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().
