linux-kernel - Re: [PATCH RFC 08/26] locking: Remove spin_unlock

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Date:   Mon, 3 Jul 2017 15:30:11 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Will Deacon <will.deacon@....com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        NetFilter <netfilter-devel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Manfred Spraul <manfred@...orfullife.com>,
        Tejun Heo <tj@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>
Subject: Re: [PATCH RFC 08/26] locking: Remove spin_unlock_wait() generic
 definitions

On Mon, Jul 03, 2017 at 06:13:38PM +0100, Will Deacon wrote:
> On Mon, Jul 03, 2017 at 09:40:22AM -0700, Linus Torvalds wrote:
> > On Mon, Jul 3, 2017 at 9:18 AM, Paul E. McKenney
> > <paulmck@...ux.vnet.ibm.com> wrote:
> > >
> > > Agreed, and my next step is to look at spin_lock() followed by
> > > spin_is_locked(), not necessarily the same lock.
> > 
> > Hmm. Most (all?) "spin_is_locked()" really should be about the same
> > thread that took the lock (ie it's about asserts and lock debugging).
> > 
> > The optimistic ABBA avoidance pattern for spinlocks *should* be
> > 
> >     spin_lock(inner)
> >     ...
> >     if (!try_lock(outer)) {
> >            spin_unlock(inner);
> >            .. do them in the right order ..
> > 
> > so I don't think spin_is_locked() should have any memory barriers.
> > 
> > In fact, the core function for spin_is_locked() is arguably
> > arch_spin_value_unlocked() which doesn't even do the access itself.
> 
> Yeah, but there's some spaced-out stuff going on in kgdb_cpu_enter where
> it looks to me like raw_spin_is_locked is used for synchronization. My
> eyes are hurting looking at it, though.

That certainly is one interesting function, isn't it?  I wonder what
happens if you replace the raw_spin_is_locked() calls with an
unlock under a trylock check?  ;-)

							Thanx, Paul