LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Kurt Kanzenbach <kurt.kanzenbach@linutronix.de>
To: David Miller <davem@davemloft.net>
Cc: anirudh@xilinx.com, John.Linn@xilinx.com,
	michal.simek@xilinx.com, radhey.shyam.pandey@xilinx.com,
	andrew@lunn.ch, yuehaibing@huawei.com, netdev@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] net: axienet: recheck condition after timeout in mdio_wait()
Date: Wed, 31 Oct 2018 14:06:51 +0100	[thread overview]
Message-ID: <20181031130651.l4toezvyg4etevnh@linutronix.de> (raw)
In-Reply-To: <20181030.112511.957438343014682098.davem@davemloft.net>

On Tue, Oct 30, 2018 at 11:25:11AM -0700, David Miller wrote:
> From: Kurt Kanzenbach <kurt@linutronix.de>
> Date: Tue, 30 Oct 2018 10:31:38 +0100
>
> > The function could report a false positive if it gets preempted between reading
> > the XAE_MDIO_MCR_OFFSET register and checking for the timeout.  In such a case,
> > the condition has to be rechecked to avoid false positives.
> >
> > Therefore, check for expected condition even after the timeout occurred.
> >
> > Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
>  ...
> >  		if (time_before_eq(end, jiffies)) {
> > -			WARN_ON(1);
> > -			return -ETIMEDOUT;
> > +			val = axienet_ior(lp, XAE_MDIO_MCR_OFFSET);
> > +			break;
> >  		}
> > +
> >  		udelay(1);
> >  	}
> > -	return 0;
> > +	if (val & XAE_MDIO_MCR_READY_MASK)
> > +		return 0;
> > +
> > +	WARN_ON(1);
> > +	return -ETIMEDOUT;
>
> You are not fundamentally changing the situation at all.
>
> The condtion could change right after your last read of
> XAR_MDIO_MCR_OFFSET, which is the same thing that happens before your
> modifications to this code.

That's true. The problem is different: If the current task gets
preempted by a higher priority task between checking the condition and
the timeout code, then a timeout might be falsely detected. Consider the
following events:

loop:
 check mdio condition
 ------------------------
 task with real time priority may run for a long time
 ------------------------
 check for timeout
 wait

That's why I've added the recheck of the condition in the timeout case.

>
> It sounds more like the timeout is slightly too short, and that's the
> real problem that causes whatever behavior you think you are fixing
> here.

The timeout value is not the problem here.

Thanks,
Kurt

  reply	other threads:[~2018-10-31 13:07 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-30  9:31 [PATCH 0/2] net: xlinx: mdio: recheck condition after timeout Kurt Kanzenbach
2018-10-30  9:31 ` [PATCH 1/2] net: axienet: recheck condition after timeout in mdio_wait() Kurt Kanzenbach
2018-10-30 18:25   ` David Miller
2018-10-31 13:06     ` Kurt Kanzenbach [this message]
2018-11-05  9:16     ` Sebastian Andrzej Siewior
2018-10-30  9:31 ` [PATCH 2/2] net: xilinx_emaclite: " Kurt Kanzenbach
2018-10-30 12:10   ` Andrew Lunn
2018-10-30 12:58     ` Radhey Shyam Pandey
2018-10-30 18:25   ` David Miller
2018-10-30 12:08 ` [PATCH 0/2] net: xlinx: mdio: recheck condition after timeout Andrew Lunn
2018-10-30 13:47   ` Kurt Kanzenbach

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20181031130651.l4toezvyg4etevnh@linutronix.de \
    --to=kurt.kanzenbach@linutronix.de \
    --cc=John.Linn@xilinx.com \
    --cc=andrew@lunn.ch \
    --cc=anirudh@xilinx.com \
    --cc=davem@davemloft.net \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=michal.simek@xilinx.com \
    --cc=netdev@vger.kernel.org \
    --cc=radhey.shyam.pandey@xilinx.com \
    --cc=yuehaibing@huawei.com \
    --subject='Re: [PATCH 1/2] net: axienet: recheck condition after timeout in mdio_wait()' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).