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    SubjectRe: [PATCH v2 1/3] locking: Document the semantics of spin_is_locked()
    On Tue, Apr 03, 2018 at 04:23:07PM +0100, David Howells wrote:
    > Andrea Parri <andrea.parri@amarulasolutions.com> wrote:
    >
    > > Sorry, but I don't understand your objection: are you suggesting to add
    > > something like "Always return 0 on !SMP" to the comment? what else?
    >
    > Something like that, possibly along with a warning that this might not be what
    > you want. You might actually want it to return true on !SMP, it depends on
    > what you're using it for.

    I ended up with the following revision. I hesitated on whether to refer
    to 'include/linux/spinlock_up.h' or not, but in the end I decided to not
    include the reference. Please let me know what you think about this.

    Andrea


    From 85f2d12d4ad9769cc9f69cc5f447fdb8c5ed4d14 Mon Sep 17 00:00:00 2001
    From: Andrea Parri <andrea.parri@amarulasolutions.com>
    Date: Tue, 3 Apr 2018 21:23:07 +0200
    Subject: [PATCH v3 1/3] locking: Document the semantics of spin_is_locked()

    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().

    A recent auditing [1] of the callers of the primitive confirmed that
    none of them are relying on particular ordering guarantees; document
    this semantics by adding a docbook header to spin_is_locked().

    [1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2

    Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
    Cc: Alan Stern <stern@rowland.harvard.edu>
    Cc: Will Deacon <will.deacon@arm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Boqun Feng <boqun.feng@gmail.com>
    Cc: Nicholas Piggin <npiggin@gmail.com>
    Cc: David Howells <dhowells@redhat.com>
    Cc: Jade Alglave <j.alglave@ucl.ac.uk>
    Cc: Luc Maranget <luc.maranget@inria.fr>
    Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    Cc: Akira Yokosawa <akiyks@gmail.com>
    Cc: Ingo Molnar <mingo@redhat.com>
    ---
    include/linux/spinlock.h | 14 ++++++++++++++
    1 file changed, 14 insertions(+)

    diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
    index 4894d322d2584..636a4436191c1 100644
    --- a/include/linux/spinlock.h
    +++ b/include/linux/spinlock.h
    @@ -380,6 +380,20 @@ 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.
    + *
    + * Remark that this primitve can return a fixed value
    + * under certain !SMP configurations.
    + */
    static __always_inline int spin_is_locked(spinlock_t *lock)
    {
    return raw_spin_is_locked(&lock->rlock);
    --
    2.7.4


    >
    > David

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