Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier

Paul E. McKenney
2010-01-10T05:25:10Z

On Sat, Jan 09, 2010 at 09:12:58PM -0500, Steven Rostedt wrote:
> On Sat, 2010-01-09 at 20:44 -0500, Mathieu Desnoyers wrote:
>
> > > So what if we have a linear decrease in performance with the number of
> > > threads on the write side?
> >
> > Hrm, looking at arch/x86/include/asm/mmu_context.h
> >
> > switch_mm(), which is basically called each time the scheduler needs to
> > change the current task, does a
> >
> > cpumask_clear_cpu(cpu, mm_cpumask(prev));
> >
> > and
> >
> > cpumask_set_cpu(cpu, mm_cpumask(next));
> >
> > which precise goal is to stop the flush ipis for the previous mm. The
> > 100$ question is : why do we have to confirm that the thread is indeed
> > on the runqueue (taking locks and everything) when we could simply just
> > bluntly use the mm_cpumask for our own IPIs ?
>
> I was just looking at that code, and was thinking the same thing ;-)
>
> > cpumask_clear_cpu and cpumask_set_cpu translate into clear_bit/set_bit.
> > cpumask_next does a find_next_bit on the cpumask.
> >
> > clear_bit/set_bit are atomic and not reordered on x86. PowerPC also uses
> > ll/sc loops in bitops.h, so I think it should be pretty safe to assume
> > that mm_cpumask is, by design, made to be used as cpumask to send a
> > broadcast IPI to all CPUs which run threads belonging to a given
> > process.
> >
> > So, how about just using mm_cpumask(current) for the broadcast ? Then we
> > don't even need to allocate our own cpumask neither.
> >
> > Or am I missing something ? I just sounds too simple.
>
> I think we can use it. If for some reason it does not satisfy what you
> need then I also think the TLB flushing is also broken.
>
> IIRC, (Paul help me out on this), what Paul said earlier, we are trying
> to protect against this scenario:
>
> (from Paul's email:)
>
>
> >
> > CPU 1 CPU 2
> > ----------- -------------
> >
> > <user space> <kernel space, switching to task>
> >
> > ->curr updated
> >
> > <long code path, maybe mb?>
> >
> > <user space>
> >
> > rcu_read_lock(); [load only]
> >
> > obj = list->next
> >
> > list_del(obj)
> >
> > sys_membarrier();
> > < kernel space >
> >
> > if (task_rq(task)->curr != task)
> > < but load to obj reordered before store to ->curr >
> >
> > < user space >
> >
> > < misses that CPU 2 is in rcu section >
>
>
> If the TLB flush misses that CPU 2 has a threaded task, and does not
> flush CPU 2s TLB, it can also risk the same type of crash.

But isn't the VM's locking helping us out in that case?

> > [CPU 2's ->curr update now visible]
> >
> > [CPU 2's rcu_read_lock() store now visible]
> >
> > free(obj);
> >
> > use_object(obj); <=== crash!
> >
>
> Think about it. If you change a process mmap, say you updated a mmap of
> a file by flushing out one page and replacing it with another. If the
> above missed sending to CPU 2, then CPU 2 may still be accessing the old
> page of the file, and not the new one.
>
> I think this may be the safe bet.

You might well be correct that we can access that bitmap locklessly,
but there are additional things (like the loading of the arch-specific
page-table register) that are likely to be helping in the VM case, but
not necessarily helping in this case.

Thanx, Paul