Messages in this thread | | | Date | Mon, 16 Aug 2010 15:19:47 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH tip/core/rcu 08/10] rcu: Add a TINY_PREEMPT_RCU |
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* 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.
> > > We might need to audit > > TREE PREEMPT RCU for the same kind of behavior. > > The version of __rcu_read_unlock() in kernel/rcutree_plugin.h is as > follows: > > void __rcu_read_unlock(void) > { > struct task_struct *t = current; > > barrier(); /* needed if we ever invoke rcu_read_unlock in rcutree.c */ > if (--ACCESS_ONCE(t->rcu_read_lock_nesting) == 0 && > unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
This seem to work because we have:
volatile access (read/update t->rcu_read_lock_nesting) && (sequence point) volatile access (t->rcu_read_unlock_special)
The C standard seems to forbid reordering of volatile accesses across sequence points, so this should be fine. But it would probably be good to document this implied ordering explicitly.
> rcu_read_unlock_special(t); > #ifdef CONFIG_PROVE_LOCKING > WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0); > #endif /* #ifdef CONFIG_PROVE_LOCKING */ > } > > The ACCESS_ONCE() calls should cover this. I believe that the first > ACCESS_ONCE() is redundant, and have checking this more closely on my > todo list.
I doubt so, see explanation above.
> > > But I might be (again ?) missing something. I've got the feeling you > > already convinced me that this was OK for some reason, but I trip on > > this every time I read the code. > > > > [...] > > > > > +/* > > > + * Check for a task exiting while in a preemptible -RCU read-side > > > + * critical section, clean up if so. No need to issue warnings, > > > + * as debug_check_no_locks_held() already does this if lockdep > > > + * is enabled. > > > + */ > > > +void exit_rcu(void) > > > +{ > > > + struct task_struct *t = current; > > > + > > > + if (t->rcu_read_lock_nesting == 0) > > > + return; > > > + t->rcu_read_lock_nesting = 1; > > > + rcu_read_unlock(); > > > +} > > > + > > > > The interaction with preemption is unclear here. exit.c disables > > preemption around the call to exit_rcu(), but if, for some reason, > > rcu_read_unlock_special was set earlier by preemption, then the > > rcu_read_unlock() code might block and cause problems. > > But rcu_read_unlock_special() does not block. In fact, it disables > interrupts over almost all of its execution. Or am I missing some > subtlety here?
I am probably the one who was missing a subtlety about how rcu_read_unlock_special() works.
> > > Maybe we should consider clearing rcu_read_unlock_special here ? > > If the task blocked in an RCU read-side critical section just before > exit_rcu() was called, we need to remove the task from the ->blkd_tasks > list. If we fail to do so, we might get a segfault later on. Also, > we do need to handle any RCU_READ_UNLOCK_NEED_QS requests from the RCU > core. > > So I really do like the current approach of calling rcu_read_unlock() > to do this sort of cleanup.
It looks good then, I just wanted to ensure that the side-effects of calling rcu_read_unlock() in this code path were well-thought.
Thanks,
Mathieu
> > Thanx, Paul > > > Thanks, > > > > Mathieu > > > > -- > > Mathieu Desnoyers > > Operating System Efficiency R&D Consultant > > EfficiOS Inc. > > http://www.efficios.com > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > > the body of a message to majordomo@vger.kernel.org > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > Please read the FAQ at http://www.tux.org/lkml/
-- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com
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