linux-kernel - Re: uart : lost characters when system is busy

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20110610102026.01f7da05@lxorguk.ukuu.org.uk>
Date:	Fri, 10 Jun 2011 10:20:26 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Matthieu CASTET <matthieu.castet@...rot.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	<linux-serial@...r.kernel.org>
Subject: Re: uart : lost characters when system is busy

> uart_throttle/uart_unthrottle is called from a workqueue.
> If the system is busy, and the uart receive lot's of data, we fill the tty
> buffer, but the workqueue doesn't run and we never have a chance to call
> uart_throttle. So the uart is never slow down.

You should have around 64K of buffering (actually n_tty worst case
should be 63.5Kbyte) that's a lot of slack so what is holding off the
work queue for so long on your problem system ? I think that should be
answered first as it sounds like some other part of your kernel is
misbehaving.

> A workaround could be to check the buffer threshold in tty_flip_buffer_push and
> call throttle callback if needed.

tty_flip_buffer_push can be called from an IRQ, the throttle callback
needs to be able to sleep.

What might work if it is needed though is to provide a tty_is_throttled()
method that a driver can use to check the instantaneous throttle state.
Trouble is that will require a lot of care on the drivers part to deal
with asynchronus throttle/unthrottle events while peering at the state in
its IRQ handler as well.

Anyway - question for the case you hit, what tasks or work held off the
serial work queue for 63.5Kbytes of data ?
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/