Re: [PATCH RFC 08/26] locking: Remove spin_unlock_wait() generic definitions

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Will Deacon <will.deacon@arm.com>
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
Date: Mon, 3 Jul 2017 09:18:51 -0700	[thread overview]
Message-ID: <20170703161851.GY2393@linux.vnet.ibm.com> (raw)
In-Reply-To: <20170703131514.GE1573@arm.com>

On Mon, Jul 03, 2017 at 02:15:14PM +0100, Will Deacon wrote:
> On Fri, Jun 30, 2017 at 03:18:40PM -0700, Paul E. McKenney wrote:
> > On Fri, Jun 30, 2017 at 02:13:39PM +0100, Will Deacon wrote:
> > > On Fri, Jun 30, 2017 at 05:38:15AM -0700, Paul E. McKenney wrote:
> > > > 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.
> > 
> > That is indeed an interesting one...  But my first round will be what
> > semantics the implementations seem to provide:
> > 
> > Acquire courtesy of TSO: s390, sparc, x86.
> > Acquire: ia64 (in reality fully ordered).
> > Control dependency: alpha, arc, arm, blackfin, hexagon, m32r, mn10300, tile,
> > 	xtensa.
> > Control dependency plus leading full barrier: arm64, powerpc.
> > UP-only: c6x, cris, frv, h8300, m68k, microblaze nios2, openrisc, um, unicore32.
> > 
> > Special cases:
> > 	metag: Acquire if !CONFIG_METAG_SMP_WRITE_REORDERING.
> > 	       Otherwise control dependency?
> > 	mips: Control dependency, acquire if CONFIG_CPU_CAVIUM_OCTEON.
> > 	parisc: Acquire courtesy of TSO, but why barrier in smp_load_acquire?
> > 	sh: Acquire if one of SH4A, SH5, or J2, otherwise acquire?  UP-only?
> > 
> > Are these correct, or am I missing something with any of them?
> 
> That looks about right but, at least on ARM, I think we have to consider
> the semantics of spin_is_locked with respect to the other spin_* functions,
> rather than in isolation.
> 
> For example, ARM only has a control dependency, but spin_lock has a trailing
> smp_mb() and spin_unlock has both leading and trailing smp_mb().

Agreed, and my next step is to look at spin_lock() followed by
spin_is_locked(), not necessarily the same lock.

							Thanx, Paul