linux-kernel - Re: [PATCH 2/2] tty: Fix lockless tty buffer race

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Date:	Tue, 06 May 2014 10:00:18 +0200
From:	Manfred Schlaegl <manfred.schlaegl@....at>
To:	Peter Hurley <peter@...leysoftware.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	Jiri Slaby <jslaby@...e.cz>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 2/2] tty: Fix lockless tty buffer race

On 2014-05-02 17:05, Peter Hurley wrote:
> On 05/02/2014 10:56 AM, Peter Hurley wrote:
>> Commit 6a20dbd6caa2358716136144bf524331d70b1e03,
>> "tty: Fix race condition between __tty_buffer_request_room and flush_to_ldisc"
>> correctly identifies an unsafe race condition between
>> __tty_buffer_request_room() and flush_to_ldisc(), where the consumer
>> flush_to_ldisc() prematurely advances the head before consuming the
>> last of the data committed. For example:
>>
>>             CPU 0                     |            CPU 1
>> __tty_buffer_request_room            | flush_to_ldisc
>>    ...                                |   ...
>>                                       |   count = head->commit - head->read
>>    n = tty_buffer_alloc()             |
>>    b->commit = b->used                |
>>    b->next = n                        |
>>                                       |   if (!count)                /* T */
>>                                       |     if (head->next == NULL)  /* F */
>>                                       |     buf->head = head->next
>>
>> In this case, buf->head has been advanced but head->commit may have
>> been updated with a new value.
>>
>> Instead of reintroducing an unnecessary lock, fix the race locklessly.
>> Read the commit-next pair in the reverse order of writing, which guarantees
>> the commit value read is the latest value written if the head is
>> advancing.

This is a fine solution! I'll verify this against my previous experimental setup
(3.12.x and 3.12.x-rt25), but I dont't expect any problems.

>>
>> Reported-by: Manfred Schlaegl <manfred.schlaegl@....at>
>> Cc: <stable@...r.kernel.org> # 3.12.x+
> 
> The patch submitted by Manfred notes the commits which introduced the
> race [1], but attributes those commits to the 3.11 cycle. Those commits
> were merged in the 3.12 cycle.

You are right. I'm sorry for this.


Regars,
Manfred
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