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

Andev
2015-04-10T02:43:51Z

On Thu, Apr 9, 2015 at 4:36 PM, Jason Low <jason.low2@hp.com> 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".
>
> So then something like the following (for rwsem)?
>
> We can also run some tests to see how the worst case "access
> stale memory for one iteration" to the heuristic can have an affect on
> performance, though that probably wouldn't be much of an issue in
> practice.
>
> ---
> kernel/locking/rwsem-xadd.c | 43 ++++++++++++++++++++++++++++---------------
> 1 files changed, 28 insertions(+), 15 deletions(-)
>
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index 3417d01..870c574 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -295,6 +295,31 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
> }
> }
>
> +static inline bool owner_running(struct rw_semaphore *sem, struct task_struct *owner)
> +{
> + bool ret;
> +
> +#ifdef CONFIG_DEBUG_PAGEALLOC
> + rcu_read_lock();
> +#endif

Please use 'if (IS_ENABLED(CONFIG_DEBUG_PAGEALLOC)) {}' here. Makes
code much readable IMHO.

--
Andev