On Sun, Sep 22, 2013 at 2:56 PM, Benjamin Herrenschmidt <b...@kernel.crashing.org> wrote: > On Sun, 2013-09-22 at 18:24 +0200, Peter Zijlstra wrote: >> >> We use a segment offset. Something like: >> >> inc %gs:var; >> > > And gcc makes no stupid assumptions that this gs doesn't change ? That's > the main problem we have with using r13 for PACA.
Since gcc doesn't really know about segment registers at all (modulo %fs as TLS on x86), we do everything like that using inline asm. It's not *too* painful if you have a number of macro helpers to build up all the different versions. And r13 isn't volatile if you are preempt-safe, so I'm wondering if you could just make the preempt disable code mark %r13 as modified ("+r"). Then gcc won't ever cache r13 across one of those. And if you don't have preemption disabled, then you cannot do multiple ops using %r13 anyway, since on a load-store architecture it might change even between the load and store, so a per-cpu "add" operation *has* to cache the %r13 value in *another* register anyway, because using memory ops with just an offset off %r13 would be buggy. So I don't think this is a gcc issue. gcc can't fix those kinds of problems. Personally, I'd suggest something like: - the paca stuff is just insane. Try to get rid of it. - use %r13 for the per-thread thread-info pointer instead. A per-thread pointer is *not* volatile like the per-cpu base is. - Now you can make the per-cpu offset be loaded off the per-thread pointer (update it at context switch). gcc knows to not cache it across function calls, since it's a memory access. Use ACCESS_ONCE() or something to make sure it's only loaded once for the cpu offset ops. Alternatively, make %r13 point to the percpu side, but make sure that you always use an asm accessor to fetch the value. In particular, I think you need to make __my_cpu_offset be an inline asm that fetches %r13 into some other register. Otherwise you can never get it right. Linus -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/