Messages in this thread | | | Date | Mon, 28 Nov 2011 09:15:13 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu 24/28] rcu: Introduce bulk reference count |
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On Mon, Nov 28, 2011 at 01:41:11PM +0100, Peter Zijlstra wrote: > On Wed, 2011-11-02 at 13:30 -0700, Paul E. McKenney wrote: > > The RCU implementations, including SRCU, are designed to be used in a > > lock-like fashion, so that the read-side lock and unlock primitives must > > execute in the same context for any given read-side critical section. > > This constraint is enforced by lockdep-RCU. However, there is a need for > > something that acts more like a reference count than a lock, in order > > to allow (for example) the reference to be acquired within the context > > of an exception, while that same reference is released in the context of > > the task that encountered the exception. The cost of this capability is > > that the read-side operations incur the overhead of disabling interrupts. > > Some optimization is possible, and will be carried out if warranted. > > > > Note that although the current implementation allows a given reference to > > be acquired by one task and then released by another, all known possible > > implementations that allow this have scalability problems. Therefore, > > a given reference must be released by the same task that acquired it, > > though perhaps from an interrupt or exception handler running within > > that task's context. > > I'm having trouble with the naming as well as the need for an explicit > new API. > > To me this looks like a regular (S)RCU variant, nothing to do with > references per-se (aside from the fact that SRCU is a refcounted rcu > variant). Also WTF is this bulk stuff about? Its still a single ref at a > time, not 10s or 100s or whatnot.
It is a bulk reference in comparison to a conventional atomic_inc()-style reference count, which is normally associated with a specific structure. In contrast, doing a bulkref_get() normally protects a group of structures, everything covered by the bulkref_t.
Yes, in theory you could have a global reference counter that protected a group of structures, but in practice we both know that this would not end well. ;-)
> > +static inline int bulkref_get(bulkref_t *brp) > > +{ > > + unsigned long flags; > > + int ret; > > + > > + local_irq_save(flags); > > + ret = __srcu_read_lock(brp); > > + local_irq_restore(flags); > > + return ret; > > +} > > + > > +static inline void bulkref_put(bulkref_t *brp, int idx) > > +{ > > + unsigned long flags; > > + > > + local_irq_save(flags); > > + __srcu_read_unlock(brp, idx); > > + local_irq_restore(flags); > > +} > > This seems to be the main gist of the patch, which to me sounds utterly > ridiculous. Why not document that srcu_read_{un,}lock() aren't IRQ safe > and if you want to use it from those contexts you have to fix it up > yourself.
I thought I had documented this, but I guess not. I will add that.
I lost you on the "fix it up yourself" -- what are you suggesting that someone needing to use RCU in this manner actually do?
> RCU lockdep doesn't do the full validation so it won't actually catch it > if you mess up the irq states, but I guess if you want we could look at > adding that.
Ah, I had missed that. Yes, it would be very good if that could be added. The vast majority of the uses exit the RCU read-side critical section in the same context that they enter it, so it would be good to check.
> > diff --git a/kernel/srcu.c b/kernel/srcu.c > > index 73ce23f..10214c8 100644 > > --- a/kernel/srcu.c > > +++ b/kernel/srcu.c > > @@ -34,13 +34,14 @@ > > #include <linux/delay.h> > > #include <linux/srcu.h> > > > > -static int init_srcu_struct_fields(struct srcu_struct *sp) > > +int init_srcu_struct_fields(struct srcu_struct *sp) > > { > > sp->completed = 0; > > mutex_init(&sp->mutex); > > sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array); > > return sp->per_cpu_ref ? 0 : -ENOMEM; > > } > > +EXPORT_SYMBOL_GPL(init_srcu_struct_fields); > > What do we need this export for? Usually we don't add exports unless > there's a use-case. Since Srikar requested this nonsense, I guess the > user is uprobes, but that isn't a module, so no export needed.
Yep, the user is uprobes. The export is for rcutorture, which can run as a module.
Thanx, Paul
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