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SubjectRe: [PATCH 2/2] locking/rwsem: Use a return variable in rwsem_spin_on_owner()
On Thu, Apr 09, 2015 at 12:43:38PM -0700, Jason Low wrote:
> On Thu, 2015-04-09 at 11:16 -0700, Linus Torvalds wrote:
> > On Thu, Apr 9, 2015 at 11:08 AM, Linus Torvalds
> > <torvalds@linux-foundation.org> wrote:
> > >
> > > The pointer is a known-safe kernel pointer - it's just that it was
> > > "known safe" a few instructions ago, and might be rcu-free'd at any
> > > time.
> >
> > Actually, we could even do something like this:
> >
> > static inline int sem_owner_on_cpu(struct semaphore *sem, struct
> > task_struct *owner)
> > {
> > int on_cpu;
> >
> > #ifdef CONFIG_DEBUG_PAGEALLOC
> > rcu_read_lock();
> > #endif
> > on_cpu = sem->owner == owner && owner->on_cpu;
> > #ifdef CONFIG_DEBUG_PAGEALLOC
> > rcu_read_unlock();
> > #endif
> > return on_cpu;
> > }
> >
> > because we really don't need to hold the RCU lock over the whole loop,
> > we just need to validate that the semaphore owner still matches, and
> > if so, check that it's on_cpu.
> >
> > And if CONFIG_DEBUG_PAGEALLOC is set, we don't care about performance
> > *at*all*. We will have worse performance problems than doing some RCU
> > read-locking inside the loop.
> >
> > And if CONFIG_DEBUG_PAGEALLOC isn't set, we don't really care about
> > locking, since at worst we just access stale memory for one iteration.
> >
> > Hmm. It's not pretty, but neither is the current "let's just take a
> > rcu lock that we don't really need over a loop that doesn't have very
> > strict bounding".
> >
> > Comments?
>
> So that looks more similar to how the original code was where the
> rcu_read_lock() and rcu_read_unlock() was done inside the owner_running
> helper function (though without the CONFIG_DEBUG_PAGEALLOC), before
> commit 307bf9803f25 ("sched: Simplify mutex_spin_on_owner()") modified
> it to be done outside the loop.

Another approach would be to post a timer before entering the spinloop,
and have the timer handler set the resched bit. Then the loop would
be bounded, safe, and would run at full speed.

Thanx, Paul



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