linux-kernel - Re: [PATCH] test_rhashtable: remove semaphore usage

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Message-ID: <20181211073825.GT30790@orbyte.nwl.cc>
Date:   Tue, 11 Dec 2018 08:38:25 +0100
From:   Phil Sutter <phil@....cc>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Arnd Bergmann <arnd@...db.de>, Thomas Graf <tgraf@...g.ch>,
        "David S. Miller" <davem@...emloft.net>,
        NeilBrown <neilb@...e.com>, Tom Herbert <tom@...ntonium.net>,
        Kees Cook <keescook@...omium.org>,
        Paul Blakey <paulb@...lanox.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] test_rhashtable: remove semaphore usage

Hi,

On Tue, Dec 11, 2018 at 01:45:52PM +0800, Herbert Xu wrote:
> On Mon, Dec 10, 2018 at 10:17:20PM +0100, Arnd Bergmann wrote:
> > This is one of only two files that initialize a semaphore to a negative
> > value. We don't really need the two semaphores here at all, but can do
> > the same thing in more conventional and more effient way, by using a
> > single waitqueue and an atomic thread counter.
> > 
> > This gets us a little bit closer to eliminating classic semaphores from
> > the kernel. It also fixes a corner case where we fail to continue after
> > one of the threads fails to start up.
> > 
> > An alternative would be to use a split kthread_create()+wake_up_process()
> > and completely eliminate the separate synchronization.
> > 
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > ---
> > This is part of a longer, untested, series to remove semaphores
> > from the kernel, please review as such before applying.
> > ---
> >  lib/test_rhashtable.c | 28 ++++++++++++++++------------
> >  1 file changed, 16 insertions(+), 12 deletions(-)
> 
> This was created by Phil Sutter so I am adding him to the cc list.

Thanks, I would have missed it otherwise. Just a minor nit:

> > diff --git a/lib/test_rhashtable.c b/lib/test_rhashtable.c
> > index 18de5ff1255b..12bdea4f6c20 100644
> > --- a/lib/test_rhashtable.c
> > +++ b/lib/test_rhashtable.c
[...]
> > @@ -635,8 +636,9 @@ static int threadfunc(void *data)
> >  	int i, step, err = 0, insert_retries = 0;
> >  	struct thread_data *tdata = data;
> >  
> > -	up(&prestart_sem);
> > -	if (down_interruptible(&startup_sem))
> > +	if (atomic_dec_and_test(&startup_count))
> > +		wake_up(&startup_wait);
> > +	if (wait_event_interruptible(startup_wait, atomic_read(&startup_count) == -1))
> >  		pr_err("  thread[%d]: down_interruptible failed\n", tdata->id);

The error message should probably be adjusted as well. Apart from that:

Acked-by: Phil Sutter <phil@....cc>

Thanks, Phil