Re: removing volatile qualifiers from lwlock.c

Hi,

On 2014-09-10 14:53:07 -0400, Robert Haas wrote:
> As discussed on the thread "Spinlocks and compiler/memory barriers",
> now that we've made the spinlock primitives function as compiler
> barriers (we think), it should be possible to remove volatile
> qualifiers from many places in the source code.  The attached patch
> does this in lwlock.c.  If the changes in commit
> 0709b7ee72e4bc71ad07b7120acd117265ab51d0 (and follow-on commits) are
> correct and complete, applying this shouldn't break anything, while
> possibly giving the compiler room to optimize things better than it
> does today.
>
> However, demonstrating the necessity of that commit for these changes
> seems to be non-trivial.  I tried applying this patch and reverting
> commits 5b26278822c69dd76ef89fd50ecc7cdba9c3f035,
> b4c28d1b92c81941e4fc124884e51a7c110316bf, and
> 0709b7ee72e4bc71ad07b7120acd117265ab51d0 on a PPC64 POWER8 box with a
> whopping 192 hardware threads (thanks, IBM!).  I then ran the
> regression tests repeatedly, and I ran several long pgbench runs with
> as many as 350 concurrent clients.  No failures.

There's actually one more commit to revert. What I used was:
git revert 5b26278822c69dd76ef89fd50ecc7cdba9c3f035 \
    b4c28d1b92c81941e4fc124884e51a7c110316bf \
    68e66923ff629c324e219090860dc9e0e0a6f5d6 \
    0709b7ee72e4bc71ad07b7120acd117265ab51d0

> So I'm posting this patch in the hope that others can help.  The
> relevant tests are:
>
> 1. If you apply this patch to master and run tests of whatever kind
> strikes your fancy, does anything break under high concurrency?  If it
> does, then the above commits weren't enough to make this safe on your
> platform.
>
> 2. If you apply this patch to master, revert the commits mentioned
> above, and again run tests, does anything now break?  If it does (but
> the first tests were OK), then that shows that those commits did
> something useful on your platform.

I just tried this on my normal x86 workstation. I applied your lwlock
patch and ontop I removed most volatiles (there's a couple still
required) from xlog.c. Works for 100 seconds. Then I reverted the above
commits. Breaks within seconds:
master:
LOG:  request to flush past end of generated WAL; request 2/E5EC3DE0, currpos 2/E5EC1E60
standby:
LOG:  record with incorrect prev-link 4/684C3108 at 4/684C3108
and similar.

So at least for x86 the compiler barriers are obviously required and
seemingly working.

I've attached the very quickly written xlog.c de-volatizing patch.

Greetings,

Andres Freund

--
 Andres Freund                       http://www.2ndQuadrant.com/
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