On Fri, Dec 13, 2024 at 05:41:15PM -0500, Andres Freund wrote:
> Afaics we didn't do anything about this issue?
postgr.es/c/e656657 fixed this. I thought this was longstanding, but it
turned out to have started on 2024-04-02.
> Hm. Leaving RBM_ZERO_AND_LOCK aside, is it actually always safe to do
> RestoreBlockImage() into a buffer that currently is pinned? Not sure if
> there's actually all that much guarantee what transient state one can read
> when reading a page concurrently to a memcpy(). I suspect it's practically
> rare to see a problem, but one could imagine an memcpy implementation that
> uses non-temporal writes, which afaict would leave you open to seeing quite
> random states when reading concurrently, as the cache coherence protocol
> doesn't protect anymore.
I wondered about that, too. I didn't dig too deep.
https://pubs.opengroup.org/onlinepubs/9799919799/functions/memcpy.html and
https://en.cppreference.com/w/cpp/string/byte/memcpy were both silent about
the topic.
> On 2024-05-12 10:16:58 -0700, Noah Misch wrote:
> > I suspect the fix is to add a ReadBufferMode specified as, "If the block is
> > already in shared_buffers, do RBM_NORMAL and exclusive-lock the buffer.
> > Otherwise, do RBM_ZERO_AND_LOCK."
>
> I think that should work. At least in the current code it looks near trivial
> to implement, although the branch differences are going to be annoying.
>
> As usual the hardest part would probably be the naming. Maybe
> RBM_ZERO_ON_MISS_LOCK? RBM_LOCK_ZERO_ON_MISS? RBM_DWIM?
It turned out RBM_ZERO_AND_LOCK long worked that way, and postgr.es/c/e656657
just had to restore that longstanding behavior. The existing comment "Don't
read from disk, caller will initialize." does allude to this (but I didn't
originally catch the subtle point).
If RBM_ZERO_AND_LOCK hadn't existed so long, I'd rename it. Perhaps it
deserves a rename anyway? Of those, I'd pick RBM_ZERO_ON_MISS_LOCK. I also
considered RBM_RECENT_OR_ZERO, borrowing a term from ReadRecentBuffer().
