Messages in this thread | | | Date | Sun, 23 Dec 2012 01:47:37 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context |
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On 12/20/2012 07:12 PM, Oleg Nesterov wrote: > 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). >
Hey, IIUC, we actually don't need mb() in the reader!! Just an rmb() will do.
This is the reader code I have so far:
#define reader_nested_percpu() \ (__this_cpu_read(reader_percpu_refcnt) & READER_REFCNT_MASK)
#define writer_active() \ (__this_cpu_read(writer_signal))
#define READER_PRESENT (1UL << 16) #define READER_REFCNT_MASK (READER_PRESENT - 1)
void get_online_cpus_atomic(void) { preempt_disable();
/* * First and foremost, make your presence known to the writer. */ this_cpu_add(reader_percpu_refcnt, READER_PRESENT);
/* * If we are already using per-cpu refcounts, it is not safe to switch * the synchronization scheme. So continue using the refcounts. */ if (reader_nested_percpu()) { this_cpu_inc(reader_percpu_refcnt); } else { smp_rmb(); if (unlikely(writer_active())) { ... //take hotplug_rwlock } }
...
/* Prevent reordering of any subsequent reads of cpu_online_mask. */ smp_rmb(); }
The smp_rmb() before writer_active() ensures that LOAD(writer_signal) follows LOAD(reader_percpu_refcnt) (at the 'if' condition). And in turn, that load is automatically going to follow the STORE(reader_percpu_refcnt) (at this_cpu_add()) due to the data dependency. So it is something like a transitive relation.
So, the result is that, we mark ourselves as active in reader_percpu_refcnt before we check writer_signal. This is exactly what we wanted to do right? And luckily, due to the dependency, we can achieve it without using the heavy smp_mb(). And, we can't crib about the smp_rmb() because it is unavoidable anyway (because we want to prevent reordering of the reads to cpu_online_mask, like you pointed out earlier).
I hope I'm not missing anything...
Regards, Srivatsa S. Bhat
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