So here's my try at fixing READ_ONCE() so that it is happy with 'const' sources.
It is entirely untested. Comments/testing? Christian, I guess I could just have forced a cast instead of the union. I'd like you to take a look at this, because right now it's holding up me pulling from Ingo. And Ingo, I think you need to add some kind of test for "horrible new warnings". I think your pull request *worked*, but the tens of lines of new warnings it generates is unacceptable, and will just cause me to undo the pull if I notice in time (like I did this time). Linus On Fri, Feb 20, 2015 at 4:03 PM, Linus Torvalds <torva...@linux-foundation.org> wrote: > How does this work for you at all? > > On Fri, Feb 20, 2015 at 5:37 AM, Ingo Molnar <mi...@kernel.org> wrote: >> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c >> index 94f643484300..e354cc6446ab 100644 >> --- a/arch/x86/kernel/kvm.c >> +++ b/arch/x86/kernel/kvm.c >> @@ -803,8 +808,8 @@ static void kvm_unlock_kick(struct arch_spinlock *lock, >> __ticket_t ticket) >> add_stats(RELEASED_SLOW, 1); >> for_each_cpu(cpu, &waiting_cpus) { >> const struct kvm_lock_waiting *w = &per_cpu(klock_waiting, >> cpu); >> - if (ACCESS_ONCE(w->lock) == lock && >> - ACCESS_ONCE(w->want) == ticket) { >> + if (READ_ONCE(w->lock) == lock && >> + READ_ONCE(w->want) == ticket) { >> add_stats(RELEASED_SLOW_KICKED, 1); >> kvm_kick_cpu(cpu); >> break; > > I get horrible compile warnings from this, because of how 'w' is a > pointer to a 'const' structure, which then causes things like > > include/linux/compiler.h:262:39: warning: passing argument 1 of > ‘__read_once_size’ discards ‘const’ qualifier from pointer target type > ({ typeof(x) __val; __read_once_size(&x, &__val, sizeof(__val)); __val; > }) > > which is fairly hard to avoid (looks like it might need a union) > > Linus
commit 1cb20581b0a32673fa3d056829ca89db4fa0de09 Author: Linus Torvalds <torva...@linux-foundation.org> Date: Fri Feb 20 15:46:31 2015 -0800 kernel: make READ_ONCE() valid on const pointers There is certainly nothing wrong with using READ_ONCE() on a const pointer, but the helper function __read_once_size() would cause warnings because it would drop the 'const' qualifier. Signed-off-by: Linus Torvalds <torva...@linux-foundation.org> --- include/linux/compiler.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/include/linux/compiler.h b/include/linux/compiler.h index d1ec10a940ff..312cf710e26a 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -202,7 +202,7 @@ static __always_inline void data_access_exceeds_word_size(void) { } -static __always_inline void __read_once_size(volatile void *p, void *res, int size) +static __always_inline void __read_once_size(const volatile void *p, void *res, int size) { switch (size) { case 1: *(__u8 *)res = *(volatile __u8 *)p; break; @@ -259,7 +259,7 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s */ #define READ_ONCE(x) \ - ({ typeof(x) __val; __read_once_size(&x, &__val, sizeof(__val)); __val; }) + ({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof((x))); __u.__val; }) #define WRITE_ONCE(x, val) \ ({ typeof(x) __val; __val = val; __write_once_size(&x, &__val, sizeof(__val)); __val; })