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Message-ID: <1425694046.19505.71.camel@stgolabs.net>
Subject: Re: softlockups in multi_cpu_stop
From: Davidlohr Bueso <dave@stgolabs.net>
To: Ming Lei <ming.lei@canonical.com>
Cc: Jason Low <jason.low2@hp.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Sasha Levin <sasha.levin@oracle.com>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Dave Jones <davej@codemonkey.org.uk>
Date: Fri, 06 Mar 2015 18:07:26 -0800
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On Sat, 2015-03-07 at 09:55 +0800, Ming Lei wrote:
> On Fri, 06 Mar 2015 14:15:37 -0800
> Davidlohr Bueso <dave@stgolabs.net> wrote:
> 
> > On Fri, 2015-03-06 at 13:12 -0800, Jason Low wrote:
> > > In owner_running() there are 2 conditions that would make it return
> > > false: if the owner changed or if the owner is not running. However,
> > > that patch continues spinning if there is a "new owner" but it does not
> > > take into account that we may want to stop spinning if the owner is not
> > > running (due to getting rescheduled).
> > 
> > So you're rationale is that we're missing this need_resched:
> > 
> > 	while (owner_running(sem, owner)) {
> > 		/* abort spinning when need_resched */
> > 		if (need_resched()) {
> > 			rcu_read_unlock();
> > 			return false;
> > 		}
> > 	}
> > 
> > Because the owner_running() would return false, right? Yeah that makes
> > sense, as missing a resched is a bug, as opposed to our heuristics being
> > so painfully off.
> > 
> > Sasha, Ming (Cc'ed), does this address the issues you guys are seeing?
> 
> For the xfstest lockup, what matters is that the owner isn't running, since
> the following simple change does fix the issue:

I much prefer Jason's approach, which should also take care of the
issue, as it includes the !owner->on_cpu stop condition to stop
spinning.

> 
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index 06e2214..5e08705 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -358,8 +358,9 @@ bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
>  	}
>  	rcu_read_unlock();
>  
> -	if (READ_ONCE(sem->owner))
> -		return true; /* new owner, continue spinning */
> +	owner = READ_ONCE(sem->owner);
> +	if (owner && owner->on_cpu)
> +		return true;
>  
>  	/*
>  	 * When the owner is not set, the lock could be free or
> 
> 
> Thanks,
> Ming Lei