linux-kernel - Re: [PATCH 29/34] net: smc: fix opencoded find_and_set_bit() in smc_wr_tx_get_free_slot

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Date:   Tue, 21 Nov 2023 05:41:22 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     Alexandra Winter <wintera@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, Karsten Graul <kgraul@...ux.ibm.com>,
        Wenjia Zhang <wenjia@...ux.ibm.com>,
        Jan Karcher <jaka@...ux.ibm.com>,
        "D. Wythe" <alibuda@...ux.alibaba.com>,
        Tony Lu <tonylu@...ux.alibaba.com>,
        Wen Gu <guwen@...ux.alibaba.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, linux-s390@...r.kernel.org,
        netdev@...r.kernel.org, Jan Kara <jack@...e.cz>,
        Mirsad Todorovac <mirsad.todorovac@....unizg.hr>,
        Matthew Wilcox <willy@...radead.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>,
        Alexey Klimov <klimov.linux@...il.com>
Subject: Re: [PATCH 29/34] net: smc: fix opencoded find_and_set_bit() in
 smc_wr_tx_get_free_slot_index()

On Mon, Nov 20, 2023 at 09:43:54AM +0100, Alexandra Winter wrote:
> 
> 
> On 18.11.23 16:51, Yury Norov wrote:
> > The function opencodes find_and_set_bit() with a for_each() loop. Fix
> > it, and make the whole function a simple almost one-liner.
> > 
> > Signed-off-by: Yury Norov <yury.norov@...il.com>
> > ---
> >  net/smc/smc_wr.c | 10 +++-------
> >  1 file changed, 3 insertions(+), 7 deletions(-)
> > 
> > diff --git a/net/smc/smc_wr.c b/net/smc/smc_wr.c
> > index 0021065a600a..b6f0cfc52788 100644
> > --- a/net/smc/smc_wr.c
> > +++ b/net/smc/smc_wr.c
> > @@ -170,15 +170,11 @@ void smc_wr_tx_cq_handler(struct ib_cq *ib_cq, void *cq_context)
> >  
> >  static inline int smc_wr_tx_get_free_slot_index(struct smc_link *link, u32 *idx)
> >  {
> > -	*idx = link->wr_tx_cnt;
> >  	if (!smc_link_sendable(link))
> >  		return -ENOLINK;
> > -	for_each_clear_bit(*idx, link->wr_tx_mask, link->wr_tx_cnt) {
> > -		if (!test_and_set_bit(*idx, link->wr_tx_mask))
> > -			return 0;
> > -	}
> > -	*idx = link->wr_tx_cnt;
> > -	return -EBUSY;
> > +
> > +	*idx = find_and_set_bit(link->wr_tx_mask, link->wr_tx_cnt);
> > +	return *idx < link->wr_tx_cnt ? 0 : -EBUSY;
> >  }
> >  
> >  /**
> 
> 
> My understanding is that you can omit the lines with
> > -	*idx = link->wr_tx_cnt;
> because they only apply to the error paths and you checked that the calling function
> does not use the idx variable in the error cases. Do I understand this correct?
> 
> If so the removal of these 2 lines is not related to your change of using find_and_set_bit(),
> do I understand that correctly?
> 
> If so, it may be worth mentioning that in the commit message.

I'll add:

        If find_and_set_bit() doesn't acquire a bit, it returns
        ->wr_tx_cnt, and so explicit initialization of *idx with
        the same value is unneeded.

Makes sense?