linux-kernel - Re: sched: softlockups in multi_cpu

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Message-ID: <54F9EBCA.1060300@oracle.com>
Date:	Fri, 06 Mar 2015 13:02:50 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...nel.org>
CC:	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 03/06/2015 12:19 PM, Davidlohr Bueso wrote:
>> 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.

There's always the possibility that bisect went wrong. I did it twice, but
since I don't have a sure way of reproducing it I was basing my good/bad
decisions on whether I saw it within a reasonable amount of time.

I can go redo that again if you suspect that that commit is not the cause.


Thanks,
Sasha

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