Re: [PATCH -v4 5/7] locking, arch: Update spin_unlock_wait()

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

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On Mon, Jun 06, 2016 at 06:08:36PM +0200, Peter Zijlstra wrote:
> On Thu, Jun 02, 2016 at 06:57:00PM +0100, Will Deacon wrote:
> > > This 'replaces' commit:
> > > 
> > >   54cf809b9512 ("locking,qspinlock: Fix spin_is_locked() and spin_unlock_wait()")
> > > 
> > > and seems to still work with the test case from that thread while
> > > getting rid of the extra barriers.
> 
> New version :-)
> 
> This one has moar comments; and also tries to address an issue with
> xchg_tail(), which is its own consumer. Paul, Will said you'd love the
> address dependency through union members :-)
> 
> (I should split this in at least 3 patches I suppose)
> 
> ARM64 and PPC should provide private versions of is_locked and
> unlock_wait; because while this one deals with the unordered store as
> per qspinlock construction, it still relies on cmpxchg_acquire()'s store
> being visible.
> 

[snip]

> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index ce2f75e32ae1..e1c29d352e0e 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -395,6 +395,8 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>  	 * pending stuff.
>  	 *
>  	 * p,*,* -> n,*,*
> +	 *
> +	 * RELEASE, such that the stores to @node must be complete.
>  	 */
>  	old = xchg_tail(lock, tail);
>  	next = NULL;
> @@ -405,6 +407,15 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>  	 */
>  	if (old & _Q_TAIL_MASK) {
>  		prev = decode_tail(old);
> +		/*
> +		 * The above xchg_tail() is also load of @lock which generates,
> +		 * through decode_tail(), a pointer.
> +		 *
> +		 * The address dependency matches the RELEASE of xchg_tail()
> +		 * such that the access to @prev must happen after.
> +		 */
> +		smp_read_barrier_depends();

Should this barrier be put before decode_tail()? Because it's the
dependency old -> prev that we want to protect here.

Regards,
Boqun

> +
>  		WRITE_ONCE(prev->next, node);
>  
>  		pv_wait_node(node, prev);
[snip]

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