Messages in this thread | | | Date | Mon, 16 Aug 2010 11:33:55 -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 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.
> 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))) 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.
> 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?
> 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.
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/
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