On Wed, 24 Oct 2012, Paul E. McKenney wrote:
> On Wed, Oct 24, 2012 at 04:22:17PM -0400, Mikulas Patocka wrote: > > > > > > On Wed, 24 Oct 2012, Paul E. McKenney wrote: > > > > > On Tue, Oct 23, 2012 at 05:39:43PM -0400, Mikulas Patocka wrote: > > > > > > > > > > > > On Tue, 23 Oct 2012, Paul E. McKenney wrote: > > > > > > > > > On Tue, Oct 23, 2012 at 01:29:02PM -0700, Paul E. McKenney wrote: > > > > > > On Tue, Oct 23, 2012 at 08:41:23PM +0200, Oleg Nesterov wrote: > > > > > > > On 10/23, Paul E. McKenney wrote: > > > > > > > > > > > > > > > > * Note that this guarantee implies a further memory-ordering > > > > > > > > guarantee. > > > > > > > > * On systems with more than one CPU, when synchronize_sched() > > > > > > > > returns, > > > > > > > > * each CPU is guaranteed to have executed a full memory > > > > > > > > barrier since > > > > > > > > * the end of its last RCU read-side critical section > > > > > > > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > > > > > > > > > > > > > > Ah wait... I misread this comment. > > > > > > > > > > > > And I miswrote it. It should say "since the end of its last > > > > > > RCU-sched > > > > > > read-side critical section." So, for example, RCU-sched need not > > > > > > force > > > > > > a CPU that is idle, offline, or (eventually) executing in user mode > > > > > > to > > > > > > execute a memory barrier. Fixed this. > > > > > > > > Or you can write "each CPU that is executing a kernel code is > > > > guaranteed > > > > to have executed a full memory barrier". > > > > > > Perhaps I could, but it isn't needed, nor is it particularly helpful. > > > Please see suggestions in preceding email. > > > > It is helpful, because if you add this requirement (that already holds for > > the current implementation), you can drop rcu_read_lock_sched() and > > rcu_read_unlock_sched() from the following code that you submitted. > > > > static inline void percpu_up_read(struct percpu_rw_semaphore *p) > > { > > /* > > * Decrement our count, but protected by RCU-sched so that > > * the writer can force proper serialization. > > */ > > rcu_read_lock_sched(); > > this_cpu_dec(*p->counters); > > rcu_read_unlock_sched(); > > } > > > > > > The current implementation fulfills this requirement, you can just add > > > > it > > > > to the specification so that whoever changes the implementation keeps > > > > it. > > > > > > I will consider doing that if and when someone shows me a situation where > > > adding that requirement makes things simpler and/or faster. From what I > > > can see, your example does not do so. > > > > > > Thanx, Paul > > > > If you do, the above code can be simplified to: > > { > > barrier(); > > this_cpu_dec(*p->counters); > > } > > The readers are lightweight enough that you are worried about the overhead > of rcu_read_lock_sched() and rcu_read_unlock_sched()? Really??? > > Thanx, Paul There was no lock in previous kernels, so we should make it as simple as possible. Disabling and reenabling preemption is probably not a big deal, but if don't have to do it, why do it? Mikulas -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/