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

    thanks,

    greg k-h
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