linux-kernel - Re: [tip:locking/urgent] locking/static_key: Fix concurrent static_key_slow

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Date:   Mon, 31 Jul 2017 17:35:40 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, mingo@...nel.org, dvyukov@...gle.com,
        hpa@...or.com, linux-tip-commits@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Dima Zavin <dmitriyz@...mo.com>
Subject: Re: [tip:locking/urgent] locking/static_key: Fix concurrent
 static_key_slow_inc()

On 31/07/2017 15:33, Peter Zijlstra wrote:
> On Mon, Jul 31, 2017 at 03:04:06PM +0200, Paolo Bonzini wrote:
>> - 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.

I see.  Then yes, I agree a smp_wmb would be nicer here.

>> 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.
> 
>> - 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.

Yes, they should be fixed and net/ can then use static_key_enable.

Paolo