linux-kernel - Re: [PATCH v2 1/3] locking: Document the semantics of spin_is

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

Alan Stern <stern@...land.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