Messages in this thread | | | Date | Wed, 24 Oct 2012 16:22:17 -0400 (EDT) | From | Mikulas Patocka <> | Subject | Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers |
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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); }
Mikulas
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