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. Oh, I didn't know that, thanks a lot for this! > Anyway, because of this, no need for the wmb() calls, just rearrange the > logic and all should be good, right? I came up with the wmb() stuff after getting scared from reading Documentation/memory-barriers.txt (which I didn't understand) and Documentation/circular-buffers.txt (which I understood partly). But it is actually a circular buffer and circular-buffers.txt says I need memory barriers for circular buffers. Though I do not know how function calls fit into this picture. > Christian, can you test that > instead? Sure, but it will probably not happen before Monday. Thanks a lot for your help! Christian -- 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