Messages in this thread | | | Date | Mon, 17 Mar 2014 12:18:07 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] [RFC] perf: Fix a race between ring_buffer_detach() and ring_buffer_wakeup() |
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> rcu: Provide grace-period piggybacking API > > The following pattern is currently not well supported by RCU: > > 1. Make data element inaccessible to RCU readers. > > 2. Do work that probably lasts for more than one grace period. > > 3. Do something to make sure RCU readers in flight before #1 above > have completed. > > Here are some things that could currently be done: > > a. Do a synchronize_rcu() unconditionally at either #1 or #3 above. > This works, but imposes needless work and latency. > > b. Post an RCU callback at #1 above that does a wakeup, then > wait for the wakeup at #3. This works well, but likely results > in an extra unneeded grace period. Open-coding this is also > a bit more semi-tricky code than would be good. > > This commit therefore adds get_state_synchronize_rcu() and > cond_synchronize_rcu() APIs. Call get_state_synchronize_rcu() at #1 > above and pass its return value to cond_synchronize_rcu() at #3 above. > This results in a call to synchronize_rcu() if no grace period has > elapsed between #1 and #3, but requires only a load, comparison, and > memory barrier if a full grace period did elapse. > > Reported-by: Peter Zijlstra <peterz@infradead.org>
More a requested-by, I'd say.
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > +/** > + * get_state_synchronize_rcu - Snapshot current RCU state > + * > + * Returns a cookie that is used by a later call to cond_synchronize_rcu() > + * to determine whether or not a full grace period has elapsed in the > + * meantime. > + */ > +unsigned long get_state_synchronize_rcu(void) > +{
/* * Make sure this load happens before anything following it; such that * ... ? */
The way I imaged using this is taking this snapshot after the RCU operation, such that we err towards seeing a later grace period and synchronizing too much in stead of seeing an earlier and sync'ing too little.
Such use would suggest the barrier the other way around.
> + return smp_load_acquire(&rcu_state->gpnum); > +} > +EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
I can't say I'm excited about that function name; but I'm also completely lacking in alternatives.
> + > +/** > + * cond_synchronize_rcu - Conditionally wait for an RCU grace period > + * > + * @oldstate: return value from earlier call to get_state_synchronize_rcu() > + * > + * If a full RCU grace period has elapsed since the earlier call to > + * get_state_synchronize_rcu(), just return. Otherwise, invoke > + * synchronize_rcu() to wait for a full grace period. > + */ > +void cond_synchronize_rcu(unsigned long oldstate) > +{ > + unsigned long newstate = smp_load_acquire(&rcu_state->completed);
Again, uncommented barriers; the load_acquire seems to suggest you want to make sure the sync_rcu() bits happen after this read. But seeing how sync_rcu() will invariably read the same data again and you get an address dep this seems somewhat superfluous.
Then again, can't hurt I suppose :-)
> + if (ULONG_CMP_GE(oldstate, newstate))
So if the observed active gp (oldstate) is ahead or equal to the last completed gp, then we wait?
I thought the double grace period thing was for preemptible rcu; is it done here because you don't want to special case the preemptible rcu and make do with a single implementation?
And I suppose that on wrap around; we do extra sync_rcu() calls, which can never be wrong.
> + synchronize_rcu(); > +} > +EXPORT_SYMBOL_GPL(cond_synchronize_rcu);
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