Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

["Paul E. McKenney" <paulmck@linux.ibm.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
> 
> 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.

That does seem like a very good idea!

							Thanx, Paul