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? thanks, greg k-h -- 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