linux-kernel - Re: [PATCH] Documentation/atomic_t: Document cmpxchg() vs try

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Message-ID: <YOM9emXjk2aHYosu@hirez.programming.kicks-ass.net>
Date:   Mon, 5 Jul 2021 19:12:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Xu, Yanfei" <yanfei.xu@...driver.com>
Cc:     mingo@...hat.com, longman@...hat.com, boqun.feng@...il.com,
        will@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Documentation/atomic_t: Document cmpxchg() vs
 try_cmpxchg()

On Mon, Jul 05, 2021 at 11:25:29PM +0800, Xu, Yanfei wrote:
> > +CMPXHG vs TRY_CMPXCHG
> 
> CMPXHG -> CMPXCHG

Yeah, already fixed. I spotted it a minute after sending :/

> > +---------------------
> > +
> > +  int atomic_cmpxchg(atomic_t *ptr, int old, int new);
> > +  bool atomic_try_cmpxchg(atomic_t *ptr, int *oldp, int new);
> > +
> > +Both provide the same functionality, but try_cmpxchg() can lead to more
> > +compact code. The functions relate like:
> > +
> > +  bool atomic_try_cmpxchg(atomic_t *ptr, int *oldp, int new)
> > +  {
> > +    int ret, old = *oldp;
> > +    ret = atomic_cmpxchg(ptr, old, new);
> > +    if (ret != old)
> > +      *oldp = ret;
> > +    return ret == old;
> > +  }
> 
> I tried to search some comments about atomic_try_cmpxchg(), but failed.
> Maybe I missed it. With your this document, it is more clear now.

OK, good, thanks!

> > +
> > +and:
> > +
> > +  int atomic_cmpxchg(atomic_t *ptr, int old, int new)
> > +  {
> > +    (void)atomic_try_cmpxchg(ptr, &old, new);
> > +    return old;
> > +  }
> > +
> > +Usage:
> > +
> > +  old = atomic_read(&v);                       old = atomic_read(&v);
> > +  for (;;) {                                   do {
> > +    new = func(old);                             new = func(old);
> > +    tmp = atomic_cmpxchg(&v, old, new);                } while (!atomic_try_cmpxchg(&v, &old, new));
> 
> Some unnecessary spaces before "while".

That's due to the diff prepending the line with "+ " which offsets the
tabstop. If you apply the patch the actual document is fine.