On Wed, Mar 09, 2016 at 03:07:01PM +0900, Byungchul Park wrote:
> 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?
I am sorry for bothering you. I found that mutex trylock can be used in
interrupt context. Let me think more.
>
> >
> > >
> > > Thanks,
> > >
> > > Ingo
