Subject: [PATCH 00/22] cpu,sched: Mark arch_cpu_idle_dead() __noreturn

From: Josh Poimboeuf <[email-masked]>
Date: Fri, 3 Feb 2023 14:05:09 -0800

These are some minor changes to enable the __noreturn attribute for
arch_cpu_idle_dead().  (If there are no objections, I can merge the
entire set through the tip tree.)

Until recently [1], in Xen, when a previously offlined CPU was brought
back online, it unexpectedly resumed execution where it left off in the
middle of the idle loop by returning from play_dead() and its caller
arch_cpu_idle_dead().

There were some clever hacks to make that work, but the behavior was
surprising as do_idle() doesn't expect an offlined CPU to return from
the dead in the middle of the function.

Now that Xen has been fixed, make sure arch_cpu_idle_dead() never
returns by marking it __noreturn.  This causes the compiler to complain
if an arch-specific implementation might return.  It also improves code
generation for both caller and callee.

Patches 1-20 update the arch-specific implementations of the function so
they don't actually return (even in error cases), and make that visible
to the compiler.  That's typically done by ending the function with a
call to another noreturn function, or with a BUG().

Patch 21 fixes the weak implementation.

Patch 22 actually adds the __noreturn attribute.

[1] 076cbf5d2163 ("x86/xen: don't let xen_pv_play_dead() return")

Josh Poimboeuf (22):
  alpha/cpu: Expose arch_cpu_idle_dead()'s prototype declaration
  alpha/cpu: Make sure arch_cpu_idle_dead() doesn't return
  arm/cpu: Make sure arch_cpu_idle_dead() doesn't return
  arm64/cpu: Mark cpu_die() __noreturn
  csky/cpu: Make sure arch_cpu_idle_dead() doesn't return
  ia64/cpu: Mark play_dead() __noreturn
  loongarch/cpu: Make sure play_dead() doesn't return
  loongarch/cpu: Mark play_dead() __noreturn
  mips/cpu: Expose play_dead()'s prototype definition
  mips/cpu: Make sure play_dead() doesn't return
  mips/cpu: Mark play_dead() __noreturn
  powerpc/cpu: Mark start_secondary_resume() __noreturn
  sh/cpu: Make sure play_dead() doesn't return
  sh/cpu: Mark play_dead() __noreturn
  sh/cpu: Expose arch_cpu_idle_dead()'s prototype definition
  sparc/cpu: Mark cpu_play_dead() __noreturn
  x86/cpu: Make sure play_dead() doesn't return
  x86/cpu: Mark play_dead() __noreturn
  xtensa/cpu: Make sure cpu_die() doesn't return
  xtensa/cpu: Mark cpu_die() __noreturn
  sched/idle: Make sure weak version of arch_cpu_idle_dead() doesn't
    return
  sched/idle: Mark arch_cpu_idle_dead() __noreturn

 arch/alpha/kernel/process.c      | 2 ++
 arch/arm/kernel/smp.c            | 2 ++
 arch/arm64/include/asm/smp.h     | 2 +-
 arch/csky/kernel/smp.c           | 2 ++
 arch/ia64/kernel/process.c       | 4 ++--
 arch/loongarch/include/asm/smp.h | 2 +-
 arch/loongarch/kernel/smp.c      | 2 +-
 arch/mips/include/asm/smp.h      | 2 +-
 arch/mips/kernel/smp-bmips.c     | 3 +++
 arch/mips/loongson64/smp.c       | 1 +
 arch/powerpc/include/asm/smp.h   | 2 +-
 arch/sh/include/asm/smp-ops.h    | 5 +++--
 arch/sh/kernel/idle.c            | 1 +
 arch/sparc/include/asm/smp_64.h  | 2 +-
 arch/x86/include/asm/smp.h       | 3 ++-
 arch/x86/kernel/process.c        | 2 +-
 arch/xtensa/include/asm/smp.h    | 2 +-
 arch/xtensa/kernel/smp.c         | 2 ++
 include/linux/cpu.h              | 2 +-
 kernel/sched/idle.c              | 2 +-
 tools/objtool/check.c            | 1 +
 21 files changed, 31 insertions(+), 15 deletions(-)

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2.39.0