From: Martin Schwidefsky <schwidefsky@de.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
manfred@colorfullife.com, dave@stgolabs.net,
paulmck@linux.vnet.ibm.com, will.deacon@arm.com,
boqun.feng@gmail.com, Waiman.Long@hpe.com, tj@kernel.org,
pablo@netfilter.org, kaber@trash.net, davem@davemloft.net,
oleg@redhat.com, netfilter-devel@vger.kernel.org,
sasha.levin@oracle.com, hofrat@osadl.org, rth@twiddle.net,
vgupta@synopsys.com, linux@armlinux.org.uk, realmz6@gmail.com,
rkuo@codeaurora.org, tony.luck@intel.com, james.hogan@imgtec.com,
ralf@linux-mips.org, dhowells@redhat.com, jejb@parisc-linux.org,
mpe@ellerman.id.au, ysato@users.sourceforge.jp,
cmetcalf@mellanox.com, chris@zankel.net,
Heiko Carstens <heiko.carstens@de.ibm.com>
Subject: Re: [PATCH -v2 4/6] locking, arch: Update spin_unlock_wait()
Date: Fri, 27 May 2016 08:46:49 +0200 [thread overview]
Message-ID: <20160527084649.065c7609@mschwide> (raw)
In-Reply-To: <20160526142354.293350777@infradead.org>
On Thu, 26 May 2016 16:19:26 +0200
Peter Zijlstra <peterz@infradead.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.
next prev parent reply other threads:[~2016-05-27 8:37 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-26 14:19 [PATCH -v2 0/6] spin_unlock_wait borkage Peter Zijlstra
2016-05-26 14:19 ` [PATCH -v2 1/6] locking: Replace smp_cond_acquire with smp_cond_load_acquire Peter Zijlstra
2016-05-26 14:19 ` [PATCH -v2 2/6] locking: Introduce cmpwait() Peter Zijlstra
2016-05-26 14:19 ` [PATCH -v2 3/6] locking: Introduce smp_acquire__after_ctrl_dep Peter Zijlstra
2016-05-26 14:19 ` [PATCH -v2 4/6] locking, arch: Update spin_unlock_wait() Peter Zijlstra
2016-05-26 21:10 ` Chris Metcalf
2016-05-27 9:05 ` Peter Zijlstra
2016-05-27 19:34 ` Chris Metcalf
2016-05-30 9:25 ` Peter Zijlstra
2016-05-27 6:46 ` Martin Schwidefsky [this message]
2016-05-27 9:02 ` Peter Zijlstra
2016-05-26 14:19 ` [PATCH -v2 5/6] locking: Update spin_unlock_wait users Peter Zijlstra
2016-05-26 14:19 ` [PATCH -v2 6/6] locking,netfilter: Fix nf_conntrack_lock() Peter Zijlstra
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