linux-kernel - Re: n_tty_write() going into schedule but NOT coming out

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Date:	Tue, 02 Apr 2013 09:38:42 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	lkml <linux-kernel@...r.kernel.org>, linux-serial@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Jiri Slaby <jslaby@...e.cz>
Subject: Re: n_tty_write() going into schedule but NOT coming out

On Tue, 2013-04-02 at 18:56 +0530, Vineet Gupta wrote:
> On 04/01/2013 08:40 PM, Peter Hurley wrote:
> > On Mon, 2013-04-01 at 19:27 +0530, Vineet Gupta wrote:
> >> On 03/30/2013 06:05 PM, Vineet Gupta wrote:
> >>> Hung find task is sitting in the schedule() call in n_tty_write()
> >>>
> >>> [ARCLinux]$ cat /proc/281/stack
> >>> [<8065945e>] n_tty_write+0x23a/0x424
> >>> [<80655cd4>] tty_write+0x1ac/0x2d4
> >>> [<805976ba>] vfs_write+0x92/0x110
> >>> [<80597816>] sys_write+0x4e/0x88
> >>> [<8050e780>] ret_from_system_call+0x0/0x4
> > Likely the writer is stuck because the receive buffer is full and the
> > reader is hung. What are the respective shells and telnetd doing?
> 
> I don't undersand how the receive buffer full/empty is coming into play - that
> schedule() call is not a wait queue or something

If n_tty_write() blocks [the schedule() call], then it is on a wait
queue (tty->write_wait) which is woken by pty's unthrottle() called from
n_tty_read from the linked pty when space becomes available in the
receive buffer.



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