Messages in this thread | | | Date | Mon, 16 Aug 2010 14:32:00 -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 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.
> > > 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)
Yep!!! ;-)
> 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.
I should probably review commenting globally, and this might be one place needing help.
> > 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.
Ditto! ;-)
> > > 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.
Long ago on the first RCU priority-boosting implementation I tried doing the rcu_read_unlock() by hand. The unhappy lessons learned caused me to just use rcu_read_unlock() when I encountered similar situations later on. ;-)
Thanx, Paul
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