linux-kernel - Re: [PATCH 1/5] locking: Add rwsem_is_write

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Message-ID: <20230907190810.GA14243@noisy.programming.kicks-ass.net>
Date:   Thu, 7 Sep 2023 21:08:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Chandan Babu R <chandan.babu@...cle.com>,
        "Darrick J . Wong" <djwong@...nel.org>, linux-xfs@...r.kernel.org
Subject: Re: [PATCH 1/5] locking: Add rwsem_is_write_locked()

On Thu, Sep 07, 2023 at 06:47:01PM +0100, Matthew Wilcox (Oracle) wrote:
> Several places want to know whether the lock is held by a writer, instead
> of just whether it's held.  We can implement this for both normal and
> rt rwsems.  RWSEM_WRITER_LOCKED is declared in rwsem.c and exposing
> it outside that file might tempt other people to use it, so just use
> a comment to note that's what the 1 means, and help anybody find it if
> they're looking to change the implementation.

I'm presuming this is deep in a callchain where they know they hold the
lock, but they lost in what capacity?

In general I strongly dislike the whole _is_locked family, because it
gives very poorly defined semantics if used by anybody but the owner.

If these new functions are indeed to be used only by lock holders to
determine what kind of lock they hold, could we please put:

	lockdep_assert_held()

in them?