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.p...@lge.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
