Re: [PATCH tip/core/rcu 03/17] rcu/tree: Skip entry into the page allocator for PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2020-06-24 13:12:12 [-0700], paulmck@kernel.org wrote:
> From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
> 
> To keep the kfree_rcu() code working in purely atomic sections on RT,
> such as non-threaded IRQ handlers and raw spinlock sections, avoid
> calling into the page allocator which uses sleeping locks on RT.
> 
> In fact, even if the  caller is preemptible, the kfree_rcu() code is
> not, as the krcp->lock is a raw spinlock.
> 
> Calling into the page allocator is optional and avoiding it should be
> Ok, especially with the page pre-allocation support in future patches.
> Such pre-allocation would further avoid the a need for a dynamically
> allocated page in the first place.
> 
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Reviewed-by: Uladzislau Rezki <urezki@gmail.com>
> Co-developed-by: Uladzislau Rezki <urezki@gmail.com>
> Signed-off-by: Uladzislau Rezki <urezki@gmail.com>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---
>  kernel/rcu/tree.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 64592b4..dbdd509 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3184,6 +3184,18 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
>  		if (!bnode) {
>  			WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE);
>  
> +			/*
> +			 * To keep this path working on raw non-preemptible
> +			 * sections, prevent the optional entry into the
> +			 * allocator as it uses sleeping locks. In fact, even
> +			 * if the caller of kfree_rcu() is preemptible, this
> +			 * path still is not, as krcp->lock is a raw spinlock.
> +			 * With additional page pre-allocation in the works,
> +			 * hitting this return is going to be much less likely.
> +			 */
> +			if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +				return false;

This is not going to work together with the "wait context validator"
(CONFIG_PROVE_RAW_LOCK_NESTING). As of -rc3 it should complain about
printk() which is why it is still disabled by default.

So assume that this is fixed and enabled then on !PREEMPT_RT it will
complain that you have a raw_spinlock_t acquired (the one from patch
02/17) and attempt to acquire a spinlock_t in the memory allocator.

>  			bnode = (struct kfree_rcu_bulk_data *)
>  				__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
>  		}

Sebastian