linux-kernel - Re: [PATCH] tty: hold lock across tty buffer finding and buffer filling

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Date:	Fri, 16 Mar 2012 10:57:01 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	"Du, Alek" <alek.du@...el.com>
CC:	Jiri Slaby <jirislaby@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"Tu, Xiaobing" <xiaobing.tu@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"Zhang, Yanmin" <yanmin.zhang@...el.com>,
	"Zuo, Jiao" <jiao.zuo@...el.com>
Subject: Re: [PATCH] tty: hold lock across tty buffer finding and buffer filling

On 03/16/2012 10:49 AM, Du, Alek wrote:
> The tty structure is protect and not null, the tty->buf.tail is null. Many people think the tty reference count isn't protected and cause this bug, it is not true. :-)

You protect the pointer, not what is inside that structure. If you
increment a tty reference count, tty->buf.tail won't be NULL iff tty is
not NULL. IOW, you have to use tty_port_tty_set/get all around.

> For the flush case, it need acquire the spinlock to free the buffer and put buf.tail to NULL. So this patch will help:
> 
> Here is the example place, you can see the __tty_buffer_flush is inside the spinlock.

I'm not talking about flush. I'm talking about prepare. It returns a
pointer to a tty buffer which may be concurrently freed by flush.

     A                                   B
================================================
prepare(&chars);
                                        flush();
memcpy(chars, data_from_HW); <- chars is freed now
flip();

thanks,
-- 
js
suse labs
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