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

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Steven Rostedt (rostedt@goodmis.org) wrote:
> On Sat, 2010-01-09 at 21:25 -0800, Paul E. McKenney wrote:
> > On Sat, Jan 09, 2010 at 09:12:58PM -0500, Steven Rostedt wrote:
> 
> > > >         < 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.
> 
> 
> Then perhaps the sys_membarrier() should just do a flush_tlb()? That
> should guarantee the synchronization, right?
> 

The way I see it, TLB can be seen as read-only elements (a local
read-only cache) on the processors. Therefore, we don't care if they are
in a stale state while performing the cpumask update, because the fact
that we are executing switch_mm() means that these TLB entries are not
being used locally anyway and will be dropped shortly. So we have the
equivalent of a full memory barrier (load_cr3()) _after_ the cpumask
updates.

However, in sys_membarrier(), we also need to flush the write buffers
present on each processor running threads which belong to our current
process. Therefore, we would need, in addition, a smp_mb() before the
mm cpumask modification. For x86, cpumask_clear_cpu/cpumask_set_cpu
implies a LOCK-prefixed operation, and hence does not need any added
barrier, but this could be different for other architectures.

So, AFAIK, doing a flush_tlb() would not guarantee the kind of
synchronization we are looking for because an uncommitted write buffer
could still sit on the remote CPU when we return from sys_membarrier().

Thanks,

Mathieu


-- 
Mathieu Desnoyers
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