Re: [PATCH 04/11] rcu/nocb: Make rcu_core() callbacks acceleration preempt-safe

[Valentin Schneider]

On 30/09/21 00:10, Frederic Weisbecker wrote:
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>
> While reporting a quiescent state for a given CPU, rcu_core() takes
> advantage of the freshly loaded grace period sequence number and the
> locked rnp to accelerate the callbacks whose sequence number have been
> assigned a stale value.
>
> This action is only necessary when the rdp isn't offloaded, otherwise
> the NOCB kthreads already take care of the callbacks progression.
>
> However the check for the offloaded state is volatile because it is
> performed outside the IRQs disabled section. It's possible for the
> offloading process to preempt rcu_core() at that point on PREEMPT_RT.
>
> This is dangerous because rcu_core() may end up accelerating callbacks
> concurrently with NOCB kthreads without appropriate locking.
>
> Fix this with moving the offloaded check inside the rnp locking section.
>
> Reported-by: Valentin Schneider <valentin.schneider@xxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Valentin Schneider <valentin.schneider@xxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
> Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>

Reviewed-by: Valentin Schneider <valentin.schneider@xxxxxxx>