linux-kernel - Re: [GIT PULL] NOHZ updates for v4.6

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Date:	Tue, 15 Mar 2016 10:49:21 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] NOHZ updates for v4.6


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Mon, Mar 14, 2016 at 07:44:14PM -0700, Linus Torvalds wrote:
> > On Mon, Mar 14, 2016 at 5:32 AM, Ingo Molnar <mingo@...nel.org> wrote:
> > > +/**
> > > + * fetch_or - perform *ptr |= mask and return old value of *ptr
> > > + * @ptr: pointer to value
> > > + * @mask: mask to OR on the value
> > > + *
> > > + * cmpxchg based fetch_or, macro so it works for different integer types
> > > + */
> > > +#ifndef fetch_or
> > > +#define fetch_or(ptr, mask)                                            \
> > > +({     typeof(*(ptr)) __old, __val = *(ptr);                           \
> > > +       for (;;) {                                                      \
> > > +               __old = cmpxchg((ptr), __val, __val | (mask));          \
> > > +               if (__old == __val)                                     \
> > > +                       break;                                          \
> > > +               __val = __old;                                          \
> > > +       }                                                               \
> > > +       __old;                                                          \
> > > +})
> > > +#endif
> > 
> > This is garbage.
> > 
> > This macro re-uses the "mask" argument potentially many many times, so
> > semantically it's very dubious.
> 
> So the below cures that; but do we want to maybe pull this back into
> sched.c and expose a version that operates on a fixed type instead?

So there are other problems with the macro - see the patch I just sent that tries 
to fix all of them.

> Although with xchg() and cmpxchg() we've already set a precedence for
> multi-width operators.

Yes, and if we name the new API 'xchg_or()' as I did it in my patch then it all 
nicely fits into the existing scheme.

Thanks,

	Ingo