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Message-ID: <20170618131056.5df46232@inria.fr>
Date: Sun, 18 Jun 2017 13:10:56 +0200
From: Jens Gustedt <jens.gustedt@...ia.fr>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] a new lock algorithm with lock value and CS
counts in the same atomic int
Hello Alexander,
On Sun, 18 Jun 2017 13:01:18 +0300 (MSK) Alexander Monakov
<amonakov@...ras.ru> wrote:
> On Fri, 16 Jun 2017, Jens Gustedt wrote:
> > void __lock(volatile int *l)
> > {
> > - if (libc.threads_minus_1)
> > - while (a_swap(l, 1)) __wait(l, l+1, 1, 1);
> > + /* This test is mostly useless, now. Leaving it to the
> > first CAS is
> > + probably just as efficient. */
> > + if (libc.threads_minus_1) {
> [...]
> > void __unlock(volatile int *l)
> > {
> > - if (l[0]) {
> > - a_store(l, 0);
> > - if (l[1]) __wake(l, 1, 1);
> > + if (a_fetch_add(l, INT_MAX) != -INT_MAX) {
> > + __syscall(SYS_futex, l, FUTEX_WAKE|FUTEX_PRIVATE,
> > 1); }
> > }
>
> This looks wrong in single-threaded case, __lock doesn't touch the
> lock, but __unlock modifies it unconditionally.
Right, probably there should be the same test as for the lock case. Or
we should drop that test all along. I don't think that it still serves
much purpose here. This is just trading one memory access against
another.
Thanks
Jens
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