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

Måns Rullgård
2014-11-10T09:26:04Z

Christian Riesch <christian.riesch@omicron.at> writes:

> 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.
>>
>> Ah, ok, time to dig out the C spec...
>>
>> Anyway, because of this, no need for the wmb() calls, just rearrange the
>> logic and all should be good, right? Christian, can you test that
>> instead?
>
> I ran a test with the patch that I posted in my first email for the
> last 4 days. No communication errors occurred so the patch actually
> fixes my problem. I will run another test as suggested by Greg, just
> with rearranging the logic.

What hardware are you running on? If it's a single-processor system,
it won't break without barriers even if they are required for SMP.

--
Måns Rullgård
mans@mansr.com