linux-kernel - Re: [PATCH v3] lock/semaphore: Avoid an unnecessary deadlock within up()

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Date:	Wed, 9 Mar 2016 15:07:01 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	willy@...ux.intel.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, akinobu.mita@...il.com, jack@...e.cz,
	sergey.senozhatsky.work@...il.com, peter@...leysoftware.com
Subject: Re: [PATCH v3] lock/semaphore: Avoid an unnecessary deadlock within
 up()

On Wed, Mar 09, 2016 at 11:00:37AM +0900, Byungchul Park wrote:
> On Wed, Feb 17, 2016 at 10:28:29AM +0100, Ingo Molnar wrote:
> > 
> > * Byungchul Park <byungchul.park@....com> wrote:
> > 
> > > diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
> > > index b8120ab..6634b68 100644
> > > --- a/kernel/locking/semaphore.c
> > > +++ b/kernel/locking/semaphore.c
> > > @@ -130,13 +130,14 @@ EXPORT_SYMBOL(down_killable);
> > >  int down_trylock(struct semaphore *sem)
> > >  {
> > >  	unsigned long flags;
> > > -	int count;
> > > +	int count = -1;
> > >  
> > > -	raw_spin_lock_irqsave(&sem->lock, flags);
> > > -	count = sem->count - 1;
> > > -	if (likely(count >= 0))
> > > -		sem->count = count;
> > > -	raw_spin_unlock_irqrestore(&sem->lock, flags);
> > > +	if (raw_spin_trylock_irqsave(&sem->lock, flags)) {
> > > +		count = sem->count - 1;
> > > +		if (likely(count >= 0))
> > > +			sem->count = count;
> > > +		raw_spin_unlock_irqrestore(&sem->lock, flags);
> > > +	}
> > 
> > I still don't really like it: two parallel trylocks will cause one of them to fail 
> > - while with the previous code they would both succeed.
> > 
> > None of these changes are necessary with all the printk robustification 
> > changes/enhancements we talked about, right?
> 
> Not only printk() but any code using a semaphore, mutex and so on, can also
> cause a deadlock if wake_up_process() eventually tries to acquire the lock.
> There are several ways to solve this problem.
> 
> 1. ensure wake_up_process() does not try to acquire the locks.
> 2. ensure wake_up_process() isn't protected by a spinlock of the locks.
> 3. ensure any kind of trylock stuff never cause waiting and deadlock.
> 4. and so on..
> 
> I am not sure which one is the best. But I think 3rd one is the one since
> it can be done by a generic way, even though it might decrease the success
> ratio as Ingo said, but IMHO it's not a big problem since a trylock user 
> only uses the trylock when it doesn't need to be cared whether it succeed
> or fail.
> 
> Which one among those do you think the best approach? Please let me know,
> then I will try to solve this problem by the appoach.

Or what do you think about this approach in which I replace the semaphore
with mutex and apply this patch to mutex trylock? Since the parallelism
does not mean that much to mutex trylock.. Right?

> 
> > 
> > Thanks,
> > 
> > 	Ingo