linux-kernel - Re: sched: softlockups in multi_cpu

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Date:	Fri, 06 Mar 2015 09:19:02 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...emonkey.org.uk>, jason.low2@...com,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: sched: softlockups in multi_cpu_stop

On Fri, 2015-03-06 at 13:32 +0100, Ingo Molnar wrote:
> * Sasha Levin <sasha.levin@...cle.com> wrote:
> 
> > I've bisected this to "locking/rwsem: Check for active lock before bailing on spinning". Relevant parties Cc'ed.
> 
> That would be:
> 
>   1a99367023f6 ("locking/rwsem: Check for active lock before bailing on spinning")
> 
> attached below.

[...]

> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index 1c0d11e8ce34..e4ad019e23f5 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -298,23 +298,30 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
>  static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
>  {
>  	struct task_struct *owner;
> -	bool on_cpu = false;
> +	bool ret = true;
>  
>  	if (need_resched())
>  		return false;
>  
>  	rcu_read_lock();
>  	owner = ACCESS_ONCE(sem->owner);
> -	if (owner)
> -		on_cpu = owner->on_cpu;
> -	rcu_read_unlock();
> +	if (!owner) {
> +		long count = ACCESS_ONCE(sem->count);
> +		/*
> +		 * If sem->owner is not set, yet we have just recently entered the
> +		 * slowpath with the lock being active, then there is a possibility
> +		 * reader(s) may have the lock. To be safe, bail spinning in these
> +		 * situations.
> +		 */
> +		if (count & RWSEM_ACTIVE_MASK)
> +			ret = false;
> +		goto done;

Hmmm so the lockup would be due to this (when owner is non-nil the patch
has no effect), telling users to spin instead of sleep -- _except_ for
this condition. And when spinning we're always checking for need_resched
to be safe. So even if this function was completely bogus, we'd end up
needlessly spinning but I'm surprised about the lockup. Maybe coffee
will make things clearer.

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