Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes: > On Thu, Nov 06, 2014 at 10:12:54PM +0000, Måns Rullgård wrote: >> Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes: >> >> > 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... >> >> The compiler can't. The hardware can. All the hardware promises is >> that at some unspecified time in the future, both memory locations will >> have the correct values. Another CPU might see 'read_head' updated >> before it sees the corresponding data value. A wmb() between the writes >> forces the CPU to complete preceding stores before it begins subsequent >> ones. > > Yes, sorry, I'm not talking about other CPUs and what they see, I'm > talking about the local one. I'm not assuming that this is SMP "safe" > at all. If it is supposed to be, then yes, we do have problems, but > there should be a lock _somewhere_ protecting this. Within the confines of a single CPU + memory, barriers are never needed. The moment another CPU or master-capable peripheral enters the mix, proper ordering must be enforced somehow. If the buffer is already protected by a lock of some kind, this will provide the necessary barriers, so nothing further is necessary. If it's a lock-less design, there will need to be barriers somewhere. >From the patch context, I can't tell which category this case falls into, and I'm far too lazy to read the entire file. -- 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