Messages in this thread | | | Date | Wed, 18 Feb 2009 18:40:30 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 2/3] generic-ipi: remove kmalloc() |
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On Wed, Feb 18, 2009 at 09:12:52PM +0100, Oleg Nesterov wrote: > On 02/18, Paul E. McKenney wrote: > > > > 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. > > Yes, probably you are right... > > But since almost nobody (except you ;) really understands this magic, > it would be nice to have a comment which explains exactly what is the > reason for mb(). Otherwise it is so hard to read the code, if you > don't understand mb(), then you probably missed something important.
An hour-long stress test on both Power 5 and Power 6 failed to locate a problem, though that of course does not prove lack of a problem, particularly for CPUs that don't pay as much attention to control-dependency ordering as Power does. :-/
The test may be found in CodeSamples/advsync/special/mbtest/mb_lhs_ws.c in git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git, if anyone wants to take a look.
In the meantime, I nominate the following comment at the end of csd_lock():
/* * prevent CPU from reordering the above assignment to ->flags * with any subsequent assignments to other fields of the * specified call_single_data structure. */
smp_mb(); /* See above block comment. */
> > > 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). > > > > You are quite correct. I guess I should have gone home early instead of > > reviewing Peter's patch... :-/ > > In that case I shouldn't even try to read this series ;) I was wrong so > many times...
I suppose that I should be happy to be wrong nine times if I find a subtle bug on the tenth attempt, but somehow it doesn't feel that way. ;-)
Thanx, Paul
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