Re: [PATCH v2 07/21] srcutree: Disable preemption across synchronize_srcu_atomic()

[Joel Fernandes <joelagnelf@nvidia.com>]

Hi Paul,

On Wed, Oct 07, 2026 at 01:59:10PM -0700, Paul E. McKenney wrote:
> Because synchronize_srcu_atomic() cannot sleep, if a pair of them run
> concurrently pinned to the same CPU, it is possible that one will spin
> uselessly waiting for the other while at the same time preventing that
> other from running.  This commit therefore disables preemption to prevent
> this failure mode.
> 
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---
>  kernel/rcu/srcutree.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 7ff45e992c011..672fb41345c91 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -2123,11 +2123,14 @@ void synchronize_srcu_atomic(struct srcu_struct *ssp)
>  
>  	// Perhaps others will do our work for us.
>  	srcu_state = get_state_synchronize_srcu(ssp);
> +	preempt_disable();

Should this be upgraded to local_irq_disable()?


>  	while (atomic_read(&sup->srcu_atomic_gp_flag) ||
>  	       atomic_xchg(&sup->srcu_atomic_gp_flag, 1)) {
> +		preempt_enable();
>  		if (poll_state_synchronize_srcu(ssp, srcu_state))
>  			return;

Otherwise, it is possible that an interrupt handler comes in here while the
srcu_atomic_gp_flag is already acquired and then spins forever?

If we disabled IRQs as well, along with preempt, that might add other
problems, though, such as a reader calling smp_call_function waiting for the
IPI handler to run on this CPU which is spinning with interrupts disabled.

thanks,

Joel Fernandes





>  		cpu_relax();
> +		preempt_disable();
>  	}
>  
>  	// One last check for others doing our work for us under the lock.
> @@ -2135,6 +2138,7 @@ void synchronize_srcu_atomic(struct srcu_struct *ssp)
>  	if (poll_state_synchronize_srcu(ssp, srcu_state)) {
>  		raw_spin_unlock_irq_rcu_node(sup);
>  		atomic_set(&sup->srcu_atomic_gp_flag, 0);
> +		preempt_enable();
>  		return;
>  	}
>  
> @@ -2152,6 +2156,7 @@ void synchronize_srcu_atomic(struct srcu_struct *ssp)
>  	}
>  	ASSERT_EXCLUSIVE_WRITER(sup->srcu_atomic_gp_flag);
>  	atomic_set_release(&sup->srcu_atomic_gp_flag, 0);
> +	preempt_enable();
>  	non_block_end();
>  }
>  EXPORT_SYMBOL_GPL(synchronize_srcu_atomic);
> -- 
> 2.40.1
>