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

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Message-ID: <20160218080031.GE5972@X58A-UD3R>
Date:	Thu, 18 Feb 2016 17:00:31 +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, 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?

Right. I expect that Jan's patch which Sergey informed can make printk
robuster. Actually I'm waiting for the patch done. And I thought that it's
also a problem that a trylock implementation can make a system deadlock.
Don't you think it need to make a trylock only either acquire or fail in
any case? IMHO, waiting something within trylock is wrong. I'm just
curious.

Thanks,
Byungchul

> 
> Thanks,
> 
> 	Ingo