On 12/20, Srivatsa S. Bhat wrote: > > On 12/20/2012 12:44 AM, Oleg Nesterov wrote: > > > > We need 2 helpers for writer, the 1st one does synchronize_sched() and the > > 2nd one takes rwlock. A generic percpu_write_lock() simply calls them both. > > > > Ah, that's the problem no? Users of reader-writer locks expect to run in > atomic context (ie., they don't want to sleep).
Ah, I misunderstood. Sure, percpu_write_lock() should be might_sleep(), and this is not symmetric to percpu_read_lock(). > We can't expose an API that > can make the task go to sleep under the covers! Why? Just this should be documented. However I would not worry until we find another user. Until then we do not even need to add percpu_write_lock or try to generalize this code too much. > > To me, the main question is: can we use synchronize_sched() in cpu_down? > > It is slow. > > > > Haha :-) So we don't want smp_mb() in the reader, We need mb() + rmb(). Plust cli/sti unless this arch has optimized this_cpu_add() like x86 (as you pointed out). > *and* also don't want > synchronize_sched() in the writer! Sounds like saying we want to have the cake > and eat it too ;-) :P Personally I'd vote for synchronize_sched() but I am not sure. And I do not really understand the problem space. > And moreover, since I'm still not convinced about the writer API part if use > synchronize_sched(), I'd rather avoid synchronize_sched().) Understand. And yes, synchronize_sched() adds more problems. For example, where should we call it? I do not this _cpu_down() should do this, in this case, say, disable_nonboot_cpus() needs num_online_cpus() synchronize_sched's. So probably cpu_down() should call it before cpu_maps_update_begin(), this makes the locking even less obvious. In short. What I am trying to say is, don't ask me I do not know ;) Oleg. -- 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/
