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Date:   Wed, 20 May 2020 00:23:48 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Stephen Hemminger <stephen@...workplumber.org>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        "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

Stephen Hemminger <stephen@...workplumber.org> writes:
> On Tue, 19 May 2020 23:45:23 +0200
> "Ahmed S. Darwish" <a.darwish@...utronix.de> 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>
>
> Have your performance tested this with 1000's of network devices?

No. We did not. -ENOTESTCASE

> The reason seqcount logic is was done here was to achieve scaleablity
> and a semaphore does not scale as well.

That still does not make the livelock magically going away. Just make a
reader with real-time priority preempt the writer and the system stops
dead. The net result is perfomance <= 0.

This was observed on RT kernels without a special 1000's of network
devices test case.

Just for the record: This is not a RT specific problem. You can
reproduce that w/o an RT kernel as well. Just run the reader with
real-time scheduling policy.

As much as you hate it from a performance POV the only sane rule of
programming is: Correctness first.

And this code clearly violates that rule.

Thanks,

        tglx

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