Messages in this thread | | | Date | Thu, 9 Apr 2015 12:58:50 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 2/2] locking/rwsem: Use a return variable in rwsem_spin_on_owner() |
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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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