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From: David Howells <dhowells@redhat.com>
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To: Alan Stern <stern@rowland.harvard.edu>
Cc: dhowells@redhat.com, Andrea Parri <andrea.parri@amarulasolutions.com>,
        paulmck@linux.vnet.ibm.com, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
        Andrea Parri <parri.andrea@gmail.com>,
        Will Deacon <will.deacon@arm.com>, Boqun Feng <boqun.feng@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
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Subject: Re: [PATCH v2 1/3] locking: Document the semantics of spin_is_locked()
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Date: Tue, 03 Apr 2018 22:43:14 +0100
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Alan Stern <stern@rowland.harvard.edu> wrote:

> + * Returns: 1 if @lock is locked, 0 otherwise.
> + * However, on !CONFIG_SMP builds with !CONFIG_DEBUG_SPINLOCK,
> + * the return value is always 0 (see include/linux/spinlock_up.h).
> + * Therefore you should not rely heavily on the return value.

Seems reasonable.

It might also want to include a note that the lock isn't necessarily held by
your own CPU.  I would also use "=n" rather than "!", so maybe something like:

 * Returns: 1 if @lock is locked, 0 otherwise.
 *
 * Note that the function only tells you that the CPU is seen to be locked,
 * not that it is locked on your CPU.
 *
 * Further, on CONFIG_SMP=n builds with CONFIG_DEBUG_SPINLOCK=n, the return
 * value is always 0 (see include/linux/spinlock_up.h).  Therefore you should
 * not rely heavily on the return value.

David