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

[Boqun Feng <boqun@kernel.org>]

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!

Regards,
Boqun

> thanks,
> 
> -- 
> Joel Fernandes
> 
[..]