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From: Johan Hovold <johan@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	"Ahmed S. Darwish" <a.darwish@linutronix.de>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Johan Hovold <johan@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	linux-usb@vger.kernel.org,
	Thomas Winischhofer <thomas@winischhofer.net>,
	Mathias Nyman <mathias.nyman@intel.com>,
	Valentina Manea <valentina.manea.m@gmail.com>,
	Shuah Khan <shuah@kernel.org>,
	Alan Stern <stern@rowland.harvard.edu>,
	linux-omap@vger.kernel.org, Kukjin Kim <kgene@kernel.org>,
	Krzysztof Kozlowski <krzk@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-samsung-soc@vger.kernel.org,
	Felipe Balbi <balbi@kernel.org>,
	Duncan Sands <duncan.sands@free.fr>
Subject: Re: [patch V2 02/13] USB: serial: keyspan_pda: Replace in_interrupt() usage
Date: Sun, 25 Oct 2020 17:56:47 +0100	[thread overview]
Message-ID: <20201025165647.GR26280@localhost> (raw)
In-Reply-To: <20201019101109.753597069@linutronix.de>

On Mon, Oct 19, 2020 at 12:06:31PM +0200, Thomas Gleixner wrote:
> 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@linutronix.de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Johan Hovold <johan@kernel.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: linux-usb@vger.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);

There's a ton of issues with this driver, but this is arguably making
things worse. A line discipline may call write() from just about any
context so we cannot rely on tty being non-NULL here (e.g. PPP).

I've prepared a series fixing up the driver's write implementation
that does away with this room check in the write path instead.

Johan

  reply	other threads:[~2020-10-25 16:56 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-19 10:06 [patch V2 00/13] USB: Cleanup in_interupt/in_irq/in_atomic() usage Thomas Gleixner
2020-10-19 10:06 ` [patch V2 01/13] USB: sisusbvga: Make console support depend on BROKEN Thomas Gleixner
2020-10-19 10:06 ` [patch V2 02/13] USB: serial: keyspan_pda: Replace in_interrupt() usage Thomas Gleixner
2020-10-25 16:56   ` Johan Hovold [this message]
2020-10-26 12:47     ` Sebastian Andrzej Siewior
2020-10-27  8:16       ` Johan Hovold
2020-10-19 10:06 ` [patch V2 03/13] USB: serial: keyspan_pda: Consolidate room query Thomas Gleixner
2020-10-25 17:05   ` Johan Hovold
2020-10-19 10:06 ` [patch V2 04/13] USB: serial: digi_acceleport: Remove in_interrupt() usage Thomas Gleixner
2020-10-25 17:16   ` Johan Hovold
2020-10-26 14:03     ` [PATCH v3 04/13 ] " Sebastian Andrzej Siewior
2020-10-27  8:26       ` Johan Hovold
2020-10-19 10:06 ` [patch V2 05/13] usb: xhci: Remove in_interrupt() checks Thomas Gleixner
2020-10-19 10:06 ` [patch V2 06/13] usb: host: isp1362: Replace in_interrupt() usage Thomas Gleixner
2020-10-28 11:27   ` Greg Kroah-Hartman
2020-10-19 10:06 ` [patch V2 07/13] usbip: Remove in_interrupt() check Thomas Gleixner
2020-10-19 10:06 ` [patch V2 08/13] usb: hosts: Remove in_interrupt() from comments Thomas Gleixner
2020-10-19 16:28   ` Alan Stern
2020-10-19 10:06 ` [patch V2 09/13] USB: host: ehci-pmcmsp: Cleanup usb_hcd_msp_remove() Thomas Gleixner
2020-10-19 16:28   ` Alan Stern
2020-10-19 10:06 ` [patch V2 10/13] usb: gadget: pxa27x_udc: Replace in_interrupt() usage in comments Thomas Gleixner
2020-10-19 10:06 ` [patch V2 11/13] usb: gadget: udc: Remove in_interrupt()/in_irq() from comments Thomas Gleixner
2020-10-19 16:28   ` Alan Stern
2020-10-19 10:06 ` [patch V2 12/13] usb: core: Replace in_interrupt() in comments Thomas Gleixner
2020-10-19 16:28   ` Alan Stern
2020-10-19 10:06 ` [patch V2 13/13] usb: atm: Replace in_interrupt() usage in comment Thomas Gleixner
2020-10-19 10:28   ` Duncan Sands

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