Messages in this thread | | | Date | Mon, 16 Aug 2010 14:55:55 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 08/10] rcu: Add a TINY_PREEMPT_RCU |
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On Mon, Aug 16, 2010 at 05:41:23PM -0400, Mathieu Desnoyers wrote: > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > On Mon, Aug 16, 2010 at 03:19:47PM -0400, Mathieu Desnoyers wrote: > > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > > > On Mon, Aug 16, 2010 at 11:07:37AM -0400, Mathieu Desnoyers wrote: > > > > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > > > > [...] > > > > > > + > > > > > > +/* > > > > > > + * Tiny-preemptible RCU implementation for rcu_read_unlock(). > > > > > > + * Decrement ->rcu_read_lock_nesting. If the result is zero (outermost > > > > > > + * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then > > > > > > + * invoke rcu_read_unlock_special() to clean up after a context switch > > > > > > + * in an RCU read-side critical section and other special cases. > > > > > > + */ > > > > > > +void __rcu_read_unlock(void) > > > > > > +{ > > > > > > + struct task_struct *t = current; > > > > > > + > > > > > > + barrier(); /* needed if we ever invoke rcu_read_unlock in rcutiny.c */ > > > > > > + if (--t->rcu_read_lock_nesting == 0 && > > > > > > + unlikely(t->rcu_read_unlock_special)) > > > > > > > > First, thank you for looking this over!!! > > > > > > > > > Hrm I think we discussed this in a past life, but would the following > > > > > sequence be possible and correct ? > > > > > > > > > > CPU 0 > > > > > > > > > > read t->rcu_read_unlock_special > > > > > interrupt comes in, preempts. sets t->rcu_read_unlock_special > > > > > <preempted> > > > > > <scheduled back> > > > > > iret > > > > > decrement and read t->rcu_read_lock_nesting > > > > > test both old "special" value (which we have locally on the stack) and > > > > > detect that rcu_read_lock_nesting is 0. > > > > > > > > > > We actually missed a reschedule. > > > > > > > > > > I think we might need a barrier() between the t->rcu_read_lock_nesting > > > > > and t->rcu_read_unlock_special reads. > > > > > > > > You are correct -- I got too aggressive in eliminating synchronization. > > > > > > > > Good catch!!! > > > > > > > > I added an ACCESS_ONCE() to the second term of the "if" condition so > > > > that it now reads: > > > > > > > > if (--t->rcu_read_lock_nesting == 0 && > > > > unlikely((ACCESS_ONCE(t->rcu_read_unlock_special))) > > > > > > > > This prevents the compiler from reordering because the ACCESS_ONCE() > > > > prohibits accessing t->rcu_read_unlock_special unless the value of > > > > t->rcu_read_lock_nesting is known to be zero. > > > > > > Hrm, --t->rcu_read_lock_nesting does not have any globally visible > > > side-effect, so the compiler is free to reorder the memory access across > > > the rcu_read_unlock_special access. I think we need the ACCESS_ONCE() > > > around the t->rcu_read_lock_nesting access too. > > > > Indeed, it is free to reorder that access. This has the effect of > > extending the scope of the RCU read-side critical section, which is > > harmless as long as it doesn't pull a lock or some such into it. > > > > So what happens if we get: > > CPU 0 > > read t->rcu_read_lock_nesting > check if equals to 1 > read t->rcu_read_unlock_special > interrupt comes in, preempts. sets t->rcu_read_unlock_special > <preempted> > <scheduled back> > iret > decrement t->rcu_read_lock_nesting
Moving this down past the check of t->rcu_read_lock_special (which is now covered by ACCESS_ONCE()) would violate the C standard, as it would be equivalent to moving a volatile up past a sequence point.
Thanx, Paul
> test rcu_read_unlock_special value (read prior to interrupt) > -> fails to notice the preemption that came in after the > rcu_read_unlock_special read. > > Thanks, > > Mathieu > > -- > Mathieu Desnoyers > Operating System Efficiency R&D Consultant > EfficiOS Inc. > http://www.efficios.com
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