Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752494AbdF3NNn (ORCPT ); Fri, 30 Jun 2017 09:13:43 -0400 Received: from foss.arm.com ([217.140.101.70]:42222 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751668AbdF3NNj (ORCPT ); Fri, 30 Jun 2017 09:13:39 -0400 Date: Fri, 30 Jun 2017 14:13:39 +0100 From: Will Deacon To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, netfilter-devel@vger.kernel.org, netdev@vger.kernel.org, oleg@redhat.com, akpm@linux-foundation.org, mingo@redhat.com, dave@stgolabs.net, manfred@colorfullife.com, tj@kernel.org, arnd@arndb.de, linux-arch@vger.kernel.org, peterz@infradead.org, stern@rowland.harvard.edu, parri.andrea@gmail.com, torvalds@linux-foundation.org Subject: Re: [PATCH RFC 08/26] locking: Remove spin_unlock_wait() generic definitions Message-ID: <20170630131339.GA14118@arm.com> References: <20170629235918.GA6445@linux.vnet.ibm.com> <1498780894-8253-8-git-send-email-paulmck@linux.vnet.ibm.com> <20170630091928.GC9726@arm.com> <20170630123815.GT2393@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170630123815.GT2393@linux.vnet.ibm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3837 Lines: 88 On Fri, Jun 30, 2017 at 05:38:15AM -0700, Paul E. McKenney wrote: > On Fri, Jun 30, 2017 at 10:19:29AM +0100, Will Deacon wrote: > > On Thu, Jun 29, 2017 at 05:01:16PM -0700, Paul E. McKenney wrote: > > > There is no agreed-upon definition of spin_unlock_wait()'s semantics, > > > and it appears that all callers could do just as well with a lock/unlock > > > pair. This commit therefore removes spin_unlock_wait() and related > > > definitions from core code. > > > > > > Signed-off-by: Paul E. McKenney > > > Cc: Arnd Bergmann > > > Cc: Ingo Molnar > > > Cc: Will Deacon > > > Cc: Peter Zijlstra > > > Cc: Alan Stern > > > Cc: Andrea Parri > > > Cc: Linus Torvalds > > > --- > > > include/asm-generic/qspinlock.h | 14 ----- > > > include/linux/spinlock.h | 31 ----------- > > > include/linux/spinlock_up.h | 6 --- > > > kernel/locking/qspinlock.c | 117 ---------------------------------------- > > > 4 files changed, 168 deletions(-) > > > > [...] > > > > > diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c > > > index b2caec7315af..64a9051e4c2c 100644 > > > --- a/kernel/locking/qspinlock.c > > > +++ b/kernel/locking/qspinlock.c > > > @@ -267,123 +267,6 @@ static __always_inline u32 __pv_wait_head_or_lock(struct qspinlock *lock, > > > #define queued_spin_lock_slowpath native_queued_spin_lock_slowpath > > > #endif > > > > > > -/* > > > - * Various notes on spin_is_locked() and spin_unlock_wait(), which are > > > - * 'interesting' functions: > > > - * > > > - * PROBLEM: some architectures have an interesting issue with atomic ACQUIRE > > > - * operations in that the ACQUIRE applies to the LOAD _not_ the STORE (ARM64, > > > - * PPC). Also qspinlock has a similar issue per construction, the setting of > > > - * the locked byte can be unordered acquiring the lock proper. > > > - * > > > - * This gets to be 'interesting' in the following cases, where the /should/s > > > - * end up false because of this issue. > > > - * > > > - * > > > - * CASE 1: > > > - * > > > - * So the spin_is_locked() correctness issue comes from something like: > > > - * > > > - * CPU0 CPU1 > > > - * > > > - * global_lock(); local_lock(i) > > > - * spin_lock(&G) spin_lock(&L[i]) > > > - * for (i) if (!spin_is_locked(&G)) { > > > - * spin_unlock_wait(&L[i]); smp_acquire__after_ctrl_dep(); > > > - * return; > > > - * } > > > - * // deal with fail > > > - * > > > - * Where it is important CPU1 sees G locked or CPU0 sees L[i] locked such > > > - * that there is exclusion between the two critical sections. > > > - * > > > - * The load from spin_is_locked(&G) /should/ be constrained by the ACQUIRE from > > > - * spin_lock(&L[i]), and similarly the load(s) from spin_unlock_wait(&L[i]) > > > - * /should/ be constrained by the ACQUIRE from spin_lock(&G). > > > - * > > > - * Similarly, later stuff is constrained by the ACQUIRE from CTRL+RMB. > > > > Might be worth keeping this comment about spin_is_locked, since we're not > > removing that guy just yet! > > Ah, all the examples had spin_unlock_wait() in them. So what I need to > do is to create a spin_unlock_wait()-free example to illustrate the > text starting with "The load from spin_is_locked(", correct? Yeah, I think so. > I also need to check all uses of spin_is_locked(). There might no > longer be any that rely on any particular ordering... Right. I think we're looking for the "insane case" as per 38b850a73034 (which was apparently used by ipc/sem.c at the time, but no longer). There's a usage in kernel/debug/debug_core.c, but it doesn't fill me with joy. Will