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Date:   Fri, 24 May 2019 13:52:31 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Will Deacon <will.deacon@arm.com>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Andrea Parri <andrea.parri@amarulasolutions.com>,
        linux-kernel@vger.kernel.org, aou@eecs.berkeley.edu, arnd@arndb.de,
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Subject: Re: [PATCH 00/18] locking/atomic: atomic64 type cleanup
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On Fri, May 24, 2019 at 12:42:20PM +0100, Will Deacon wrote:

> > diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
> > index dca3fb0554db..125c95ddbbc0 100644
> > --- a/Documentation/atomic_t.txt
> > +++ b/Documentation/atomic_t.txt
> > @@ -83,6 +83,9 @@ The non-RMW ops are (typically) regular LOADs and STOREs and are canonically
> >  implemented using READ_ONCE(), WRITE_ONCE(), smp_load_acquire() and
> >  smp_store_release() respectively.
> >  
> 
> Not sure you need a new paragraph here.
> 
> > +Therefore, if you find yourself only using the Non-RMW operations of atomic_t,
> > +you do not in fact need atomic_t at all and are doing it wrong.
> > +
> 
> That makes sense to me, although I now find that the sentence below is a bit
> confusing because it sounds like it's a caveat relating to only using
> Non-RMW ops.
> 
> >  The one detail to this is that atomic_set{}() should be observable to the RMW
> >  ops. That is:
> 
> How about changing this to be:
> 
>   "A subtle detail of atomic_set{}() is that it should be observable..."

Done, find below.

---
Subject: Documentation/atomic_t.txt: Clarify pure non-rmw usage

Clarify that pure non-RMW usage of atomic_t is pointless, there is
nothing 'magical' about atomic_set() / atomic_read().

This is something that seems to confuse people, because I happen upon it
semi-regularly.

Acked-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 Documentation/atomic_t.txt | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
index dca3fb0554db..89eae7f6b360 100644
--- a/Documentation/atomic_t.txt
+++ b/Documentation/atomic_t.txt
@@ -81,9 +81,11 @@ SEMANTICS
 
 The non-RMW ops are (typically) regular LOADs and STOREs and are canonically
 implemented using READ_ONCE(), WRITE_ONCE(), smp_load_acquire() and
-smp_store_release() respectively.
+smp_store_release() respectively. Therefore, if you find yourself only using
+the Non-RMW operations of atomic_t, you do not in fact need atomic_t at all
+and are doing it wrong.
 
-The one detail to this is that atomic_set{}() should be observable to the RMW
+A subtle detail of atomic_set{}() is that it should be observable to the RMW
 ops. That is:
 
   C atomic-set