Re: [PATCH] n_tty: Add memory barrier to fix race condition in receive path

Christian Riesch
2014-11-06T21:40:41Z

On Thu, Nov 6, 2014 at 9:56 PM, Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
> On Thu, Nov 06, 2014 at 08:49:01PM +0000, Måns Rullgård wrote:
>> Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
>>
>> > On Thu, Nov 06, 2014 at 12:39:59PM +0100, Christian Riesch wrote:
>> >> The current implementation of put_tty_queue() causes a race condition
>> >> when re-arranged by the compiler.
>> >>
>> >> On my build with gcc 4.8.3, cross-compiling for ARM, the line
>> >>
>> >> *read_buf_addr(ldata, ldata->read_head++) = c;
>> >>
>> >> was re-arranged by the compiler to something like
>> >>
>> >> x = ldata->read_head
>> >> ldata->read_head++
>> >> *read_buf_addr(ldata, x) = c;
>> >>
>> >> which causes a race condition. Invalid data is read if data is read
>> >> before it is actually written to the read buffer.
>> >
>> > Really? A compiler can rearange things like that and expect things to
>> > actually work? How is that valid?
>>
>> This is actually required by the C spec. There is a sequence point
>> before a function call, after the arguments have been evaluated. Thus
>> all side-effects, such as the post-increment, must be complete before
>> the function is called, just like in the example.
>>
>> There is no "re-arranging" here. The code is simply wrong.

Oh, I didn't know that, thanks a lot for this!

> Anyway, because of this, no need for the wmb() calls, just rearrange the
> logic and all should be good, right?

I came up with the wmb() stuff after getting scared from reading
Documentation/memory-barriers.txt (which I didn't understand) and
Documentation/circular-buffers.txt (which I understood partly). But it
is actually a circular buffer and circular-buffers.txt says I need
memory barriers for circular buffers. Though I do not know how
function calls fit into this picture.

> Christian, can you test that
> instead?

Sure, but it will probably not happen before Monday. Thanks a lot for your help!

Christian