Messages in this thread | | | Date | Wed, 21 Aug 2013 06:42:04 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 5/8] rcu: eliminate deadlock for rcu read site |
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On Wed, Aug 21, 2013 at 11:25:55AM +0800, Lai Jiangshan wrote: > On 08/21/2013 11:17 AM, Paul E. McKenney wrote: > > On Sat, Aug 10, 2013 at 08:07:15AM -0700, Paul E. McKenney wrote: > >> On Sat, Aug 10, 2013 at 11:43:59AM +0800, Lai Jiangshan wrote: > > > > [ . . . ] > > > >>> So I have to narrow the range of suspect locks. Two choices: > >>> A) don't call rt_mutex_unlock() from rcu_read_unlock(), only call it > >>> from rcu_preempt_not_context_switch(). we need to rework these > >>> two functions and it will add complexity to RCU, and it also still > >>> adds some probability of deferring. > >> > >> One advantage of bh-disable locks is that enabling bh checks > >> TIF_NEED_RESCHED, so that there is no deferring beyond that > >> needed by bh disable. The same of course applies to preempt_disable(). > >> > >> So one approach is to defer when rcu_read_unlock_special() is entered > >> with either preemption or bh disabled. Your current set_need_resched() > >> trick would work fine in this case. Unfortunately, re-enabling interrupts > >> does -not- check TIF_NEED_RESCHED, which is why we have latency problems > >> in that case. (Hence my earlier question about making self-IPI safe > >> on all arches, which would result in an interrupt as soon as interrupts > >> were re-enabled.) > >> > >> Another possibility is to defer only when preemption or bh are disabled > >> on entry ro rcu_read_unlock_special(), but to retain the current > >> (admittedly ugly) nesting rules for the scheduler locks. > > > > Would you be willing to do a patch that deferred rt_mutex_unlock() in > > the preempt/bh cases? This of course does not solve the irq-disable > > case, but it should at least narrow the problem to the scheduler locks. > > > > Not a big hurry, given the testing required, this is 3.13 or 3.14 material, > > I think. > > > > If you are busy, no problem, I can do it, just figured you have priority > > if you want it. > > I'm writing a special rt_mutex_unlock() for rcu deboost only. > I hope Steven accept it.
That would be very cool, though if I understand the requirements, especially for -rt, very challenging. Looking forward to seeing it!
Thanx, Paul
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