From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752263AbeB1LYM (ORCPT ); Wed, 28 Feb 2018 06:24:12 -0500 Received: from mail-wr0-f196.google.com ([209.85.128.196]:42203 "EHLO mail-wr0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751899AbeB1LYL (ORCPT ); Wed, 28 Feb 2018 06:24:11 -0500 X-Google-Smtp-Source: AH8x225lD0XrIZ+t6HWQReAkEv1HjU5ghvNVH0NC35xbS8UcF/ti59pg1z7tqUNDVXz/HEveWVmwTA== Date: Wed, 28 Feb 2018 12:24:03 +0100 From: Andrea Parri To: Will Deacon Cc: linux-kernel@vger.kernel.org, Alan Stern , Peter Zijlstra , Boqun Feng , Nicholas Piggin , David Howells , Jade Alglave , Luc Maranget , "Paul E. McKenney" , Akira Yokosawa Subject: Re: [PATCH] Documentation/locking: Document the semantics of spin_is_locked() Message-ID: <20180228112403.GA32228@andrea> References: <1519814372-19941-1-git-send-email-parri.andrea@gmail.com> <20180228105631.GA7681@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180228105631.GA7681@arm.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 28, 2018 at 10:56:32AM +0000, Will Deacon wrote: > On Wed, Feb 28, 2018 at 11:39:32AM +0100, Andrea Parri wrote: > > There appeared to be a certain, recurrent uncertainty concerning the > > semantics of spin_is_locked(), likely a consequence of the fact that > > this semantics remains undocumented or that it has been historically > > linked to the (likewise unclear) semantics of spin_unlock_wait(). > > > > Document this semantics. > > > > Signed-off-by: Andrea Parri > > Cc: Alan Stern > > Cc: Will Deacon > > Cc: Peter Zijlstra > > Cc: Boqun Feng > > Cc: Nicholas Piggin > > Cc: David Howells > > Cc: Jade Alglave > > Cc: Luc Maranget > > Cc: "Paul E. McKenney" > > Cc: Akira Yokosawa > > --- > > include/linux/spinlock.h | 11 +++++++++++ > > 1 file changed, 11 insertions(+) > > > > diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h > > index 4894d322d2584..2639fdc9a916c 100644 > > --- a/include/linux/spinlock.h > > +++ b/include/linux/spinlock.h > > @@ -380,6 +380,17 @@ static __always_inline int spin_trylock_irq(spinlock_t *lock) > > raw_spin_trylock_irqsave(spinlock_check(lock), flags); \ > > }) > > > > +/** > > + * spin_is_locked() - Check whether a spinlock is locked. > > + * @lock: Pointer to the spinlock. > > + * > > + * This function is NOT required to provide any memory ordering > > + * guarantees; it could be used for debugging purposes or, when > > + * additional synchronization is needed, accompanied with other > > + * constructs (memory barriers) enforcing the synchronization. > > + * > > + * Return: 1, if @lock is (found to be) locked; 0, otherwise. > > + */ > > I also don't think this is quite right, since the spin_is_locked check > must be ordered after all prior lock acquisitions (to any lock) on the same > CPU. That's why we have an smp_mb() in there on arm64 (see 38b850a73034f). So, arm64 (and powerpc) complies to the semantics I _have_ in mind ... > > So this is a change in semantics and we need to audit the users before > proceeding. We should also keep spin_is_locked consistent with the versions > for mutex, rwsem, bit_spin. Well, strictly speaking, it isn't (given that the current semantics is, as reported above, currently undocumented); for the same reason, cases relying on anything more than _nothing_ (if any) are already broken ... Andrea > > Will