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.
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 > > 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 -- 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/