Messages in this thread | | | Date | Tue, 23 Oct 2012 13:29:02 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers |
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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.
> But this patch needs more? Or I misunderstood. There is no RCU unlock > in percpu_up_read(). > > IOW. Suppose the code does > > percpu_down_read(); > x = PROTECTED_BY_THIS_RW_SEM; > percpu_up_read(); > > Withoit mb() the load above can be reordered with this_cpu_dec() in > percpu_up_read(). > > However, we do not care if we can guarantee that the next > percpu_down_write() can not return (iow, the next "write" section can > not start) until this load is complete. > > And I _think_ that another synchronize_sched() in percpu_down_write() > added by this patch should work. > > But, "since the end of its last RCU read-side critical section" > does not look enough. > > Or I misundersood you/Mikulas/both ?
I clearly need to look more carefully at Mikulas's code...
Thanx, Paul
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