Re: [PATCH 2/2] locking/rwsem: Use a return variable in rwsem_spin_on_owner()

Jason Low
2015-04-09T20:58:22Z

On Thu, Apr 9, 2015 at 12:58 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> 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.

Though posting a timer, ect... would also add more overhead right?