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Message-ID: <20170625092951.4f67a70b@inria.fr>
Date: Sun, 25 Jun 2017 09:29:51 +0200
From: Jens Gustedt <jens.gustedt@...ia.fr>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] unify the use of FUTEX_PRIVATE
Hello Rich,
On Sat, 24 Jun 2017 18:16:30 -0400 Rich Felker <dalias@...c.org> wrote:
> For now let's just use a macro for FUTEX_PRIVATE.
Ok, was my guess :)
> My concern was
> mainly about whether there are cases where it's assumed that the 128
> or ^128 translates to FUTEX_PRIVATE by arithmetic ops. The __wait,
> __timedwait, and __wake interfaces just treat their argument as a
> boolean now, so I think the only places there might be remaining
> assumptions of that sort is where a flag bit from the sync object is
> passed directly into a futex __syscall -- places like the condvar
> implementation. You've looked at all this code just now so you
> probably have a better idea if there are any such places;
All the usages that I found go through a Boolean operation. The only
place where -maybe- the compiler might shortcut some of it is __wake
because it is inlined. In any case, by just using the macro the
compiler see's the same 128.
> if so I
> think the assumption that the bits match up should just be replaced by
> a conditional (e.g. flags & 128 ? 0 : FUTEX_PRIVATE or such).
There is none in the arch independent code, so you could go with my
patch as it is, I think.
I only found one other place, but which I don't understand (and I am
not sure that I want to :). This is in
src/thread/x32/syscall_cp_fixup.c
where there is a special casing for SYS_futex
Thanks
Jens
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