Messages in this thread | | | Date | Wed, 18 Feb 2009 11:47:27 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 2/3] generic-ipi: remove kmalloc() |
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On Wed, Feb 18, 2009 at 05:15:15PM +0100, Oleg Nesterov wrote: > On 02/17, Paul E. McKenney wrote: > > > > On Tue, Feb 17, 2009 at 10:59:06PM +0100, Peter Zijlstra wrote: > > > +static void csd_lock(struct call_single_data *data) > > > { > > > - /* Wait for response */ > > > - do { > > > - if (!(data->flags & CSD_FLAG_WAIT)) > > > - break; > > > + while (data->flags & CSD_FLAG_LOCK) > > > cpu_relax(); > > > - } while (1); > > > + data->flags = CSD_FLAG_LOCK; > > > > We do need an smp_mb() here, otherwise, the call from > > smp_call_function_single() could be reordered by either CPU or compiler > > as follows: > > > > data->func = func; > > data->info = info; > > csd_lock(data); > > > > This might come as a bit of a surprise to the other CPU still trying to > > use the old values for data->func and data->info. > > Could you explain a bit more here? > > The compiler can't re-order this code due to cpu_relax(). Cpu can > re-order, but this doesn't matter because both the sender and ipi > handler take call_single_queue->lock. > > And, giwen that csd_unlock() does mb() before csd_unlock(), how > it is possible that other CPU (ipi handler) still uses the old > values in *data after we see !CSD_FLAG_LOCK ?
Good point on cpu_relax(), which appears to be at least a compiler barrier on all architectures.
I must confess to being in the habit of assuming reordering unless I can prove that such reordering cannot happen. I am running tests of this code snippet on POWER, but do not have access to ARM, which some say has more tendency to ignore control-flow dependencies than does Power.
I will let people know what comes of the tests.
> > Note that this smb_mb() is required even if cpu_relax() contains a > > memory barrier, as it is possible to execute csd_lock_wait() without > > executing the cpu_relax(), if you get there at just the right time. > > Can't understand... Nobody can do csd_wait() on this per-cpu entry, > but I guess you meant something else. > > > OK... What prevents the following sequence of events? > > > > o CPU 0 calls smp_call_function_many(), targeting numerous CPUs, > > including CPU 2. CPU 0 therefore enqueues this on the global > > call_function.queue. "wait" is not specified, so CPU 0 returns > > immediately after sending the IPIs. > > > > It decrements the ->refs field, but, finding the result > > non-zero, refrains from removing the element that CPU 0 > > enqueued, and also refrains from invoking csd_unlock(). > > > > o CPU 3 also receives the IPI, and also calls the needed function. > > Now, only CPU 1 need receive the IPI for the element to be > > removed. > > so we have a single entry E0 on list, > > > o CPU 3 calls smp_call_function_many(), targeting numerous CPUs, > > but -not- including CPU 2. CPU 3 therefore also this on the > > global call_function.queue and sends the IPIs, but no IPI for > > CPU 2. Your choice as to whether CPU 3 waits or not. > > now we have E3 -> E0 > > > o CPU 2 receives CPU 3's IPI, but CPU 0's element is first on the > > list. CPU 2 fetches the pointer (via list_for_each_entry_rcu()), > > and then... > > it actually sees E3, not E0 > > > o CPU 1 finally re-enables irqs and receives the IPIs!!! It > > finds CPU 0's element on the queue, calls the function, > > decrements the ->refs field, and finds that it is zero. > > So, CPU 1 invokes list_del_rcu() to remove the element > > (OK so far, as list_del_rcu() doesn't overwrite the next > > pointer), then invokes csd_unlock() to release the element. > > > > o CPU 0 then invokes another smp_call_function_many(), also > > multiple CPUs, but -not- to CPU 2. It requeues the element > > that was just csd_unlock()ed above, carrying CPU 2 with it. > > It IPIs CPUs 1 and 3, but not CPU 2. > > and inserts the element E0 at the head of the list again, > > > > > o CPU 2 continues, and falls off the bottom of the list. > > afaics, it doesn't. > > Every time smp_call_function_many() reuses the element, it sets its > ->next pointer to the head of the list. If we race with another CPU > which fetches this pointer, this CPU has to re-scan the whole list, > but since we always modify/read data under data->lock this should > be safe, that CPU must notice (!cpumask_test_cpu(cpu, data->cpumask). > > No?
You are quite correct. I guess I should have gone home early instead of reviewing Peter's patch... :-/
Thanx, Paul
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