| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Latches with weak memory ordering (Re: max_wal_senders must die) |
| Date: | 2010-11-15 16:12:00 |
| Message-ID: | 15085.1289837520@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
> In SetLatch, is it enough to add the SpinLockAcquire() call *after*
> checking that is_set is not already set? Ie. still do the quick exit
> without holding a lock. Or do we need a memory barrier operation before
> the fetch, to ensure that we see if the other process has just cleared
> the flag with ResetLatch() ? Presumable ResetLatch() needs to call
> SpinLockAcquire() anyway to ensure that other processes see the clearing
> of the flag.
Hmm ... I just remembered the reason why we didn't use a spinlock in
these functions already. Namely, that it's unsafe for a signal handler
to try to acquire a spinlock that the interrupted code might be holding.
So I think a bit more thought is needed here. Maybe we need to bite the
bullet and do memory barriers ...
regards, tom lane
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