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

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Message-ID: <29562.1522759749@warthog.procyon.org.uk>
Date:   Tue, 03 Apr 2018 13:49:09 +0100
From:   David Howells <dhowells@...hat.com>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     dhowells@...hat.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>,
        Alan Stern <stern@...land.harvard.edu>,
        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()

Andrea Parri <andrea.parri@...rulasolutions.com> wrote:

> +/**
> + * 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.

It's more complicated than that.  This function is dangerous and should be
used with extreme care.  In the case where CONFIG_SMP=n the value is locked
one way or the other and it might be the wrong way.

David