linux-kernel - Re: Introduce atomic_dec_and_lock

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Message-ID: <20180504163826.GR12217@hirez.programming.kicks-ass.net>
Date:   Fri, 4 May 2018 18:38:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        Ingo Molnar <mingo@...hat.com>, linux-mm@...ck.org,
        Shaohua Li <shli@...nel.org>, linux-raid@...r.kernel.org
Subject: Re: Introduce atomic_dec_and_lock_irqsave()

On Fri, May 04, 2018 at 05:26:40PM +0100, Al Viro wrote:
> On Fri, May 04, 2018 at 06:21:02PM +0200, Peter Zijlstra wrote:
> > On Fri, May 04, 2018 at 06:07:26PM +0200, Sebastian Andrzej Siewior wrote:
> > 
> > > do you intend to kill refcount_dec_and_lock() in the longterm?
> > 
> > You meant to say atomic_dec_and_lock() ? Dunno if we ever get there, but
> > typically dec_and_lock is fairly refcounty, but I suppose it is possible
> > to have !refcount users, in which case we're eternally stuck with it.
> 
> Yes, there are - consider e.g.
> 
> void iput(struct inode *inode)
> { 
>         if (!inode)
>                 return;
>         BUG_ON(inode->i_state & I_CLEAR);
> retry:
>         if (atomic_dec_and_lock(&inode->i_count, &inode->i_lock)) {
> 
> inode->i_count sure as hell isn't refcount_t fodder...

Yeah, I should've remembered, I tried to convert that once ;-) i_count is
a usage count, not a refcount.