Messages in this thread | | | Date | Thu, 7 Jan 2010 12:44:35 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier |
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* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > On Thu, Jan 07, 2010 at 06:18:36PM +0100, Peter Zijlstra wrote: > > On Thu, 2010-01-07 at 08:52 -0800, Paul E. McKenney wrote: > > > On Thu, Jan 07, 2010 at 09:44:15AM +0100, Peter Zijlstra wrote: > > > > On Wed, 2010-01-06 at 22:35 -0800, Josh Triplett wrote: > > > > > > > > > > The number of threads doesn't matter nearly as much as the number of > > > > > threads typically running at a time compared to the number of > > > > > processors. Of course, we can't measure that as easily, but I don't > > > > > know that your proposed heuristic would approximate it well. > > > > > > > > Quite agreed, and not disturbing RT tasks is even more important. > > > > > > OK, so I stand un-Reviewed-by twice in one morning. ;-) > > > > > > > A simple: > > > > > > > > for_each_cpu(cpu, current->mm->cpu_vm_mask) { > > > > if (cpu_curr(cpu)->mm == current->mm) > > > > smp_call_function_single(cpu, func, NULL, 1); > > > > } > > > > > > > > seems far preferable over anything else, if you really want you can use > > > > a cpumask to copy cpu_vm_mask in and unset bits and use the mask with > > > > smp_call_function_any(), but that includes having to allocate the > > > > cpumask, which might or might not be too expensive for Mathieu. > > > > > > This would be vulnerable to the sys_membarrier() CPU seeing an old value > > > of cpu_curr(cpu)->mm, and that other task seeing the old value of the > > > pointer we are trying to RCU-destroy, right? > > > > Right, so I was thinking that since you want a mb to be executed when > > calling sys_membarrier(). If you observe a matching ->mm but the cpu has > > since scheduled, we're good since it scheduled (but we'll still send the > > IPI anyway), if we do not observe it because the task gets scheduled in > > after we do the iteration we're still good because it scheduled. > > Something like the following for sys_membarrier(), then? > > smp_mb();
This smp_mb() is redundant, as we issue it through the for_each_cpu loop on the local CPU already.
> for_each_cpu(cpu, current->mm->cpu_vm_mask) { > if (cpu_curr(cpu)->mm == current->mm) > smp_call_function_single(cpu, func, NULL, 1); > } > > Then the code changing ->mm on the other CPU also needs to have a > full smp_mb() somewhere after the change to ->mm, but before starting > user-space execution. Which it might well just due to overhead, but > we need to make sure that someone doesn't optimize us out of existence.
I believe we also need one between execution of the userspace task and change to ->mm. If we have these guarantees I think we are fine.
Mathieu
> > Thanx, Paul > > > As to needing to keep rcu_read_lock() around the iteration, for sure we > > need that to ensure the remote task_struct reference we take is valid. > >
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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