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