netdev - Re: [patch net-next RFC 0/2] net: devlink: remove devlink big lock

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Message-ID: <YrnS2tcgyI9Aqe+b@nanopsycho>
Date:   Mon, 27 Jun 2022 17:55:06 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Ido Schimmel <idosch@...dia.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        petrm@...dia.com, pabeni@...hat.com, edumazet@...gle.com,
        mlxsw@...dia.com, saeedm@...dia.com
Subject: Re: [patch net-next RFC 0/2] net: devlink: remove devlink big lock

Mon, Jun 27, 2022 at 05:41:31PM CEST, idosch@...dia.com wrote:
>On Mon, Jun 27, 2022 at 03:54:59PM +0200, Jiri Pirko wrote:
>> From: Jiri Pirko <jiri@...dia.com>
>> 
>> This is an attempt to remove use of devlink_mutex. This is a global lock
>> taken for every user command. That causes that long operations performed
>> on one devlink instance (like flash update) are blocking other
>> operations on different instances.
>
>This patchset is supposed to prevent one devlink instance from blocking
>another? Devlink does not enable "parallel_ops", which means that the
>generic netlink mutex is serializing all user space operations. AFAICT,
>this series does not enable "parallel_ops", so I'm not sure what
>difference the removal of the devlink mutex makes.

You are correct, that is missing. For me, as a side effect this patchset
resolved the deadlock for LC auxdev you pointed out. That was my
motivation for this patchset :)


>
>The devlink mutex (in accordance with the comment above it) serializes
>all user space operations and accesses to the devlink devices list. This
>resulted in a AA deadlock in the previous submission because we had a
>flow where a user space operation (which acquires this mutex) also tries
>to register / unregister a nested devlink instance which also tries to
>acquire the mutex.
>
>As long as devlink does not implement "parallel_ops", it seems that the
>devlink mutex can be reduced to only serializing accesses to the devlink
>devices list, thereby eliminating the deadlock.
>
>> 
>> The first patch makes sure that the xarray that holds devlink pointers
>> is possible to be safely iterated.
>> 
>> The second patch moves the user command mutex to be per-devlink.
>> 
>> Jiri Pirko (2):
>>   net: devlink: make sure that devlink_try_get() works with valid
>>     pointer during xarray iteration
>>   net: devlink: replace devlink_mutex by per-devlink lock
>> 
>>  net/core/devlink.c | 256 ++++++++++++++++++++++++++++-----------------
>>  1 file changed, 161 insertions(+), 95 deletions(-)
>> 
>> -- 
>> 2.35.3
>>