Hi Peter, (combining your replies)
On 06/11/17 10:32, Peter Zijlstra wrote: > On Fri, Nov 03, 2017 at 02:45:45PM +0000, James Morse wrote: >> I'm trying to work out what stops a thread being pre-empted and migrated >> between >> calling get_online_cpus() and put_online_cpus(). > Nothing; why would you think it would? To stop the this_cpu_*() operations in down/up being applied on different CPUs, affecting a different percpu:read_count. > All those functions guarantee is > that any CPU observed as being online says online (and its converse, > that a CPU observed as being offline, says offline, although less people > care about that one). >> According to __percpu_down_read(), its the pre-empt count: >>> * Due to having preemption disabled the decrement happens on >>> * the same CPU as the increment, avoiding the >>> * increment-on-one-CPU-and-decrement-on-another problem. >> >> >> So this: >>> void cpus_read_lock(void) >>> { >>> percpu_down_read(&cpu_hotplug_lock); >>> + >>> + /* Can we migrated before we release this per-cpu lock? */ >>> + WARN_ON(preemptible()); >>> } >> >> should never fire? > It should.. You're reading a comment on __percpu_down_read() and using > percpu_down_read(), _not_ the same function ;-) Yes, sorry, I thought you did a better job of describing the case I'm trying to work-out. > If you look at percpu_down_read(), you'll note it'll disable preemption > before calling __percpu_down_read(). Yes, this is how __percpu_down_read() protects the combination of it's fast/slow paths. But next percpu_down_read() calls preempt_enable(), I can't see what stops us migrating before percpu_up_read() preempt_disable()s to call __this_cpu_dec(), which now affects a different variable. > And yes, that whole percpu-rwsem code is fairly magical :-) I think I'll file this under magical. That rcu_sync_is_idle() must know something I don't! Thanks! James