Hi Måns, On Mon, Nov 10, 2014 at 10:25 AM, Måns Rullgård <m...@mansr.com> wrote: > Christian Riesch <christian.rie...@omicron.at> writes: > >> On Thu, Nov 6, 2014 at 9:56 PM, Greg Kroah-Hartman >> <gre...@linuxfoundation.org> wrote: >>> On Thu, Nov 06, 2014 at 08:49:01PM +0000, Måns Rullgård wrote: >>>> Greg Kroah-Hartman <gre...@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.
Yes, single processor. Texas Instruments AM1808 SoC. Thanks, Christian -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
