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 <[email protected]>
> 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) <[email protected]>
> ---
> 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.
>