All of lore.kernel.org
 help / color / mirror / Atom feed
From: Joel Fernandes <joel@joelfernandes.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	paulmck <paulmck@linux.ibm.com>, Will Deacon <will@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	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>
Subject: Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates
Date: Wed, 21 Aug 2019 15:03:31 -0400	[thread overview]
Message-ID: <20190821190331.GA113711@google.com> (raw)
In-Reply-To: <52600272.1909.1566402523680.JavaMail.zimbra@efficios.com>

On Wed, Aug 21, 2019 at 11:48:43AM -0400, Mathieu Desnoyers wrote:
> ----- 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

Fwiw, and, interestingly, on clang v7.0.1-8 (aarch64), I get a proper 64-bit
str with the above example (even when not using volatile):

0000000000000000 <nonvol>:
   0:	d28acf08 	mov	x8, #0x5678                	// #22136
   4:	f2a24688 	movk	x8, #0x1234, lsl #16
   8:	f2cacf08 	movk	x8, #0x5678, lsl #32
   c:	f2e24688 	movk	x8, #0x1234, lsl #48
  10:	90000009 	adrp	x9, 8 <nonvol+0x8>
  14:	91000129 	add	x9, x9, #0x0
  18:	f9000128 	str	x8, [x9]
  1c:	d65f03c0 	ret

test1.o:     file format elf64-littleaarch64


And even with -O2 it is a single store:

Disassembly of section .text:

0000000000000000 <nonvol>:
   0:	d28acf09 	mov	x9, #0x5678                	// #22136
   4:	f2a24689 	movk	x9, #0x1234, lsl #16
   8:	f2cacf09 	movk	x9, #0x5678, lsl #32
   c:	90000008 	adrp	x8, 8 <nonvol+0x8>
  10:	f2e24689 	movk	x9, #0x1234, lsl #48
  14:	f9000109 	str	x9, [x8]
  18:	d65f03c0 	ret

thanks,

 - Joel

[...]

  parent reply	other threads:[~2019-08-21 19:03 UTC|newest]

Thread overview: 60+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-18 10:29 WARNING in tracepoint_probe_register_prio (3) syzbot
2019-08-16  0:11 ` syzbot
2019-08-16 14:26   ` [PATCH 1/1] Fix: trace sched switch start/stop racy updates Mathieu Desnoyers
2019-08-16 16:25     ` Steven Rostedt
2019-08-16 16:48       ` Valentin Schneider
2019-08-16 17:04         ` Steven Rostedt
2019-08-16 17:41           ` Mathieu Desnoyers
2019-08-16 19:18             ` Steven Rostedt
2019-08-16 19:19             ` Alan Stern
2019-08-16 20:44               ` Joel Fernandes
2019-08-16 20:49                 ` Thomas Gleixner
2019-08-16 20:57                   ` Joel Fernandes
2019-08-16 22:27                     ` Valentin Schneider
2019-08-16 22:57                       ` Linus Torvalds
2019-08-17  1:41                         ` Mathieu Desnoyers
2019-08-17  4:52                         ` Paul E. McKenney
2019-08-17  8:28                           ` Linus Torvalds
2019-08-17  8:44                             ` Linus Torvalds
2019-08-17 15:02                               ` Mathieu Desnoyers
2019-08-17 20:03                                 ` Valentin Schneider
2019-08-17 23:00                                   ` Paul E. McKenney
2019-08-19 10:34                                     ` Valentin Schneider
2019-08-17 22:28                             ` Paul E. McKenney
2019-08-20 14:01                           ` Peter Zijlstra
2019-08-20 20:31                             ` Paul E. McKenney
2019-08-20 20:39                               ` Peter Zijlstra
2019-08-20 20:52                                 ` Paul E. McKenney
2019-08-16 21:04                   ` Linus Torvalds
2019-08-17  1:36                     ` Mathieu Desnoyers
2019-08-17  2:13                       ` Steven Rostedt
2019-08-17 14:40                         ` Mathieu Desnoyers
2019-08-17 15:26                           ` Steven Rostedt
2019-08-17 15:55                             ` Mathieu Desnoyers
2019-08-17 16:40                               ` Steven Rostedt
2019-08-17 22:06                                 ` Paul E. McKenney
2019-08-17  8:08                       ` Linus Torvalds
2019-08-20 13:56                         ` Peter Zijlstra
2019-08-20 20:29                           ` Paul E. McKenney
2019-08-21 10:32                             ` Will Deacon
2019-08-21 13:23                               ` Paul E. McKenney
2019-08-21 13:32                                 ` Will Deacon
2019-08-21 13:56                                   ` Paul E. McKenney
2019-08-21 16:22                                     ` Will Deacon
2019-08-21 15:33                                 ` Peter Zijlstra
2019-08-21 15:48                                   ` Mathieu Desnoyers
2019-08-21 16:14                                     ` Paul E. McKenney
2019-08-21 19:03                                     ` Joel Fernandes [this message]
2019-09-09  6:21                           ` Herbert Xu
2019-08-16 20:49                 ` Steven Rostedt
2019-08-16 20:59                   ` Joel Fernandes
2019-08-17  1:25                   ` Mathieu Desnoyers
2019-08-18  9:15                   ` stable markup was " Pavel Machek
2019-08-16 17:19       ` Mathieu Desnoyers
2019-08-16 19:15         ` Steven Rostedt
2019-08-17 14:27           ` Mathieu Desnoyers
2019-08-17 15:42             ` Steven Rostedt
2019-08-17 15:53               ` Mathieu Desnoyers
2019-08-17 16:43                 ` Steven Rostedt
2019-08-16 12:32 ` WARNING in tracepoint_probe_register_prio (3) syzbot
2019-08-16 12:41   ` Sebastian Andrzej Siewior

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20190821190331.GA113711@google.com \
    --to=joel@joelfernandes.org \
    --cc=boqun.feng@gmail.com \
    --cc=dhowells@redhat.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mathieu.desnoyers@efficios.com \
    --cc=paulmck@linux.ibm.com \
    --cc=peterz@infradead.org \
    --cc=rostedt@goodmis.org \
    --cc=stern@rowland.harvard.edu \
    --cc=tglx@linutronix.de \
    --cc=torvalds@linux-foundation.org \
    --cc=valentin.schneider@arm.com \
    --cc=will.deacon@arm.com \
    --cc=will@kernel.org \
    --subject='Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.