Re: calling context when writing to tty_driver

David McIlwraith
2002-10-01T11:08:39Z

Semaphores may sleep - therefore, they cannot be used from a 'non-sleep'
context.

Sincerely,
David McIlwraith quack@bigpond.net.au

----- Original Message -----
From: "Martin Diehl" <lists@mdiehl.de>
To: <linux-kernel@vger.kernel.org>
Cc: "Greg KH" <greg@kroah.com>
Sent: Tuesday, October 01, 2002 8:37 PM
Subject: calling context when writing to tty_driver

>
> Hi,
>
> just hitting another "sleeping on semaphore from illegal context" issue
> with 2.5.39. Happened on down() in either usbserial->write_room() or
> usbserial->write(), when invoked from bh context.
>
> Some grepping reveals no documentation of calling context requirements
> for those driver calls and existing serial code seems to be happy with bh
> context. Therefore I'm wondering whether it is permitted to call from
> don't-sleep context?
>
> Since write_room() is usually called immediately before write()'ing stuff
> to the driver it would be a good idea to keep them both callable from bh,
> IMHO. For example tty_ldisc->write_wakeup() might probably want to issue
> write_room() followed by write().
>
> Currently, usbserial calls write_wakeup() from bh (on OUT urb completion)
> but needs process context for write_room() and write(). My impression is
> the whole point of write_room() is to find out how many data can be
> accepted by the write() - if write() would be allowed to sleep it could
> just block to deal with any amount of data.
>
> TIA for any insight.
>
> Martin
>
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