Messages in this thread | | | Date | Sun, 23 Dec 2012 17:42:42 +0100 | From | Oleg Nesterov <> | Subject | Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context |
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On 12/23, Srivatsa S. Bhat wrote: > > On 12/20/2012 07:12 PM, Oleg Nesterov wrote: > > > > 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.
Well. I don't think so. But when it comes to the barriers I am never sure until Paul confirms my understanding ;)
> #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)
But why this STORE should be visible on another CPU before we LOAD(writer_signal)?
Lets discuss the simple and artificial example. Suppose we have
int X, Y;
int func(void) { X = 1; // suppose that nobody else can change it mb(); return Y; }
Now you are saying that we can change it and avoid the costly mb():
int func(void) { X = 1;
if (X != 1) BUG(); rmb(); return Y; }
I doubt. rmb() can only guarantee that the preceding LOAD's should be completed. Without mb() it is possible that this CPU won't write X to memory at all.
Oleg.
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