On Tue, 2015-04-07 at 12:29 +0200, Peter Zijlstra wrote:
> On Tue, Apr 07, 2015 at 07:09:43AM +0200, Mike Galbraith wrote:
> > On Mon, 2015-04-06 at 21:59 -0400, Steven Rostedt wrote:
> > >
> > > We really should have a rt_spin_trylock_in_irq() and not have the
> > > below if conditional.
> > >
> > > The paths that will be executed in hard irq context are static.
> > > They
> > > should be labeled as such.
> >
> > +/*
> > + * Special purpose for locks taken in interrupt context: Take and
> > hold
> > + * ->wait_lock lest PI catching us with our fingers in the cookie
> > jar.
> > + * Do NOT abuse.
> > + */
> > +int __lockfunc rt_spin_trylock_in_irq(spinlock_t *lock)
> > +{
> > + struct task_struct *owner;
> > + if (!raw_spin_trylock(&lock->lock.wait_lock))
> > + return 0;
> > + owner = idle_task(raw_smp_processor_id());
> > + if (!(rt_mutex_cmpxchg(&lock->lock, NULL, owner))) {
> > + raw_spin_unlock(&lock->lock.wait_lock);
> > + return 0;
> > + }
> > + spin_acquire(&lock->dep_map, 0, 1, _RET_IP_);
> > + return 1;
> > +}
> > +
> > +/* ONLY for use with rt_spin_trylock_in_irq(), do NOT abuse. */
> > +void __lockfunc rt_spin_trylock_in_irq_unlock(spinlock_t *lock)
> > +{
> > + struct task_struct *owner =
> > idle_task(raw_smp_processor_id());
> > + /* NOTE: we always pass in '1' for nested, for simplicity
> > */
> > + spin_release(&lock->dep_map, 1, _RET_IP_);
> > + BUG_ON(!(rt_mutex_cmpxchg(&lock->lock, owner, NULL)));
> > + raw_spin_unlock(&lock->lock.wait_lock);
> > +}
> > +
>
> Can someone explain this braindamage? You should _NOT_ take mutexes
> in
> hardirq context.
No.. really? ;-)
If you have a spot where it'd be nice to do that despite it being
somewhat (koff).. discouraged shall we say, you have to do something
funky. Thomas had a patch to not raise sirq unconditionally for -rt
to let nohz_full work, but it needs a lock that's converted to an
rtmutex in -rt, and which doesn't want to be un-converted. Ergo, get
funky.
> And if its an irq thread, then the irq thread _IS_ the right owner,
> the
> thread needs to be boosted by waiters.
No irq thread.
> The idle thread cannot ever be owner of a mutex, that's complete and
> utter bullshit.
Not if you want to hide current from the deadlock detector lest it get
upset and make box go boom.
-Mike
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