Messages in this thread | | | Date | Fri, 2 May 2008 19:30:06 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 1/10] Add generic helpers for arch IPI function calls |
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On Fri, May 02, 2008 at 07:21:39AM -0700, Paul E. McKenney wrote: > On Fri, May 02, 2008 at 02:59:29PM +0200, Peter Zijlstra wrote: > > On Fri, 2008-05-02 at 05:42 -0700, Paul E. McKenney wrote: > > > > > And here is one scenario that makes me doubt that my imagination is > > > faulty: > > > > > > 1. CPU 0 disables irqs. > > > > > > 2. CPU 1 disables irqs. > > > > > > 3. CPU 0 invokes smp_call_function(). But CPU 1 will never respond > > > because its irqs are disabled. > > > > > > 4. CPU 1 invokes smp_call_function(). But CPU 0 will never respond > > > because its irqs are disabled. > > > > > > Looks like inherent deadlock to me, requiring that smp_call_function() > > > be invoked with irqs enabled. > > > > > > So, what am I missing here? > > > > The wish to do it anyway ;-) > > > > I can imagine some situations where I'd like to try anyway and fall back > > to a slower path when failing. > > > > With the initial design we would simply allocate data, stick it on the > > queue and call the ipi (when needed). > > > > This is perfectly deadlock free when wait=0 and it just returns -ENOMEM > > on allocation failure. > > > > It it doesn't return -ENOMEM I know its been queued and will be > > processed at some point, if it does fail, I can deal with it in another > > way. > > > > I know I'd like to do that and I suspect Nick has a few use cases up his > > sleeve as well. > > OK, so one approach would be to check for irqs being disabled, > perhaps as follows, on top of my previous patch: > > struct call_single_data *data = NULL; > > if (!wait) { > data = kmalloc(sizeof(*data), GFP_ATOMIC); > if (data) > data->flags = CSD_FLAG_ALLOC; > } > if (!data) { > if (unlikely(irqs_disabled())) { > put_cpu(); > return -ENOMEM; > } > data = &d; > data->flags = CSD_FLAG_WAIT; > } > > data->func = func; > data->info = info; > generic_exec_single(cpu, data); > > That would prevent -ENOMEM unless you invoked the function with irqs > disabled. So normal callers would still see the current failure-free > semantics -- you really don't want to be inflicting failure when not > necessary, right? > > There could only be one irq-disabled caller at a time, which could be > handled using a trylock, returning -EBUSY if the lock is already held. > Otherwise, you end up with the scenario called out above (which Keith > Ownens pointed out some years ago). > > Does this approach make sense?
Actually, no... The irq-disabled callers would need to acquire the spinlock -before- disabling irqs, otherwise we end up right back in the deadlock scenario.
Thanx, Paul
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