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

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Date:   Mon, 5 Jul 2021 23:25:29 +0800
From:   "Xu, Yanfei" <yanfei.xu@...driver.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 7/5/21 11:07 PM, Peter Zijlstra wrote:
> [Please note: This e-mail is from an EXTERNAL e-mail address]
> 
> On Mon, Jul 05, 2021 at 04:00:01PM +0200, Peter Zijlstra wrote:
> 
>> No, when try_cmpxchg() fails it will update oldp. This is the reason old
>> is now a pointer too.
> 
> Since you're not the first person confused by this, does the below
> clarify?
> 
> ---
> Subject: Documentation/atomic_t: Document cmpxchg() vs try_cmpxchg()
> From: Peter Zijlstra <peterz@...radead.org>
> Date: Mon Jul  5 17:00:24 CEST 2021
> 
> There seems to be a significant amount of confusion around the 'new'
> try_cmpxchg(), despite this being more like the C11
> atomic_compare_exchange_*() family. Add a few words of clarification
> on how cmpxchg() and try_cmpxchg() relate to one another.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>   Documentation/atomic_t.txt |   41 +++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 41 insertions(+)
> 
> --- a/Documentation/atomic_t.txt
> +++ b/Documentation/atomic_t.txt
> @@ -271,3 +271,44 @@ because it would not order the W part of
>                          SC *y, t;
> 
>   is allowed.
> +
> +
> +CMPXHG vs TRY_CMPXCHG

CMPXHG -> CMPXCHG

> +---------------------
> +
> +  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.

> +
> +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".

Thanks,
Yanfei

> +    if (tmp == old)
> +      break;
> +    old = tmp;
> +  }
> +
> +NB. try_cmpxchg() also generates better code on some platforms (notably x86)
> +where the function more closely matches the hardware instruction.
>