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    SubjectRe: [PATCH] n_tty: Add memory barrier to fix race condition in receive path
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    Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:

    > 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?

    Weakly ordered SMP systems probably need some kind of barrier. I didn't
    look at it carefully.

    --
    Måns Rullgård
    mans@mansr.com
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