[PATCH] arm64: smp: Prevent raw_smp_processor_id() recursion

[Robin Murphy]

Under CONFIG_DEBUG_PREEMPT=y, this_cpu_ptr() ends up calling back into
raw_smp_processor_id(), resulting in some hilariously catastrophic
infinite recursion. In the normal case, we have:

  #define this_cpu_ptr(ptr) raw_cpu_ptr(ptr)

and everything is dandy. However for CONFIG_DEBUG_PREEMPT, this_cpu_ptr()
is defined in terms of my_cpu_offset, wherein the fun begins:

  #define my_cpu_offset per_cpu_offset(smp_processor_id())
  ...
  #define smp_processor_id() debug_smp_processor_id()
  ...
  notrace unsigned int debug_smp_processor_id(void)
  {
  	return check_preemption_disabled("smp_processor_id", "");
  ...
  notrace static unsigned int check_preemption_disabled(const char *what1,
  							const char *what2)
  {
  	int this_cpu = raw_smp_processor_id();

and bang. Use raw_cpu_ptr() directly to avoid that.

Reported-by: Marek Szyprowski <m.szyprowski at samsung.com>
Acked-by: Will Deacon <will.deacon at arm.com>
Signed-off-by: Robin Murphy <robin.murphy at arm.com>
---

Since I just reproduced this locally to verify Will's suggestion, it
seemed I might as well just write it up as a patch :)

 arch/arm64/include/asm/smp.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/smp.h b/arch/arm64/include/asm/smp.h
index a62db952ffcb..d050d720a1b4 100644
--- a/arch/arm64/include/asm/smp.h
+++ b/arch/arm64/include/asm/smp.h
@@ -41,8 +41,10 @@ DECLARE_PER_CPU_READ_MOSTLY(int, cpu_number);
  * We don't use this_cpu_read(cpu_number) as that has implicit writes to
  * preempt_count, and associated (compiler) barriers, that we'd like to avoid
  * the expense of. If we're preemptible, the value can be stale at use anyway.
+ * And we can't use this_cpu_ptr() either, as that winds up recursing back
+ * here under CONFIG_DEBUG_PREEMPT=y.
  */
-#define raw_smp_processor_id() (*this_cpu_ptr(&cpu_number))
+#define raw_smp_processor_id() (*raw_cpu_ptr(&cpu_number))
 
 struct seq_file;
 
-- 
2.10.2.dirty