Re: [tip:locking/urgent] locking/static_key: Fix concurrent static_key_slow_inc()

[Peter Zijlstra]

On Mon, Jul 31, 2017 at 03:04:06PM +0200, Paolo Bonzini wrote:
> - key->enabled cannot go from 0 to nonzero outside jump_label_mutex.
> For this reason the atomic_try_cmpxchg is unnecessary.

Agreed, the only reason was the implied barrier.

> - the (implied) smp_mb before jump_label_update is not interesting, but
> I don't think it is useful because: 1) during the jump_label_update
> there is no correspondence between what static_key_enabled returns and
> what the text look like; 2) what would it even be pairing with?

Ah, indeed. So I was worried about the text changes escaping upwards,
but you're right in that there's no harm in that because there's nothing
that cares.

Another inc would see 0 and still serialize on the mutex.

> - the smp_mb (though it could be a smp_wmb or atomic_set_release)
> initially triggered my paranoia indeed.  But even then, I can't see why
> you would need it because there's nothing it pairs with.

So this one would pair with the mb implied by the cmpxchg loop for
inc-if-positive. The ordering being that if we see a positive v, we must
then also see all the text modifications.

So if jump_label_update() were to not fully serialize things, it would
be possible for the v=1 store to appear before the last text changes.
And in that case we'd allow the fast path to complete
static_key_slow_inc() before it was in fact done with changing all text.

Now, I suspect (but did not audit) that anything that changes text must
in fact serialize world, but I wasn't sure.

> Rather, it's *any use of key->enabled outside jump_label_lock*
> (meaning: any use of static_key_enabled and static_key_count outside
> the core jump_label.c code) that should be handled with care.

> And indeed, while there aren't many, I think two of them are wrong (and
> not fixed by your patch):
> 
> - include/linux/cpuset.h defines nr_cpusets which uses static_key_count.
>  It makes no sense to call it outside cpuset_mutex, and indeed that's
> how kernel/cgroup/cpuset.c uses it (nr_cpusets <- generate_sched_domains
> <- rebuild_sched_domains_locked).  The function should be moved inside
> kernel/cgroup/cpuset.c since the mutex is static.

Dima was poking at that code.

> - net/ipv4/udp.c and net/ipv6/udp.c want to implement a "once-only"
> increment of the static key.  It's racy and maybe we should provide a
> new API static_key_enable_forever:
> 
> 	void static_key_enable_forever(struct static_key *key)
> 	{
> 	        STATIC_KEY_CHECK_USE();
> 	        if (atomic_read(&key->enabled) > 0)
> 	                return;
> 
> 	        cpus_read_lock();
> 	        jump_label_lock();
> 	        if (atomic_read(&key->enabled) == 0) {
> 	                atomic_set(&key->enabled, -1);
> 	                jump_label_update(key);
> 	                atomic_set(&key->enabled, 1);
> 	        }
> 	        jump_label_unlock();
> 	        cpus_read_unlock();
> 	}
> 	EXPORT_SYMBOL_GPL(static_key_enable_forever);
> 
> I can prepare a patch if you agree.

Isn't that what we have static_key_enable() for? Which btw also uses
static_key_count() outside of the mutex.