linux-kernel - Re: [PATCH] synclink_gt fix transmit race and timeout

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Date:	Tue, 23 Jun 2009 12:29:01 -0600
From:	Paul Fulghum <paulkf@...rogate.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] synclink_gt fix transmit race and timeout

Andrew Morton wrote:
> I'm referring to
> 
>   Where did the "7" come from?

Oops, I did not scroll down far enough.

+	/*
+	 * use worst case speed of 1200bps to calculate transmit timeout
+	 * based on data in buffers (tbuf_bytes) and FIFO (128 bytes)
+	 */
+	if (info->params.mode == MGSL_MODE_HDLC) {
+		int timeout  = (tbuf_bytes(info) * 7) + 1000;
+		mod_timer(&info->tx_timer, jiffies + msecs_to_jiffies(timeout));
+	}

7 is roughly the number of milliseconds to send a byte at 1200bps.

The problem with externally provided data clocks is that you
don't necessarily know the data rate before hand, so a somewhat
arbitrary worst case assumption is used.

> and
> 
>   I have a suspicion that tx_stop() should use del_timer_sync(), not
>   del_timer().  What happens if the timer handler is concurrently
>   running?

Everything is synchronized with info->lock spin_lock,
so nothing critical runs concurrently. tx_stop() is sometimes
called in interrupt context so it can't call del_timer_sync().
If the timer has already fired but has not
run yet it does nothing more than call tx_stop() itself and wake
any transmit waiters so there are no ill effects.

-- 
Paul Fulghum
MicroGate Systems, Ltd.
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