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Date: Fri, 18 Sep 2015 10:28:20 +0100
From: Will Deacon <will.deacon@arm.com>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Oleg Nesterov <oleg@redhat.com>,
        "ebiederm@xmission.com" <ebiederm@xmission.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
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        Andrey Konovalov <andreyknvl@google.com>,
        Alexander Potapenko <glider@google.com>,
        Hans Boehm <hboehm@google.com>
Subject: Re: [PATCH] kernel: fix data race in put_pid
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On Fri, Sep 18, 2015 at 10:06:46AM +0100, Dmitry Vyukov wrote:
> On Fri, Sep 18, 2015 at 10:51 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > As an alternative patch, could we not do:
> >
> >   void put_pid(struct pid *pid)
> >   {
> >         struct pid_namespace *ns;
> >
> >         if (!pid)
> >                 return;
> >
> >         ns = pid->numbers[pid->level].ns;
> >         if ((atomic_read(&pid->count) == 1) ||
> >              atomic_dec_and_test(&pid->count)) {
> >
> > +               smp_read_barrier_depends(); /* ctrl-dep */
> >
> >                 kmem_cache_free(ns->pid_cachep, pid);
> >                 put_pid_ns(ns);
> >         }
> >   }
> >
> > That would upgrade the atomic_read() path to a full READ_ONCE_CTRL(),
> > and thereby avoid any of the kmem_cache_free() stores from leaking out.
> > And its free, except on Alpha. Whereas the atomic_read_acquire() will
> > generate a full memory barrier on whole bunch of archs.
> 
> 
> What you propose makes sense.
> 
> +Will, Paul
> 
> Can we have something along the lines of:
> 
> #define atomic_read_ctrl(v) READ_ONCE_CTRL(&(v)->counter)

Funnily enough, I had this exact same discussion off-list yesterday
afternoon, since I wrote some code relying on a ctrl dependency from
an atomic_read to an atomic_xchg_relaxed.

So I guess I'm for the addition, but at the same time, could we make
atomic_read and atomic_set generic too?

Will