Re: sem_lock() vs qspinlocks

[Boqun Feng <boqun.feng@gmail.com>]

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Hi Peter,

On Fri, May 20, 2016 at 01:58:19PM +0200, Peter Zijlstra wrote:
> On Thu, May 19, 2016 at 10:39:26PM -0700, Davidlohr Bueso wrote:
> > As such, the following restores the behavior of the ticket locks and 'fixes'
> > (or hides?) the bug in sems. Naturally incorrect approach:
> > 
> > @@ -290,7 +290,8 @@ static void sem_wait_array(struct sem_array *sma)
> > 
> > 	for (i = 0; i < sma->sem_nsems; i++) {
> > 		sem = sma->sem_base + i;
> > -               spin_unlock_wait(&sem->lock);
> > +               while (atomic_read(&sem->lock))
> > +                       cpu_relax();
> > 	}
> > 	ipc_smp_acquire__after_spin_is_unlocked();
> > }
> 
> The actual bug is clear_pending_set_locked() not having acquire
> semantics. And the above 'fixes' things because it will observe the old
> pending bit or the locked bit, so it doesn't matter if the store
> flipping them is delayed.
> 
> The comment in queued_spin_lock_slowpath() above the smp_cond_acquire()
> states that that acquire is sufficient, but this is incorrect in the
> face of spin_is_locked()/spin_unlock_wait() usage only looking at the
> lock byte.
> 
> The problem is that the clear_pending_set_locked() is an unordered
> store, therefore this store can be delayed until no later than
> spin_unlock() (which orders against it due to the address dependency).
> 
> This opens numerous races; for example:
> 
> 	ipc_lock_object(&sma->sem_perm);
> 	sem_wait_array(sma);
> 
> 				false   ->	spin_is_locked(&sma->sem_perm.lock)
> 
> is entirely possible, because sem_wait_array() consists of pure reads,
> so the store can pass all that, even on x86.
> 
> The below 'hack' seems to solve the problem.
> 
> _However_ this also means the atomic_cmpxchg_relaxed() in the locked:
> branch is equally wrong -- although not visible on x86. And note that
> atomic_cmpxchg_acquire() would not in fact be sufficient either, since
> the acquire is on the LOAD not the STORE of the LL/SC.
> 
> I need a break of sorts, because after twisting my head around the sem
> code and then the qspinlock code I'm wrecked. I'll try and make a proper
> patch if people can indeed confirm my thinking here.
> 

I think your analysis is right, however, the problem only exists if we
have the following use pattern, right?

	CPU 0			CPU 1
	====================	==================
	spin_lock(A);		spin_lock(B);
	spin_unlock_wait(B);	spin_unlock_wait(A);
	do_something();		do_something();

, which ends up CPU 0 and 1 both running do_something(). And actually
this can be simply fixed by add smp_mb() between spin_lock() and
spin_unlock_wait() on both CPU, or add an smp_mb() in spin_unlock_wait()
as PPC does in 51d7d5205d338 "powerpc: Add smp_mb() to arch_spin_is_locked()".

So if relaxed/acquire atomics and clear_pending_set_locked() work fine
in other situations, a proper fix would be fixing the
spin_is_locked()/spin_unlock_wait() or their users?

Regards,
Boqun

> ---
>  kernel/locking/qspinlock.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index ce2f75e32ae1..348e172e774f 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -366,6 +366,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>  	 * *,1,0 -> *,0,1
>  	 */
>  	clear_pending_set_locked(lock);
> +	smp_mb();
>  	return;
>  
>  	/*

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