From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Jann Horn <jannh@google.com>, Ingo Molnar <mingo@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
linux-api <linux-api@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Andrea Parri <parri.andrea@gmail.com>,
Andrew Hunter <ahh@google.com>, Andy Lutomirski <luto@kernel.org>,
Avi Kivity <avi@scylladb.com>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Boqun Feng <boqun.feng@gmail.com>,
Dave Watson <davejwatson@fb.com>, David Sehr <sehr@google.com>,
Greg Hackmann <ghackmann@google.com>,
"H. Peter Anvin" <hpa@zytor.com>,
maged michael <maged.michael@gmail.com>,
Michael Ellerman <mpe@ellerman.id.au>,
Paul Mackerras <paulus@samba.org>,
"Russell King, ARM Linux" <linux@armlinux.org.uk>,
Will Deacon <will.deacon@arm.com>,
stable <stable@vger.kernel.org>
Subject: Re: [RFC PATCH] Fix: membarrier: racy access to p->mm in membarrier_global_expedited()
Date: Mon, 28 Jan 2019 13:26:33 -0800 [thread overview]
Message-ID: <20190128212633.GC4240@linux.ibm.com> (raw)
In-Reply-To: <231707440.2765.1548709646123.JavaMail.zimbra@efficios.com>
On Mon, Jan 28, 2019 at 04:07:26PM -0500, Mathieu Desnoyers wrote:
> ----- On Jan 28, 2019, at 3:46 PM, paulmck paulmck@linux.ibm.com wrote:
>
> > On Mon, Jan 28, 2019 at 12:27:03PM -0800, Linus Torvalds wrote:
> >> On Mon, Jan 28, 2019 at 10:27 AM Mathieu Desnoyers
> >> <mathieu.desnoyers@efficios.com> wrote:
> >> >
> >> > Jann Horn identified a racy access to p->mm in the global expedited
> >> > command of the membarrier system call.
> >> >
> >> > The suggested fix is to hold the task_lock() around the accesses to
> >> > p->mm and to the mm_struct membarrier_state field to guarantee the
> >> > existence of the mm_struct.
> >>
> >> Hmm. I think this is right. You shouldn't access another threads mm
> >> pointer without proper locking.
> >>
> >> That said, we *could* make the mm_cachep be SLAB_TYPESAFE_BY_RCU,
> >> which would allow speculatively reading data off the mm pointer under
> >> RCU. It might not be the *right* mm if somebody just did an exit, but
> >> for things like this it shouldn't matter.
> >
> > That sounds much simpler and more effective than the contention-reduction
> > approach that I suggested. ;-)
>
> I'd be tempted to stick to the locking approach for a fix, and implement
> Linus' type-safe mm_cachep idea if anyone complains about the overhead
> of membarrier GLOBAL_EXPEDITED (and submit for a future merge window).
>
> I tested the KASAN splat reproducer from Jann locally, and confirmed that
> my patch fixes the issue it reproduces.
>
> Please let me know if the task_lock() approach is OK as a fix for now.
Agreed, no need for added complexity until there is a clear need.
> I'm also awaiting a Tested-by from Jann before submitting this for real.
Makes sense to me!
Thanx, Paul
> Thanks,
>
> Mathieu
>
> >
> > Thanx, Paul
> >
> >> But if this is the only case that might care, it sounds like just
> >> doing the proper locking is the right approach.
> >>
> >> Linus
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
>
next prev parent reply other threads:[~2019-01-28 21:26 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-28 18:26 [RFC PATCH] Fix: membarrier: racy access to p->mm in membarrier_global_expedited() Mathieu Desnoyers
2019-01-28 20:27 ` Linus Torvalds
2019-01-28 20:46 ` Paul E. McKenney
2019-01-28 21:07 ` Mathieu Desnoyers
2019-01-28 21:26 ` Paul E. McKenney [this message]
2019-01-28 21:33 ` Jann Horn
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