linux-kernel - Re: [RFC PATCH 01/10] thread_info: add helpers to snapshot thread flags

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Message-ID: <20210611091724.GA78056@C02TD0UTHF1T.local>
Date:   Fri, 11 Jun 2021 10:17:24 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Marco Elver <elver@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>, jonas@...thpole.se,
        juri.lelli@...hat.com,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Matt Turner <mattst88@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        Michal Simek <monstr@...str.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Paul Mackerras <paulus@...ba.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Richard Henderson <rth@...ddle.net>, shorne@...il.com,
        stefan.kristiansson@...nalahti.fi,
        Thomas Gleixner <tglx@...utronix.de>,
        vincent.guittot@...aro.org, Will Deacon <will@...nel.org>
Subject: Re: [RFC PATCH 01/10] thread_info: add helpers to snapshot thread
 flags

On Thu, Jun 10, 2021 at 11:01:34AM +0200, Marco Elver wrote:
> On Wed, 9 Jun 2021 at 14:20, Mark Rutland <mark.rutland@....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@....com>
> > Cc: Boqun Feng <boqun.feng@...il.com>
> > Cc: Dmitry Vyukov <dvyukov@...gle.com>
> > Cc: Marco Elver <elver@...gle.com>
> > Cc: Paul E. McKenney <paulmck@...nel.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Will Deacon <will@...nel.org>
> 
> Acked-by: Marco Elver <elver@...gle.com>

Thanks!

> > ---
> >  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.

That's a very good point; I agree it should be __always_inline, and I'll
fix that up for the next spin.

Thanks,
Mark.