netdev - Re: [PATCH v1 01/25] net: core: device_rename: Use rwsem instead of a seqcount

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Message-ID: <20200520064246.GA353513@debian-buster-darwi.lab.linutronix.de>
Date:   Wed, 20 May 2020 08:42:46 +0200
From:   "Ahmed S. Darwish" <a.darwish@...utronix.de>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "Sebastian A. Siewior" <bigeasy@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH v1 01/25] net: core: device_rename: Use rwsem instead of
 a seqcount

Hello Eric,

On Tue, May 19, 2020 at 07:01:38PM -0700, Eric Dumazet wrote:
>
> On 5/19/20 2:45 PM, Ahmed S. Darwish wrote:
> > Sequence counters write paths are critical sections that must never be
> > preempted, and blocking, even for CONFIG_PREEMPTION=n, is not allowed.
> >
> > Commit 5dbe7c178d3f ("net: fix kernel deadlock with interface rename and
> > netdev name retrieval.") handled a deadlock, observed with
> > CONFIG_PREEMPTION=n, where the devnet_rename seqcount read side was
> > infinitely spinning: it got scheduled after the seqcount write side
> > blocked inside its own critical section.
> >
> > To fix that deadlock, among other issues, the commit added a
> > cond_resched() inside the read side section. While this will get the
> > non-preemptible kernel eventually unstuck, the seqcount reader is fully
> > exhausting its slice just spinning -- until TIF_NEED_RESCHED is set.
> >
> > The fix is also still broken: if the seqcount reader belongs to a
> > real-time scheduling policy, it can spin forever and the kernel will
> > livelock.
> >
> > Disabling preemption over the seqcount write side critical section will
> > not work: inside it are a number of GFP_KERNEL allocations and mutex
> > locking through the drivers/base/ :: device_rename() call chain.
> >
> > From all the above, replace the seqcount with a rwsem.
> >
> > Fixes: 5dbe7c178d3f (net: fix kernel deadlock with interface rename and netdev name retrieval.)
> > Fixes: 30e6c9fa93cf (net: devnet_rename_seq should be a seqcount)
> > Fixes: c91f6df2db49 (sockopt: Change getsockopt() of SO_BINDTODEVICE to return an interface name)
> > Cc: <stable@...r.kernel.org>
> > Signed-off-by: Ahmed S. Darwish <a.darwish@...utronix.de>
> > Reviewed-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > ---
> >  net/core/dev.c | 30 ++++++++++++------------------
> >  1 file changed, 12 insertions(+), 18 deletions(-)
> >
>
> Seems fine to me, assuming rwsem prevent starvation of the writer.
>

Thanks for the review.

AFAIK, due to 5cfd92e12e13 ("locking/rwsem: Adaptive disabling of reader
optimistic spinning"), using a rwsem shouldn't lead to writer starvation
in the contended case.

--
Ahmed S. Darwish
Linutronix GmbH