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Date:   Wed, 20 May 2020 00:23:48 +0200
From:   Thomas Gleixner <>
To:     Stephen Hemminger <>,
        "Ahmed S. Darwish" <>
Cc:     Peter Zijlstra <>,
        Ingo Molnar <>, Will Deacon <>,
        "Paul E. McKenney" <>,
        "Sebastian A. Siewior" <>,
        Steven Rostedt <>,
        LKML <>,
        "David S. Miller" <>,
        Jakub Kicinski <>,
Subject: Re: [PATCH v1 01/25] net: core: device_rename: Use rwsem instead of a seqcount

Stephen Hemminger <> writes:
> On Tue, 19 May 2020 23:45:23 +0200
> "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: <>
>> Signed-off-by: Ahmed S. Darwish <>
>> Reviewed-by: Sebastian Andrzej Siewior <>
> 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.



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