Re: [PATCH 2/3] asm-generic, x86: wrap atomic operations

[Mark Rutland]

On Tue, Mar 21, 2017 at 12:25:06PM +0300, Andrey Ryabinin wrote:
> On 03/20/2017 08:17 PM, Mark Rutland wrote:
> > Hi,
> > 
> > On Tue, Mar 14, 2017 at 08:24:13PM +0100, Dmitry Vyukov wrote:
> >>  /**
> >> - * atomic_read - read atomic variable
> >> + * arch_atomic_read - read atomic variable
> >>   * @v: pointer of type atomic_t
> >>   *
> >>   * Atomically reads the value of @v.
> >>   */
> >> -static __always_inline int atomic_read(const atomic_t *v)
> >> +static __always_inline int arch_atomic_read(const atomic_t *v)
> >>  {
> >> -	return READ_ONCE((v)->counter);
> >> +	/*
> >> +	 * We use READ_ONCE_NOCHECK() because atomic_read() contains KASAN
> >> +	 * instrumentation. Double instrumentation is unnecessary.
> >> +	 */
> >> +	return READ_ONCE_NOCHECK((v)->counter);
> >>  }
> > 
> > Just to check, we do this to avoid duplicate reports, right?
> > 
> > If so, double instrumentation isn't solely "unnecessary"; it has a
> > functional difference, and we should explicitly describe that in the
> > comment.
> > 
> > ... or are duplicate reports supressed somehow?
> 
> They are not suppressed yet. But I think we should just switch kasan
> to single shot mode, i.e. report only the first error. Single bug
> quite often has multiple invalid memory accesses causing storm in
> dmesg. Also write OOB might corrupt metadata so the next report will
> print bogus alloc/free stacktraces.
> In most cases we need to look only at the first report, so reporting
> anything after the first is just counterproductive.

FWIW, that sounds sane to me.

Given that, I agree with your comment regarding READ_ONCE{,_NOCHECK}().

If anyone really wants all the reports, we could have a boot-time option
to do that.

Thanks,
Mark.