walsender >> startup adds a lot of complications, because e.g. somebody could already >> hold the spinlock because the previous walsender just disconnected, and >> they were looking at the stats.
Okay. > Even if we initialize "writtenUpto" in WalRcvShmemInit(), WalReceiverMain() > still needs to initialize (reset to 0) by using pg_atomic_write_u64(). Yes, you have to do that. > Basically we should not acquire new spinlock while holding another spinlock, > to shorten the spinlock duration. Right? If yes, we need to move > pg_atomic_read_u64() of "writtenUpto" after the release of spinlock in > pg_stat_get_wal_receiver. It would not matter much as a NULL tuple is returned as long as the WAL receiver information is not ready to be displayed. The only reason why all the fields are read before checking for ready_to_display is that we can be sure that everything is consistent with the PID. So reading writtenUpto before or after does not really matter logically. I would just move it after the check, as you did previously. + /* + * Read "writtenUpto" without holding a spinlock. So it may not be + * consistent with other WAL receiver's shared variables protected by a + * spinlock. This is OK because that variable is used only for + * informational purpose and should not be used for data integrity checks. + */ What about the following? "Read "writtenUpto" without holding a spinlock. Note that it may not be consistent with the other shared variables of the WAL receiver protected by a spinlock, but this should not be used for data integrity checks." I agree that what has been done with MyProc->waitStart in 46d6e5f is not safe, and that initialization should happen once at postmaster startup, with a write(0) when starting the backend. There are two of them in proc.c, one in twophase.c. Do you mind if I add an open item for this one? -- Michael
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