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Date:   Wed, 21 Aug 2019 11:48:43 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     paulmck <paulmck@linux.ibm.com>, Will Deacon <will@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Joel Fernandes, Google" <joel@joelfernandes.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        rostedt <rostedt@goodmis.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Will Deacon <will.deacon@arm.com>,
        David Howells <dhowells@redhat.com>
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Subject: Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates
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----- On Aug 21, 2019, at 8:33 AM, Peter Zijlstra peterz@infradead.org wrote:

> On Wed, Aug 21, 2019 at 06:23:10AM -0700, Paul E. McKenney wrote:
>> On Wed, Aug 21, 2019 at 11:32:01AM +0100, Will Deacon wrote:
> 
>> > and so it is using a store-pair instruction to reduce the complexity in
>> > the immediate generation. Thus, the 64-bit store will only have 32-bit
>> > atomicity. In fact, this is scary because if I change bar to:
>> > 
>> > void bar(u64 *x)
>> > {
>> > 	*(volatile u64 *)x = 0xabcdef10abcdef10;
>> > }
>> > 
>> > then I get:
>> > 
>> > bar:
>> > 	mov	w1, 61200
>> > 	movk	w1, 0xabcd, lsl 16
>> > 	str	w1, [x0]
>> > 	str	w1, [x0, 4]
>> > 	ret
>> > 
>> > so I'm not sure that WRITE_ONCE would even help :/
>> 
>> Well, I can have the LWN article cite your email, then.  So thank you
>> very much!
>> 
>> Is generation of this code for a 64-bit volatile store considered a bug?
>> Or does ARMv8 exclude the possibility of 64-bit MMIO registers?  And I
>> would guess that Thomas and Linus would ask a similar bugginess question
>> for normal stores.  ;-)
> 
> I'm calling this a compiler bug; the way I understand volatile this is
> very much against the intentended use case. That is, this is buggy even
> on UP vs signals or MMIO.

And here is a simpler reproducer on my gcc-8.3.0 (aarch64) compiled with O2:

volatile unsigned long a;
 
void fct(void)
{
        a = 0x1234567812345678ULL;
}

void fct(void)
{
        a = 0x1234567812345678ULL;
   0:   90000000        adrp    x0, 8 <fct+0x8>
   4:   528acf01        mov     w1, #0x5678                     // #22136
   8:   72a24681        movk    w1, #0x1234, lsl #16
   c:   f9400000        ldr     x0, [x0]
  10:   b9000001        str     w1, [x0]
  14:   b9000401        str     w1, [x0, #4]
}
  18:   d65f03c0        ret

And the non-volatile case uses stp (is it a single store to memory ?):

unsigned long a;
  
void fct(void)
{
        a = 0x1234567812345678ULL;
}

void fct(void)
{
        a = 0x1234567812345678ULL;
   0:   90000000        adrp    x0, 8 <fct+0x8>
   4:   528acf01        mov     w1, #0x5678                     // #22136
   8:   72a24681        movk    w1, #0x1234, lsl #16
   c:   f9400000        ldr     x0, [x0]
  10:   29000401        stp     w1, w1, [x0]
}
  14:   d65f03c0        ret

It would probably be a good idea to audit other architectures, since this
is done by the compiler backend.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com