Messages in this thread | | | Date | Tue, 23 Oct 2012 17:39:43 -0400 (EDT) | From | Mikulas Patocka <> | Subject | Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers |
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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".
It would be consistent with the current implementation and it would make it possible to use
barrier()-synchronize_sched() as biased memory barriers.
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In percpu-rwlocks, CPU 1 executes
...make some writes in the critical section... barrier(); this_cpu_dec(*p->counters);
and the CPU 2 executes
while (__percpu_count(p->counters)) msleep(1); synchronize_sched();
So, when CPU 2 finishes synchronize_sched(), we must make sure that all writes done by CPU 1 are visible to CPU 2.
The current implementation fulfills this requirement, you can just add it to the specification so that whoever changes the implementation keeps it.
Mikulas
> And I should hasten to add that for synchronize_sched(), disabling > preemption (including disabling irqs, further including NMI handlers) > acts as an RCU-sched read-side critical section. (This is in the > comment header for synchronize_sched() up above my addition to it.) > > Thanx, Paul
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