From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep Date: Tue, 24 May 2016 16:27:24 +0200 Message-ID: <20160524143649.523586684@infradead.org> References: <20160524142723.178148277@infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Cc: 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, "Peter Zijlstra (Intel)" To: linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, manfred@colorfullife.com, dave@stgolabs.net, paulmck@linux.vnet.ibm.com, will.deacon@arm.com Return-path: Content-Disposition: inline; filename=peterz-locking-smp_acquire__after_ctrl_dep.patch Sender: linux-kernel-owner@vger.kernel.org List-Id: netfilter-devel.vger.kernel.org Introduce smp_acquire__after_ctrl_dep(), this construct is not uncommen, but the lack of this barrier is. Signed-off-by: Peter Zijlstra (Intel) --- include/linux/compiler.h | 14 ++++++++++---- ipc/sem.c | 14 ++------------ 2 files changed, 12 insertions(+), 16 deletions(-) --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -305,20 +305,26 @@ static __always_inline void __write_once }) /** + * smp_acquire__after_ctrl_dep() - Provide ACQUIRE ordering after a control dependency + * + * A control dependency provides a LOAD->STORE order, the additional RMB + * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order, + * aka. ACQUIRE. + */ +#define smp_acquire__after_ctrl_dep() smp_rmb() + +/** * smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering * @cond: boolean expression to wait for * * Equivalent to using smp_load_acquire() on the condition variable but employs * the control dependency of the wait to reduce the barrier on many platforms. * - * The control dependency provides a LOAD->STORE order, the additional RMB - * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order, - * aka. ACQUIRE. */ #define smp_cond_acquire(cond) do { \ while (!(cond)) \ cpu_relax(); \ - smp_rmb(); /* ctrl + rmb := acquire */ \ + smp_acquire__after_ctrl_dep(); \ } while (0) #endif /* __KERNEL__ */ --- a/ipc/sem.c +++ b/ipc/sem.c @@ -260,16 +260,6 @@ static void sem_rcu_free(struct rcu_head } /* - * spin_unlock_wait() and !spin_is_locked() are not memory barriers, they - * are only control barriers. - * The code must pair with spin_unlock(&sem->lock) or - * spin_unlock(&sem_perm.lock), thus just the control barrier is insufficient. - * - * smp_rmb() is sufficient, as writes cannot pass the control barrier. - */ -#define ipc_smp_acquire__after_spin_is_unlocked() smp_rmb() - -/* * Wait until all currently ongoing simple ops have completed. * Caller must own sem_perm.lock. * New simple ops cannot start, because simple ops first check @@ -292,7 +282,7 @@ static void sem_wait_array(struct sem_ar sem = sma->sem_base + i; spin_unlock_wait(&sem->lock); } - ipc_smp_acquire__after_spin_is_unlocked(); + smp_acquire__after_ctrl_dep(); } /* @@ -350,7 +340,7 @@ static inline int sem_lock(struct sem_ar * complex_count++; * spin_unlock(sem_perm.lock); */ - ipc_smp_acquire__after_spin_is_unlocked(); + smp_acquire__after_ctrl_dep(); /* * Now repeat the test of complex_count: