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. -- 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