Re: pg_memory_barrier() doesn't compile, let alone work, for me

On Sun, Jul 14, 2013 at 8:41 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Commit 9a20a9b2 breaks the build for me, using gcc on HPPA:
>
> xlog.c:2182: macro `pg_memory_barrier' used without args
> xlog.c:2546: macro `pg_memory_barrier' used without args
> make[4]: *** [xlog.o] Error 1
>
> The reason for this is that the "fallback" definition of
> pg_memory_barrier has been wrong since day one; it needs this fix:
>
> -#define pg_memory_barrier(x) \
> +#define pg_memory_barrier() \

Uggh, sorry.

> However, fixing that doesn't yield much joy; initdb stalls and then
> crashes with
>
> PANIC:  stuck spinlock (40054a88) detected at xlog.c:2182
>
> The reason for that is that the code does not bother to initialize
> "dummy_spinlock" anywhere.  It might accidentally fail to fail
> on machines where the unlocked state of a spinlock is all-zeroes
> (given a compiler that's not picky about the incorrect macro usage)
> ... but HPPA is not such a machine.

This would not be hard to fix, I think.

> Rather than trying to think of a fix for that, I'm of the opinion that
> we should rip this out.  The fallback implementation of pg_memory_barrier
> ought to be pg_compiler_barrier(), on the theory that non-mainstream
> architectures don't have weak memory ordering anyway, or if they do you
> need to do some work to get PG to work on them.  Or maybe we ought to
> stop pretending that the code is likely to work on arbitrary machines,
> and just #error if there's not a supplied machine-specific macro.

Well, pg_memory_barrier() isn't even equivalent to
pg_compiler_barrier() on x86, which has among the strongest memory
orderings out there, so I think your first idea is a non-starter.  A
compiler barrier on x86 will fence reads from reads and writes from
writes, but it will not prevent a read from being speculated before a
subsequent write, which a full barrier must do.

I'm pretty sure we've got latent memory-ordering risks in our existing
code which we just haven't detected and fixed yet.  Consider, for
example, this exciting code from GetNewTransactionId:
                               myproc->subxids.xids[nxids] = xid;                               mypgxact->nxids = nxids
+1;
 

I don't believe that's technically safe even on an architecture like
x86, because the compiler could decide to reorder those assignments.
Of course there is probably no reason to do so, and even if it does
you'd have to get really unlucky to see a user-visible failure, and if
you did you'd probably misguess the cause.

However, I'm guessing that as we start using memory barriers more, and
as we speed up the system generally, we're going to see bugs like this
that are currently hidden start to become visible from time to time.
Especially in view of that, I think it's wise to be pessimistic rather
than optimistic about what barriers are needed, when we don't know for
certain.  That way, when we get a memory-ordering-related bug report,
we can at least hope that the problem must be something specific to
the problem area rather than a general failure to use the right memory
barrier primitives.

My preference would be to fix this in a narrow way, by initializing
dummy_spinlock somewhere.  But if not, then I think #error is the only
safe way to go.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company