linux-kernel - Re: [PATCH tip/core/rcu 1/4] rcu: Eliminate BUG

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Date:   Sun, 11 Nov 2018 21:07:04 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH tip/core/rcu 1/4] rcu: Eliminate BUG_ON() for sync.c

On Sun, 11 Nov 2018 11:32:14 -0800
"Paul E. McKenney" <paulmck@...ux.ibm.com> wrote:

> The sync.c file has a number of calls to BUG_ON(), which panics the
> kernel, which is not a good strategy for devices (like embedded) that
> don't have a way to capture console output.  This commit therefore
> changes these BUG_ON() calls to WARN_ON_ONCE(), but does so quite naively.
> 
> Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
> Acked-by: Oleg Nesterov <oleg@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> ---
>  kernel/rcu/sync.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
> index 3f943efcf61c..a6ba446a9693 100644
> --- a/kernel/rcu/sync.c
> +++ b/kernel/rcu/sync.c
> @@ -125,8 +125,7 @@ void rcu_sync_enter(struct rcu_sync *rsp)
>  		rsp->gp_state = GP_PENDING;
>  	spin_unlock_irq(&rsp->rss_lock);
>  
> -	BUG_ON(need_wait && need_sync);
> -
> +	WARN_ON_ONCE(need_wait && need_sync);
>  	if (need_sync) {
>  		gp_ops[rsp->gp_type].sync();
>  		rsp->gp_state = GP_PASSED;
> @@ -139,7 +138,7 @@ void rcu_sync_enter(struct rcu_sync *rsp)
>  		 * Nobody has yet been allowed the 'fast' path and thus we can
>  		 * avoid doing any sync(). The callback will get 'dropped'.
>  		 */
> -		BUG_ON(rsp->gp_state != GP_PASSED);
> +		WARN_ON_ONCE(rsp->gp_state != GP_PASSED);
>  	}
>  }
>  
> @@ -166,8 +165,8 @@ static void rcu_sync_func(struct rcu_head *rhp)
>  	struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head);
>  	unsigned long flags;
>  
> -	BUG_ON(rsp->gp_state != GP_PASSED);
> -	BUG_ON(rsp->cb_state == CB_IDLE);
> +	WARN_ON_ONCE(rsp->gp_state != GP_PASSED);
> +	WARN_ON_ONCE(rsp->cb_state == CB_IDLE);
>  
>  	spin_lock_irqsave(&rsp->rss_lock, flags);
>  	if (rsp->gp_count) {
> @@ -225,7 +224,7 @@ void rcu_sync_dtor(struct rcu_sync *rsp)
>  {
>  	int cb_state;
>  
> -	BUG_ON(rsp->gp_count);
> +	WARN_ON_ONCE(rsp->gp_count);
>  
>  	spin_lock_irq(&rsp->rss_lock);
>  	if (rsp->cb_state == CB_REPLAY)
> @@ -235,6 +234,6 @@ void rcu_sync_dtor(struct rcu_sync *rsp)
>  
>  	if (cb_state != CB_IDLE) {
>  		gp_ops[rsp->gp_type].wait();
> -		BUG_ON(rsp->cb_state != CB_IDLE);
> +		WARN_ON_ONCE(rsp->cb_state != CB_IDLE);
>  	}
>  }

I take it that if any of these WARN_ON_ONCE() triggers, they wont cause
immediate catastrophe, and/or there's no gentle way out like you have
with the other patches exiting the function when one is hit.

-- Steve