Re: [PATCH] READ_ONCE, WRITE_ONCE, kcsan: Perform checks in __*_ONCE variants

[Qian Cai]

On Tue, May 12, 2020 at 3:09 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Tue, May 12, 2020 at 08:38:39PM +0200, Marco Elver wrote:
> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> > index 741c93c62ecf..e902ca5de811 100644
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -224,13 +224,16 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
> >   * atomicity or dependency ordering guarantees. Note that this may result
> >   * in tears!
> >   */
> > -#define __READ_ONCE(x)       (*(const volatile __unqual_scalar_typeof(x) *)&(x))
> > +#define __READ_ONCE(x)                                                       \
> > +({                                                                   \
> > +     kcsan_check_atomic_read(&(x), sizeof(x));                       \
> > +     data_race((*(const volatile __unqual_scalar_typeof(x) *)&(x))); \
> > +})
>
> NAK
>
> This will actively insert instrumentation into __READ_ONCE() and I need
> it to not have any.

Any way to move this forward? Due to linux-next commit 6bcc8f459fe7
(locking/atomics: Flip fallbacks and instrumentation), it triggers a
lots of KCSAN warnings due to atomic ops are no longer marked. For
example,

[  197.318288][ T1041] write to 0xffff9302764ccc78 of 8 bytes by task
1048 on cpu 47:
[  197.353119][ T1041]  down_read_trylock+0x9e/0x1e0
atomic_long_set(&sem->owner, val);
__rwsem_set_reader_owned at kernel/locking/rwsem.c:205
(inlined by) rwsem_set_reader_owned at kernel/locking/rwsem.c:213
(inlined by) __down_read_trylock at kernel/locking/rwsem.c:1373
(inlined by) down_read_trylock at kernel/locking/rwsem.c:1517
[  197.374641][ T1041]  page_lock_anon_vma_read+0x19d/0x3c0
[  197.398894][ T1041]  rmap_walk_anon+0x30e/0x620

[  197.924695][ T1041] read to 0xffff9302764ccc78 of 8 bytes by task
1041 on cpu 43:
[  197.959501][ T1041]  up_read+0xb8/0x41a
arch_atomic64_read at arch/x86/include/asm/atomic64_64.h:22
(inlined by) atomic64_read at include/asm-generic/atomic-instrumented.h:838
(inlined by) atomic_long_read at include/asm-generic/atomic-long.h:29
(inlined by) rwsem_clear_reader_owned at kernel/locking/rwsem.c:242
(inlined by) __up_read at kernel/locking/rwsem.c:1433
(inlined by) up_read at kernel/locking/rwsem.c:1574
[  197.977728][ T1041]  rmap_walk_anon+0x2f2/0x620
[  197.999055][ T1041]  rmap_walk+0xb5/0xe0