Subject: Re: [PATCH v2 03/11] asm-generic/mmiowb: Mark accesses to fix KCSAN warnings

From: Michael Ellerman <[email-masked]>
Date: Fri, 12 May 2023 12:20:26 +1000

Rohan McLure <[email-masked]> writes:
> Prior to this patch, data races are detectable by KCSAN of the following
> forms:
>
> [1] Asynchronous calls to mmiowb_set_pending() from an interrupt context
>     or otherwise outside of a critical section
> [2] Interrupted critical sections, where the interrupt will itself
>     acquire a lock
>
> In case [1], calling context does not need an mmiowb() call to be
> issued, otherwise it would do so itself. Such calls to
> mmiowb_set_pending() are either idempotent or no-ops.
>
> In case [2], irrespective of when the interrupt occurs, the interrupt
> will acquire and release its locks prior to its return, nesting_count
> will continue balanced. In the worst case, the interrupted critical
> section during a mmiowb_spin_unlock() call observes an mmiowb to be
> pending and afterward is interrupted, leading to an extraneous call to
> mmiowb(). This data race is clearly innocuous.
>
> Mark all potentially asynchronous memory accesses with READ_ONCE or
> WRITE_ONCE, including increments and decrements to nesting_count. This
> has the effect of removing KCSAN warnings at consumer's callsites.
>
> Signed-off-by: Rohan McLure <[email-masked]>
> Reported-by: Michael Ellerman <[email-masked]>
> Reported-by: Gautam Menghani <[email-masked]>
> Tested-by: Gautam Menghani <[email-masked]>
> Acked-by: Arnd Bergmann <[email-masked]>
> ---
> v2: Remove extraneous READ_ONCE in mmiowb_set_pending for nesting_count
> ---
>  include/asm-generic/mmiowb.h | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)

This will need wider review, it's used by a lot of other architectures.

Probably best to pull this one out of the series and post it separately.
Add at least linux-kernel and linux-arch to Cc, and probably Will as
he's the original author.

cheers