linux-kernel - Re: [PATCH -v2 4/6] locking, arch: Update spin_unlock

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Date:	Fri, 27 May 2016 08:46:49 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	manfred@...orfullife.com, dave@...olabs.net,
	paulmck@...ux.vnet.ibm.com, will.deacon@....com,
	boqun.feng@...il.com, Waiman.Long@....com, tj@...nel.org,
	pablo@...filter.org, kaber@...sh.net, davem@...emloft.net,
	oleg@...hat.com, netfilter-devel@...r.kernel.org,
	sasha.levin@...cle.com, hofrat@...dl.org, rth@...ddle.net,
	vgupta@...opsys.com, linux@...linux.org.uk, realmz6@...il.com,
	rkuo@...eaurora.org, tony.luck@...el.com, james.hogan@...tec.com,
	ralf@...ux-mips.org, dhowells@...hat.com, jejb@...isc-linux.org,
	mpe@...erman.id.au, ysato@...rs.sourceforge.jp,
	cmetcalf@...lanox.com, chris@...kel.net,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH -v2 4/6] locking, arch: Update spin_unlock_wait()

On Thu, 26 May 2016 16:19:26 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> This patch updates/fixes all spin_unlock_wait() implementations.
> 
> The update is in semantics; where it previously was only a control
> dependency, we now upgrade to a full load-acquire to match the
> store-release from the spin_unlock() we waited on. This ensures that
> when spin_unlock_wait() returns, we're guaranteed to observe the full
> critical section we waited on.
> 
> This fixes a number of spin_unlock_wait() users that (not
> unreasonably) rely on this.

All that is missing is an smp_rmb(), no? 

> --- a/arch/s390/include/asm/spinlock.h
> +++ b/arch/s390/include/asm/spinlock.h
> @@ -95,8 +95,7 @@ static inline void arch_spin_unlock(arch
> 
>  static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
>  {
> -	while (arch_spin_is_locked(lock))
> -		arch_spin_relax(lock);
> +	smp_cond_load_acquire(&lock->lock, !VAL);
>  }
> 
>  /*

This change adds the smp_rmb() at the end of the waiting loop, but
it also replaces arch_spin_relax() alias arch_lock_relax() with a
cpu_relax(). This is not good, these two functions do *very* different
things. cpu_relax() does an undirected yield with diagnose 0x44 but
only if the system is non-SMT. arch_lock_relax() does an additional
cpu_is_preempted() to test if the target cpu is running and does a
directed yield with diagnose 0x9c. 

Why can't we just add the smp_rmb() to the arch_spin_unlock_wait()?

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.