Christian Riesch <christian.riesch@xxxxxxxxxx> writes: > On Thu, Nov 6, 2014 at 9:56 PM, Greg Kroah-Hartman > <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: >> On Thu, Nov 06, 2014 at 08:49:01PM +0000, Måns Rullgård wrote: >>> Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html