Re: [PATCH net] net: sysfs: Fix deadlock situation in sysfs accesses

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 17/12/2024 à 10:20, Eric Dumazet a écrit :
> On Tue, Dec 17, 2024 at 9:59 AM Christophe Leroy
> <christophe.leroy@csgroup.eu> wrote:
>>
>>
>>
>> Le 17/12/2024 à 09:16, Eric Dumazet a écrit :
>>> On Tue, Dec 17, 2024 at 8:18 AM Christophe Leroy
>>> <christophe.leroy@csgroup.eu> wrote:
>>>>
>>>> The following problem is encountered on kernel built with
>>>> CONFIG_PREEMPT. An snmp daemon running with normal priority is
>>>> regularly calling ioctl(SIOCGMIIPHY). Another process running with
>>>> SCHED_FIFO policy is regularly reading /sys/class/net/eth0/carrier.
>>>>
>>>> After some random time, the snmp daemon gets preempted while holding
>>>> the RTNL mutex then the high priority process is busy looping into
>>>> carrier_show which bails out early due to a non-successfull
>>>> rtnl_trylock() which implies restart_syscall(). Because the snmp
>>>> daemon has a lower priority, it never gets the chances to release
>>>> the RTNL mutex and the high-priority task continues to loop forever.
>>>>
>>>> Replace the trylock by lock_interruptible. This will increase the
>>>> priority of the task holding the lock so that it can release it and
>>>> allow the reader of /sys/class/net/eth0/carrier to actually perform
>>>> its read.
>>>>
>>
>> ...
>>
>>>>
>>>> Fixes: 336ca57c3b4e ("net-sysfs: Use rtnl_trylock in sysfs methods.")
>>>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>>>> ---
>>>
>>> At a first glance, this might resurface the deadlock issue Eric W. Biederman
>>> was trying to fix in 336ca57c3b4e ("net-sysfs: Use rtnl_trylock in
>>> sysfs methods.")
>>
>> Are you talking about the deadlock fixed (incompletely) by 5a5990d3090b
>> ("net: Avoid race between network down and sysfs"), or the complement
>> provided by 336ca57c3b4e ?
>>
>> My understanding is that mutex_lock() will return EINTR only if a signal
>> is pending so there is no need to set signal_pending like it was when
>> using mutex_trylock() which does nothing when the mutex is already locked.
>>
>> And an EINTR return is expected and documented for a read() or a
>> write(), I can't see why we want ERESTARTNOINTR instead of ERSTARTSYS.
>> Isn't it the responsibility of the user app to call again read or write
>> if it has decided to not install the necessary sigaction for an
>> automatic restart ?
>>
>> Do you think I should instead use rtnl_lock_killable() and return
>> ERESTARTNOINTR in case of failure ? In that case, is it still possible
>> to interrupt a blocked 'cat /sys/class/net/eth0/carrier' which CTRL+C ?
> 
> Issue is when no signal is pending, we have a typical deadlock situation :
> 
> One process A is :
> 
> Holding sysfs lock, then attempts to grab rtnl.
> 
> Another one (B) is :
> 
> Holding rtnl, then attempts to grab sysfs lock.

Ok, I see.

But then what can be the solution to avoid busy looping with 
mutex_trylock , not giving any chance to the task holding the rtnl to 
run and unlock it ?

> 
> Using rtnl_lock_interruptible()  in A will still block A and B, until
> a CTRL+C is sent by another thread.