Re: wake_q memory ordering

[Manfred Spraul]

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Hi Davidlohr,

On 10/10/19 9:25 PM, Davidlohr Bueso wrote:
> On Thu, 10 Oct 2019, Peter Zijlstra wrote:
>
> > On Thu, Oct 10, 2019 at 02:13:47PM +0200, Manfred Spraul wrote:
> >
> > > Therefore smp_mb__{before,after}_atomic() may be combined with
> > > cmpxchg_relaxed, to form a full memory barrier, on all archs.
> >
> > Just so.
>
> We might want something like this?
>
> ----8<---------------------------------------------------------
>
> From: Davidlohr Bueso <dave@xxxxxxxxxxxx>
> Subject: [PATCH] Documentation/memory-barriers.txt: Mention 
> smp_mb__{before,after}_atomic() and CAS
>
> Explicitly mention possible usages to guarantee serialization even upon
> failed cmpxchg (or similar) calls along with 
> smp_mb__{before,after}_atomic().
>
> Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
> ---
> Documentation/memory-barriers.txt | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/Documentation/memory-barriers.txt 
> b/Documentation/memory-barriers.txt
> index 1adbb8a371c7..5d2873d4b442 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -1890,6 +1890,18 @@ There are some more advanced barrier functions:
> ÂÂÂÂ This makes sure that the death mark on the object is perceived to 
> be set
> ÂÂÂÂ *before* the reference counter is decremented.
>
> +ÂÂÂÂ Similarly, these barriers can be used to guarantee serialization 
> for atomic
> +ÂÂÂÂ RMW calls on architectures which may not imply memory barriers 
> upon failure.
> +
> +ÂÂÂ obj->next = NULL;
> +ÂÂÂ smp_mb__before_atomic()
> +ÂÂÂ if (cmpxchg(&obj->ptr, NULL, val))
> +ÂÂÂÂÂÂÂ return;
> +
> +ÂÂÂÂ This makes sure that the store to the next pointer always has 
> smp_store_mb()
> +ÂÂÂÂ semantics. As such, smp_mb__{before,after}_atomic() calls allow 
> optimizing
> +ÂÂÂÂ the barrier usage by finer grained serialization.
> +
> ÂÂÂÂ See Documentation/atomic_{t,bitops}.txt for more information.
I don't know. The new documentation would not have answered my question 
(is it ok to combine smp_mb__before_atomic() with atomic_relaxed()?). 
And it copies content already present in atomic_t.txt.

Thus: I would prefer if the first sentence of the paragraph is replaced: 
The list of operations should end with "...", and it should match what 
is in atomic_t.txt

Ok?

--

ÂÂÂ Manfred