From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755120AbZDVQYT (ORCPT ); Wed, 22 Apr 2009 12:24:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751690AbZDVQYH (ORCPT ); Wed, 22 Apr 2009 12:24:07 -0400 Received: from mx2.redhat.com ([66.187.237.31]:48420 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750988AbZDVQYF (ORCPT ); Wed, 22 Apr 2009 12:24:05 -0400 Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 In-Reply-To: <4664.1240413126@redhat.com> References: <4664.1240413126@redhat.com> <20090422135134.GA5249@elte.hu> <20090416143351.GD6532@redhat.com> <20090415162712.342d4c07.akpm@linux-foundation.org> <1239649429.16771.9.camel@heimdal.trondhjem.org> <20090413181733.GA10424@redhat.com> <32260.1239658818@redhat.com> <20090413214852.GA1127@redhat.com> <1239659841.16771.26.camel@heimdal.trondhjem.org> <20090413222451.GA2758@redhat.com> <14561.1239873018@redhat.com> <21239.1240407420@redhat.com> To: Ingo Molnar Cc: dhowells@redhat.com, Oleg Nesterov , Andrew Morton , serue@us.ibm.com, viro@zeniv.linux.org.uk, "Paul E. McKenney" , Nick Piggin , linux-kernel@vger.kernel.org Subject: Re: [PATCH] Document that wake_up(), complete() and co. imply a full memory barrier Date: Wed, 22 Apr 2009 17:23:18 +0100 Message-ID: <5591.1240417398@redhat.com> From: David Howells Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org David Howells wrote: > So we can't assume that complete(), wake_up() and co. imply any barriers. > > All we can assume is that try_to_wake_up() implies a write barrier, but we > can't assume that that will be called via __wake_up_common(). So how about this, then? David --- From: David Howells Subject: [PATCH] Document that wake_up(), complete() and co. may not imply a memory barrier Add to the memory barriers document to note that wake_up(), complete() and co. may not be assumed to imply any sort of memory barrier, with the exception of try_to_wake_up() and things derived from that. Signed-off-by: David Howells --- Documentation/memory-barriers.txt | 34 +++++++++++++++++++++++++++++++++- kernel/sched.c | 10 ++++++++++ 2 files changed, 43 insertions(+), 1 deletions(-) diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index f5b7127..6bd626a 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -42,6 +42,7 @@ Contents: - Interprocessor interaction. - Atomic operations. + - Wake up of processes - Accessing devices. - Interrupts. @@ -1224,6 +1225,9 @@ Other functions that imply barriers: (*) schedule() and similar imply full memory barriers. + (*) try_to_wake_up() and things derived from that imply a write memory + barrier. + ================================= INTER-CPU LOCKING BARRIER EFFECTS @@ -1366,13 +1370,15 @@ WHERE ARE MEMORY BARRIERS NEEDED? Under normal operation, memory operation reordering is generally not going to be a problem as a single-threaded linear piece of code will still appear to -work correctly, even if it's in an SMP kernel. There are, however, three +work correctly, even if it's in an SMP kernel. There are, however, five circumstances in which reordering definitely _could_ be a problem: (*) Interprocessor interaction. (*) Atomic operations. + (*) Wake up of processes. + (*) Accessing devices. (*) Interrupts. @@ -1568,6 +1574,32 @@ and in such cases the special barrier primitives will be no-ops. See Documentation/atomic_ops.txt for more information. +WAKE UP OF PROCESSES +-------------------- + +An unlock, write memory barrier or a full memory barrier may be needed before a +call to wake up another processes if the waker sets some state that the sleeper +will need to see. + + complete(); + wake_up(); + wake_up_all(); + wake_up_bit(); + wake_up_interruptible(); + wake_up_interruptible_all(); + wake_up_interruptible_nr(); + wake_up_interruptible_poll(); + wake_up_interruptible_sync(); + wake_up_interruptible_sync_poll(); + wake_up_locked(); + wake_up_locked_poll(); + wake_up_nr(); + wake_up_poll(); + +The sleeper may then need to interpolate a lock, read or full memory barrier +before accessing that state. + + ACCESSING DEVICES ----------------- diff --git a/kernel/sched.c b/kernel/sched.c index b902e58..7cbc3de 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -2337,6 +2337,8 @@ static int sched_balance_self(int cpu, int flag) * runnable without the overhead of this. * * returns failure only if the task is already active. + * + * It may be assumed that this function implies a full memory barrier. */ static int try_to_wake_up(struct task_struct *p, unsigned int state, int sync) { @@ -5241,6 +5243,8 @@ void __wake_up_common(wait_queue_head_t *q, unsigned int mode, * @mode: which threads * @nr_exclusive: how many wake-one or wake-many threads to wake up * @key: is directly passed to the wakeup function + * + * It may not be assumed that this function implies any sort of memory barrier. */ void __wake_up(wait_queue_head_t *q, unsigned int mode, int nr_exclusive, void *key) @@ -5279,6 +5283,8 @@ void __wake_up_locked_key(wait_queue_head_t *q, unsigned int mode, void *key) * with each other. This can prevent needless bouncing between CPUs. * * On UP it can prevent extra preemption. + * + * It may not be assumed that this function implies any sort of memory barrier. */ void __wake_up_sync_key(wait_queue_head_t *q, unsigned int mode, int nr_exclusive, void *key) @@ -5315,6 +5321,8 @@ EXPORT_SYMBOL_GPL(__wake_up_sync); /* For internal use only */ * awakened in the same order in which they were queued. * * See also complete_all(), wait_for_completion() and related routines. + * + * It may not be assumed that this function implies any sort of memory barrier. */ void complete(struct completion *x) { @@ -5332,6 +5340,8 @@ EXPORT_SYMBOL(complete); * @x: holds the state of this particular completion * * This will wake up all threads waiting on this particular completion event. + * + * It may not be assumed that this function implies any sort of memory barrier. */ void complete_all(struct completion *x) {