linux-kernel - Re: [PATCH] cdc-acm: some enhancement on acm delayed write

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Date:	Tue, 8 Apr 2014 15:12:27 +0200
From:	Johan Hovold <johan@...old.com>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	Xiao Jin <jin.xiao@...el.com>, gregkh@...uxfoundation.org,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	david.a.cohen@...ux.intel.com, yanmin.zhang@...el.com,
	Oliver Neukum <oneukum@...e.de>
Subject: Re: [PATCH] cdc-acm: some enhancement on acm delayed write

[ +CC: Oliver ]

On Tue, Apr 08, 2014 at 12:22:29PM +0100, One Thousand Gnomes wrote:
> > (2) acm tty port ASYNCB_INITIALIZED flag will be cleared when
> > close. If acm resume callback run after ASYNCB_INITIALIZED flag
> > cleared, there will have no chance for delayed write to start.
> > That lead to acm_wb.use can't be cleared. If user space open
> > acm tty again and try to setd, tty will be blocked in
> > tty_wait_until_sent for ever.
> 
> If there is data pending when the close occurs the close path should
> block until either the close timeout occurs or the buffer is written.
> This sounds more like the implementation of the ACM chars_in_buffer
> method is wrong and not counting the deferred bytes as unsent ?

The ACM chars_in_buffer does count the deferred bytes so I guess this
can only happen if close happens fast enough that resume is never called
before shutdown (e.g. closing_wait = ASYNC_CLOSING_WAIT_NONE and
drain_delay = 0).

Johan
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