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

[Eric Dumazet]

On 5/19/20 11:42 PM, Ahmed S. Darwish wrote:
> 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@xxxxxxxxxxxxxxx>
>>> Signed-off-by: Ahmed S. Darwish <a.darwish@xxxxxxxxxxxxx>
>>> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
>>> ---
>>>  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.

Hmm this was in linux-5.3, so very recent stuff.

Has this patch been backported to stable releases ?

With all the Fixes: tags you added, stable teams will backport this networking patch to
all stable versions.

Do we have a way to tune a dedicare rwsem to 'give preference to the (unique in this case) writer" over
a myriad of potential readers ?

Thanks.