On Wed, Oct 17, 2012 at 11:07:21AM -0400, Mikulas Patocka wrote: > > > > Even the previous patch is applied, percpu_down_read() still > > needs mb() to pair with it. > > percpu_down_read uses rcu_read_lock which should guarantee that memory > accesses don't escape in front of a rcu-protected section. You do realize that rcu_read_lock() does nothing more that a barrier(), right? Paul worked really hard to get rcu_read_locks() to not call HW barriers. > > If rcu_read_unlock has only an unlock barrier and not a full barrier, > memory accesses could be moved in front of rcu_read_unlock and reordered > with this_cpu_inc(*p->counters), but it doesn't matter because > percpu_down_write does synchronize_rcu(), so it never sees these accesses > halfway through. Looking at the patch, you are correct. The read side doesn't need the memory barrier as the worse thing that will happen is that it sees the locked = false, and will just grab the mutex unnecessarily. > > > > I suggest any new synchronization should stay in -tip for 2 or more cycles > > before merged to mainline. > > But the bug that this synchronization is fixing is quite serious (it > causes random crashes when block size is being changed, the crash happens > regularly at multiple important business sites) so it must be fixed soon > and not wait half a year. I don't think Lai was suggesting to wait on this fix, but instead to totally rip out the percpu_rwsems and work on them some more, and then re-introduce them in a half a year. -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
