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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 3 Jul 2017 09:40:22 -0700
Message-ID: <CA+55aFzA+dk8P-wWFX_rYE09A-3uCL8_7ZNG4jC1+c0x4COQkQ@mail.gmail.com>
Subject: Re: [PATCH RFC 08/26] locking: Remove spin_unlock_wait() generic definitions
To: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Will Deacon <will.deacon@arm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        NetFilter <netfilter-devel@vger.kernel.org>,
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        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, Davidlohr Bueso <dave@stgolabs.net>,
        Manfred Spraul <manfred@colorfullife.com>, Tejun Heo <tj@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        Andrea Parri <parri.andrea@gmail.com>
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On Mon, Jul 3, 2017 at 9:18 AM, Paul E. McKenney
<paulmck@linux.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.

                       Linus