[-tip,1/3] x86/percpu: Fix "const_pcpu_hot" version generation failure
Commit Message
Version generation for "const_pcpu_hot" symbol failed because genksyms
doesn't know the __seg_gs keyword. Revert commit 4604c052b84d
"x86/percpu: Declare const_pcpu_hot as extern const variable" and
use this_cpu_read_const instead to avoid "sparse: dereference of
noderef expression" warning when reading const_pcpu_hot.
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
---
arch/x86/include/asm/current.h | 5 +++--
arch/x86/include/asm/percpu.h | 7 +++++++
arch/x86/include/asm/processor.h | 2 +-
3 files changed, 11 insertions(+), 3 deletions(-)
Comments
On Mon, 4 Dec 2023 at 07:12, Uros Bizjak <ubizjak@gmail.com> wrote:
>
> +/*
> + * The generic per-cpu infrastrucutre is not suitable for
> + * reading const-qualified variables.
> + */
> +#define this_cpu_read_const(pcp) ({ BUG(); (typeof(pcp))0; })
NAK. Absolutely not.
No way in hell is it acceptable to make this a run-time BUG. If it
doesn't work, it needs to be a compile failure. End of story.
Linus
On Sun, Dec 3, 2023 at 11:19 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Mon, 4 Dec 2023 at 07:12, Uros Bizjak <ubizjak@gmail.com> wrote:
> >
> > +/*
> > + * The generic per-cpu infrastrucutre is not suitable for
> > + * reading const-qualified variables.
> > + */
> > +#define this_cpu_read_const(pcp) ({ BUG(); (typeof(pcp))0; })
>
> NAK. Absolutely not.
>
> No way in hell is it acceptable to make this a run-time BUG. If it
> doesn't work, it needs to be a compile failure. End of story.
Thanks - BUILD_BUG() also works here and is indeed more appropriate:
/**
* BUILD_BUG - break compile if used.
*
* If you have some code that you expect the compiler to eliminate at
* build time, you should use BUILD_BUG to detect if it is
* unexpectedly used.
*/
v2 is in testing.
Thanks,
Uros.
@@ -37,12 +37,13 @@ static_assert(sizeof(struct pcpu_hot) == 64);
DECLARE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot);
/* const-qualified alias to pcpu_hot, aliased by linker. */
-extern const struct pcpu_hot __percpu_seg_override const_pcpu_hot;
+DECLARE_PER_CPU_ALIGNED(const struct pcpu_hot __percpu_seg_override,
+ const_pcpu_hot);
static __always_inline struct task_struct *get_current(void)
{
if (IS_ENABLED(CONFIG_USE_X86_SEG_SUPPORT))
- return const_pcpu_hot.current_task;
+ return this_cpu_read_const(const_pcpu_hot.current_task);
return this_cpu_read_stable(pcpu_hot.current_task);
}
@@ -28,6 +28,7 @@
#else /* ...!ASSEMBLY */
+#include <linux/bug.h>
#include <linux/kernel.h>
#include <linux/stringify.h>
@@ -462,6 +463,7 @@ do { \
#define this_cpu_write_8(pcp, val) __raw_cpu_write(volatile, pcp, val)
#endif
+#define this_cpu_read_const(pcp) __raw_cpu_read(, pcp)
#else /* CONFIG_USE_X86_SEG_SUPPORT */
#define raw_cpu_read_1(pcp) percpu_from_op(1, , "mov", pcp)
@@ -486,6 +488,11 @@ do { \
#define this_cpu_write_8(pcp, val) percpu_to_op(8, volatile, "mov", (pcp), val)
#endif
+/*
+ * The generic per-cpu infrastrucutre is not suitable for
+ * reading const-qualified variables.
+ */
+#define this_cpu_read_const(pcp) ({ BUG(); (typeof(pcp))0; })
#endif /* CONFIG_USE_X86_SEG_SUPPORT */
#define raw_cpu_add_1(pcp, val) percpu_add_op(1, , (pcp), val)
@@ -534,7 +534,7 @@ static __always_inline unsigned long current_top_of_stack(void)
* entry trampoline.
*/
if (IS_ENABLED(CONFIG_USE_X86_SEG_SUPPORT))
- return const_pcpu_hot.top_of_stack;
+ return this_cpu_read_const(const_pcpu_hot.top_of_stack);
return this_cpu_read_stable(pcpu_hot.top_of_stack);
}