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From:   Marco Elver <elver@google.com>
Date:   Thu, 10 Jun 2021 11:01:34 +0200
Message-ID: <CANpmjNOvnqE29mMU++K0VMibF8xMixJXRDqbDKXjXuxobAfM1Q@mail.gmail.com>
Subject: Re: [RFC PATCH 01/10] thread_info: add helpers to snapshot thread flags
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>, jonas@southpole.se,
        juri.lelli@redhat.com,
        Russell King - ARM Linux admin <linux@armlinux.org.uk>,
        Andy Lutomirski <luto@kernel.org>,
        Matt Turner <mattst88@gmail.com>,
        Ingo Molnar <mingo@redhat.com>,
        Michal Simek <monstr@monstr.eu>,
        Michael Ellerman <mpe@ellerman.id.au>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Paul Mackerras <paulus@samba.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Richard Henderson <rth@twiddle.net>, shorne@gmail.com,
        stefan.kristiansson@saunalahti.fi,
        Thomas Gleixner <tglx@linutronix.de>,
        vincent.guittot@linaro.org, Will Deacon <will@kernel.org>
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On Wed, 9 Jun 2021 at 14:20, Mark Rutland <mark.rutland@arm.com> wrote:
>
> We have common helpers to manipulate individual thread flags, but where
> code wants to check several flags at once, it must open code reading
> current_thread_info()->flags and operating on a snapshot.
>
> As some flags can be set remotely it's necessary to use READ_ONCE() to
> get a consistent snapshot even when IRQs are disabled, but some code
> forgets to do this. Generally this is unlike to cause a problem in
> practice, but it is somewhat unsound, and KCSAN will legitimately warn
> that there is a data race.
>
> To make it easier to do the right thing, and to highlight that
> concurrent modification is possible, let's add a new helpers to snapshot
> the flags, which should be used in preference to plain reads.
> Subsequent patches will move existing code to use the new helpers.
>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Marco Elver <elver@google.com>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Will Deacon <will@kernel.org>

Acked-by: Marco Elver <elver@google.com>


> ---
>  include/linux/thread_info.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
> index 157762db9d4b..f3769842046d 100644
> --- a/include/linux/thread_info.h
> +++ b/include/linux/thread_info.h
> @@ -117,6 +117,11 @@ static inline int test_ti_thread_flag(struct thread_info *ti, int flag)
>         return test_bit(flag, (unsigned long *)&ti->flags);
>  }
>
> +static inline unsigned long read_ti_thread_flags(struct thread_info *ti)
> +{
> +       return READ_ONCE(ti->flags);
> +}
> +

Are some of the callers 'noinstr'? I haven't seen it in this series
yet, but if yes, then not inlining (which some compilers may do with
heavier instrumentation) might cause issues and this could be
__always_inline.

Thanks,
-- Marco