On Thu, Nov 06, 2014 at 09:38:59PM +0000, Måns Rullgård wrote: > Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes: > > > On Thu, Nov 06, 2014 at 09:01:36PM +0000, Måns Rullgård wrote: > >> Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes: > >> > >> > 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? > >> > >> Weakly ordered SMP systems probably need some kind of barrier. I didn't > >> look at it carefully. > > > > It shouldn't need a barier, as it is a sequence point with the function > > call. Well, it's an inline function, but that "shouldn't" matter here, > > right? > > Sequence points say nothing about the order in which stores become > visible to other CPUs. That's why there are barrier instructions. Yes, but "order" matters. If I write code that does: 100 x = ldata->read_head; 101 &ldata->read_head[x & SOME_VALUE] = y; 102 ldata->read_head++; the compiler can not reorder lines 102 and 101 just because it feels like it, right? Or is it time to go spend some reading of the C spec again... 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