linux-kernel - Re: Question about TTY_DO_WRITE

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Message-ID: <20081120155502.608812d9@lxorguk.ukuu.org.uk>
Date:	Thu, 20 Nov 2008 15:55:02 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Thomas Pfaff <tpfaff@....com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Question about TTY_DO_WRITE_WAKEUP

> But n_tty_write_wakeup is only called from tty_wakeup when this bit is already 
> set, therefore it makes no sense to set this bit in n_tty_write_wakeup again.

The base code should probably really use test_and_clear_bit() when
calling that method.
> 
> The flow looks to me as
> 
> If the tty driver sets TTY_DO_WRITE_WAKEUP a SIGIO is generated on every 
> tty_wakeup.
> 
> If it is not set then n_tty_write_wakeup is never called and a SIGIO is not 
> generated.

Which isn't perfect (excess SIGIO cases) but doesn't seem incorrect. If
you've not blocked the tty output buffer then write() has not returned a
short write and no SIGIO is due.

Alan
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