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

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Message-Id: <20170703211035.GH2393@linux.vnet.ibm.com>
Date:   Mon, 3 Jul 2017 14:10:35 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Will Deacon <will.deacon@....com>,
        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 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).

Good to know, that does make things easier.  ;-)

I am not certain that it is feasible to automatically recognize
non-assert/non-debugging use cases of spin_is_locked(), but there is
aways manual inspection.

> 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.

OK, so we should rework any cases where people are relying on acquisition
of one spin_lock() being ordered with a later spin_is_locked() on some
other lock by that same thread.

							Thanx, Paul