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

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Date:   Tue, 12 Sep 2023 15:52:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Dave Chinner <david@...morbit.com>,
        Waiman Long <longman@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        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 Tue, Sep 12, 2023 at 01:28:13PM +0100, Matthew Wilcox wrote:
> On Tue, Sep 12, 2023 at 11:03:42AM +0200, Peter Zijlstra wrote:
> > If not, then sure we can do this; it's not like I managed to get rid of
> > muteX_is_locked() -- and I actually tried at some point :/
> > 
> > And just now I grepped for it, and look what I find:
> > 
> > drivers/hid/hid-nintendo.c:     if (unlikely(mutex_is_locked(&ctlr->output_mutex))) {
> > drivers/nvdimm/btt.c:           if (mutex_is_locked(&arena->err_lock)
> > 
> > And there's more :-(
> 
> Are these actually abuse?  I looked at these two, and they both seem to
> be asking "Does somebody else currently have this mutex?" rather than
> "Do I have this mutex?".

It's effectively a random number generator in that capacity. Someone
might have it or might have had it when you looked and no longer have
it, or might have it now but not when you asked.