Re: [PATCH tip/core/rcu 3/3] rcu: Correctly handle non-empty Tiny RCU callback list with none ready

[josh@joshtriplett.org]

On Tue, May 12, 2015 at 03:49:13PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> If, at the time __rcu_process_callbacks() is invoked,  there are callbacks
> in Tiny RCU's callback list, but none of them are ready to be invoked,
> the current list-management code will knit the non-ready callbacks out
> of the list.  This can result in hangs and possibly worse.  This commit
> therefore inserts a check for there being no callbacks that can be
> invoked immediately.
> 
> This bug is unlikely to occur -- you have to get a new callback between
> the time rcu_sched_qs() or rcu_bh_qs() was called, but before we get to
> __rcu_process_callbacks().  It was detected by the addition of RCU-bh
> testing to rcutorture, which in turn was instigated by Iftekhar Ahmed's
> mutation testing.  Although this bug was made much more likely by
> 915e8a4fe45e (rcu: Remove fastpath from __rcu_process_callbacks()), this
> did not cause the bug, but rather made it much more probable.   That
> said, it takes more than 40 hours of rcutorture testing, on average,
> for this bug to appear, so this fix cannot be considered an emergency.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: <stable@vger.kernel.org>

Ouch, subtle.

Reviewed-by: Josh Triplett <josh@joshtriplett.org>

>  kernel/rcu/tiny.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> index a501b4ab9b1c..591af0cb7b9f 100644
> --- a/kernel/rcu/tiny.c
> +++ b/kernel/rcu/tiny.c
> @@ -137,6 +137,11 @@ static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)
>  
>  	/* Move the ready-to-invoke callbacks to a local list. */
>  	local_irq_save(flags);
> +	if (rcp->donetail == &rcp->rcucblist) {
> +		/* No callbacks ready, so just leave. */
> +		local_irq_restore(flags);
> +		return;
> +	}
>  	RCU_TRACE(trace_rcu_batch_start(rcp->name, 0, rcp->qlen, -1));
>  	list = rcp->rcucblist;
>  	rcp->rcucblist = *rcp->donetail;
> -- 
> 1.8.1.5
>