On Tue, Nov 21, 2006 at 03:26:44PM -0500, Alan Stern wrote:
> On Tue, 21 Nov 2006, Paul E. McKenney wrote:
>
> > On Tue, Nov 21, 2006 at 07:44:20PM +0300, Oleg Nesterov wrote:
> > > On 11/20, Paul E. McKenney wrote:
> > > >
> > > > On Mon, Nov 20, 2006 at 09:57:12PM +0300, Oleg Nesterov wrote:
> > > > > >
> > > > > So, if we have global A == B == 0,
> > > > >
> > > > > CPU_0 CPU_1
> > > > >
> > > > > A = 1; B = 2;
> > > > > mb(); mb();
> > > > > b = B; a = A;
> > > > >
> > > > > It could happen that a == b == 0, yes? Isn't this contradicts with definition
> > > > > of mb?
> > > >
> > > > It can and does happen. -Which- definition of mb()? ;-)
> > >
> > > I had a somewhat similar understanding before this discussion
> > >
> > > [PATCH] Fix RCU race in access of nohz_cpu_mask
> > > http://marc.theaimsgroup.com/?t=113378060600003
> > >
> > > Semantics of smp_mb() [was : Re: [PATCH] Fix RCU race in access of nohz_cpu_mask ]
> > > http://marc.theaimsgroup.com/?t=113432312600001
> > >
> > > Could you please explain me again why that fix was correct? What we have now is:
> > >
> > > CPU_0 CPU_1
> > > rcu_start_batch: stop_hz_timer:
> > >
> > > rcp->cur++; STORE nohz_cpu_mask |= cpu
> > >
> > > smp_mb(); mb(); // missed actually
> > >
> > > ->cpumask = ~nohz_cpu_mask; LOAD if (rcu_pending()) // reads rcp->cur
> > > nohz_cpu_mask &= ~cpu
> > >
> > > So, it is possible that CPU_0 reads an empty nohz_cpu_mask and starts a grace
> > > period with CPU_1 included in rcp->cpumask. CPU_1 in turn reads an old value
> > > of rcp->cur (so rcu_pending() returns 0) and becomes CPU_IDLE.
> >
> > At this point, I am not certain that it is in fact correct. :-/
> >
> > > Take another patch,
> > >
> > > Re: Oops on 2.6.18
> > > http://marc.theaimsgroup.com/?l=linux-kernel&m=116266392016286
> > >
> > > switch_uid: __sigqueue_alloc:
> > >
> > > STORE 'new_user' to ->user STORE "locked" to ->siglock
> > >
> > > mb(); "mb()"; // sort of, wrt loads/stores above
> > >
> > > LOAD ->siglock LOAD ->siglock
> > >
> > > Agian, it is possible that switch_uid() doesn't notice that ->siglock is locked
> > > and frees ->user. __sigqueue_alloc() in turn reads an old (freed) value of ->user
> > > and does get_uid() on it.
> >
> > Ditto.
>
> > > Paul, Alan, in case it was not clear: I am not arguing, just trying to
> > > understand, and I appreciate very much your time and your explanations.
> >
> > Either way, we clearly need better definitions of what the memory barriers
> > actually do! And I expect that we will need your help.
>
> Things may not be quite as bad as they appear. On many architectures the
> store-mb-load pattern will work as expected. (In fact, I don't know which
> architectures it might fail on.)
Several weak-memory-ordering CPUs. :-/
> Furthermore this is a very difficult race to trigger. You couldn't force
> it to happen, for example, by adding a delay somewhere.
I have only seen it when explicitly forcing it, and even then it is not
easy to make happen. But how would you know whether or not it happened
in a kernel or large multithreaded application?
I am gaining increasing sympathy with anyone who might wish to reduce
the number of non-MMIO-related memory barriers in the Linux kernel!
Thanx, Paul
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