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Date:   Fri, 24 May 2019 13:18:07 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Andrea Parri <andrea.parri@amarulasolutions.com>,
        linux-kernel@vger.kernel.org, will.deacon@arm.com,
        aou@eecs.berkeley.edu, arnd@arndb.de, bp@alien8.de,
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        heiko.carstens@de.ibm.com, herbert@gondor.apana.org.au,
        ink@jurassic.park.msu.ru, jhogan@kernel.org, linux@armlinux.org.uk,
        mattst88@gmail.com, mingo@kernel.org, mpe@ellerman.id.au,
        palmer@sifive.com, paul.burton@mips.com, paulus@samba.org,
        ralf@linux-mips.org, rth@twiddle.net, stable@vger.kernel.org,
        tglx@linutronix.de, tony.luck@intel.com, vgupta@synopsys.com,
        gregkh@linuxfoundation.org, jhansen@vmware.com, vdasa@vmware.com,
        aditr@vmware.com, Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH 00/18] locking/atomic: atomic64 type cleanup
Message-ID: <20190524111807.GS2650@hirez.programming.kicks-ass.net>
References: <20190522132250.26499-1-mark.rutland@arm.com>
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 <20190523101926.GA3370@lakrids.cambridge.arm.com>
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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@lakrids:~/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: