linux-kernel - Re: [PATCH 00/18] locking/atomic: atomic64 type cleanup

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Date:   Fri, 24 May 2019 13:18:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        linux-kernel@...r.kernel.org, will.deacon@....com,
        aou@...s.berkeley.edu, arnd@...db.de, bp@...en8.de,
        catalin.marinas@....com, davem@...emloft.net, fenghua.yu@...el.com,
        heiko.carstens@...ibm.com, herbert@...dor.apana.org.au,
        ink@...assic.park.msu.ru, jhogan@...nel.org, linux@...linux.org.uk,
        mattst88@...il.com, mingo@...nel.org, mpe@...erman.id.au,
        palmer@...ive.com, paul.burton@...s.com, paulus@...ba.org,
        ralf@...ux-mips.org, rth@...ddle.net, stable@...r.kernel.org,
        tglx@...utronix.de, tony.luck@...el.com, vgupta@...opsys.com,
        gregkh@...uxfoundation.org, jhansen@...are.com, vdasa@...are.com,
        aditr@...are.com, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 00/18] locking/atomic: atomic64 type cleanup

On Fri, May 24, 2019 at 12:37:31PM +0200, Peter Zijlstra wrote:
> On Thu, May 23, 2019 at 11:19:26AM +0100, Mark Rutland wrote:
> 
> > [mark@...rids:~/src/linux]% git grep '\(return\|=\)\s\+atomic\(64\)\?_set'
> > include/linux/vmw_vmci_defs.h:  return atomic_set((atomic_t *)var, (u32)new_val);
> > include/linux/vmw_vmci_defs.h:  return atomic64_set(var, new_val);
> > 
> 
> Oh boy, what a load of crap you just did find.
> 
> How about something like the below? I've not read how that buffer is
> used, but the below preserves all broken without using atomic*_t.

Clarified by something along these lines?

---
 Documentation/atomic_t.txt | 3 +++
 1 file changed, 3 insertions(+)

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.
 
+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
 ops. That is: