Messages in this thread | | | Date | Mon, 16 Aug 2010 17:41:23 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH tip/core/rcu 08/10] rcu: Add a TINY_PREEMPT_RCU |
| |
* 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 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
| |