linux-kernel - [patch V2 02/13] USB: serial: keyspan_pda: Replace in

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Message-Id: <20201019101109.753597069@linutronix.de>
Date:   Mon, 19 Oct 2020 12:06:31 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Johan Hovold <johan@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org,
        Thomas Winischhofer <thomas@...ischhofer.net>,
        Mathias Nyman <mathias.nyman@...el.com>,
        Valentina Manea <valentina.manea.m@...il.com>,
        Shuah Khan <shuah@...nel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        linux-omap@...r.kernel.org, Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, Felipe Balbi <balbi@...nel.org>,
        Duncan Sands <duncan.sands@...e.fr>
Subject: [patch V2 02/13] USB: serial: keyspan_pda: Replace in_interrupt() usage

keyspan_pda_write() uses in_interrupt() to check whether it is safe to
invoke functions which might sleep.

The usage of in_interrupt() in drivers is phased out and Linus clearly
requested that code which changes behaviour depending on context should
either be seperated or the context be conveyed in an argument passed by the
caller, which usually knows the context.

Aside of that it does not cover all contexts which cannot sleep,
e.g. preempt disabled regions which cannot be reliably detected on all
kernel configurations.

With the current printk() implementation console->write() can be invoked
from almost any context. The upcoming rework of the console core will
provide thread context for console drivers which require to sleep.

For now, restrict the room query which can sleep to tty writes which happen
from preemptible task context.

The usability for dmesg output is limited anyway because it's almost
guaranteed to drop the 'LF' which is submitted after the dmesg line because
the device supports only one transfer on flight. Same for any printk()
which is coming in before the previous transfer has been done.

This new restriction does not make it worse than before, but it makes the
condition correct under all circumstances.

Signed-off-by: Ahmed S. Darwish <a.darwish@...utronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Johan Hovold <johan@...nel.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-usb@...r.kernel.org

---
 drivers/usb/serial/keyspan_pda.c |   10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

--- a/drivers/usb/serial/keyspan_pda.c
+++ b/drivers/usb/serial/keyspan_pda.c
@@ -477,10 +477,12 @@ static int keyspan_pda_write(struct tty_

 	count = (count > port->bulk_out_size) ? port->bulk_out_size : count;

-	/* Check if we might overrun the Tx buffer.   If so, ask the
-	   device how much room it really has.  This is done only on
-	   scheduler time, since usb_control_msg() sleeps. */
-	if (count > priv->tx_room && !in_interrupt()) {
+	/*
+	 * Check if we might overrun the Tx buffer. If so, ask the device
+	 * how much room it really has. This can only be invoked for tty
+	 * usage because the console write can't sleep.
+	 */
+	if (count > priv->tx_room && tty) {
 		u8 *room;

 		room = kmalloc(1, GFP_KERNEL);