Messages in this thread | | | Date | Thu, 9 Apr 2015 11:39:26 -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 11:16:24AM -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.
Much better from an RCU grace-period-latency perspective.
> 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.
But if we are running on a hypervisor, mightn't our VCPU be preempted just before accessing ->on_cpu, the task exit and its structures be freed and unmapped? Or is the task structure in memory that is never unmapped? (If the latter, clearly not a problem.)
Thanx, Paul
> 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? > > Linus >
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