Re: [PATCH v2] rcu: Use an intermediate irq_work to start process_srcu()

[Joel Fernandes <joelagnelf@nvidia.com>]

On 3/23/2026 6:18 PM, Boqun Feng wrote:
> On Mon, Mar 23, 2026 at 05:09:53PM -0400, Joel Fernandes wrote:
>>
>>
>> On 3/20/2026 6:29 PM, Boqun Feng wrote:
>>> Since commit c27cea4416a3 ("rcu: Re-implement RCU Tasks Trace in terms
>>> of SRCU-fast") we switched to SRCU in BPF. However as BPF instrument can
>>> happen basically everywhere (including where a scheduler lock is held),
>>> call_srcu() now needs to avoid acquiring scheduler lock because
>>> otherwise it could cause deadlock [1]. Fix this by following what the
>>> previous RCU Tasks Trace did: using an irq_work to delay the queuing of
>>> the work to start process_srcu().
>>>
>>> [boqun: Apply Joel's feedback]
>>> [boqun: Apply Andrea's test feedback]
>>>
>>> Reported-by: Andrea Righi <arighi@nvidia.com>
>>> Closes: https://lore.kernel.org/all/abjzvz_tL_siV17s@gpd4/
>>> Fixes: commit c27cea4416a3 ("rcu: Re-implement RCU Tasks Trace in terms of SRCU-fast")
>>> Link: https://lore.kernel.org/rcu/3c4c5a29-24ea-492d-aeee-e0d9605b4183@nvidia.com/ [1]
>>> Suggested-by: Zqiang <qiang.zhang@linux.dev>
>>> Tested-by: Andrea Righi <arighi@nvidia.com>
>>> Signed-off-by: Boqun Feng <boqun@kernel.org>
>>
>> Tested-by: Joel Fernandes <joelagnelf@nvidia.com>
>>
>> I have the following in the shared rcu tree rcu/dev branch, but if you're
>> sending it for 7.0 -rc cycle I will drop them:
>>
>> 998608c o rcu: Use an intermediate irq_work to start process_srcu()
>> 15d921a o rcutorture: Test call_srcu() with preemption disabled and not
>>
> 
> Thank you. I think Paul has another one we need for v7.0:
> 
> 	https://lore.kernel.org/rcu/ad67e723-7f7d-4350-b886-c04e56c0b78d@paulmck-laptop/
> 
> I've made an fixes.v7.0-rc4 branch at:
> 
> 	git://git.kernel.org/pub/scm/linux/kernel/git/rcu/linux.git fixes.v7.0-rc4
> 
> with these commits on top of -rc4:
> 
> d0f816d5e872 rcu: Use an intermediate irq_work to start process_srcu()
> 6b7fc20ab878 srcu: Push srcu_node allocation to GP when non-preemptible
> f627bbab8fd0 srcu: Use raw spinlocks so call_srcu() can be used under preempt_disable()
> 
> I will send a PR at Wednesday if I don't hear any objection. Thanks!
> 
Btw, I am getting lockdep splats on SRCU-T now for some reason.

rcu_gp_start_if_needed() calls schedule_work() acquiring pool->lock

I believe this is because of Paul's rcutorture patch. I will debug it more and
send a patch but all other tests scenarios are looking good.

--
Joel Fernandes