From mboxrd@z Thu Jan 1 00:00:00 1970 From: Martin Schwidefsky Subject: Re: [PATCH -v2 4/6] locking, arch: Update spin_unlock_wait() Date: Fri, 27 May 2016 08:46:49 +0200 Message-ID: <20160527084649.065c7609@mschwide> References: <20160526141922.163198062@infradead.org> <20160526142354.293350777@infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit 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 To: Peter Zijlstra Return-path: In-Reply-To: <20160526142354.293350777@infradead.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: netfilter-devel.vger.kernel.org On Thu, 26 May 2016 16:19:26 +0200 Peter Zijlstra 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.