Christophe LEROY [off-list ref] writes:

Le 22/11/2017 =C3=A0 00:07, Balbir Singh a =C3=A9crit=C2=A0:

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On Wed, Nov 22, 2017 at 1:28 AM, Christophe Leroy
[off-list ref] wrote:

quoted

On powerpc32, patch_instruction() is called by apply_feature_fixups()
which is called from early_init()

There is the following note in front of early_init():
  * Note that the kernel may be running at an address which is different
  * from the address that it was linked at, so we must use RELOC/PTRREL=

OC

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  * to access static data (including strings).  -- paulus

Therefore, slab_is_available() cannot be called yet, and
text_poke_area must be addressed with PTRRELOC()

Fixes: 37bc3e5fd764f ("powerpc/lib/code-patching: Use alternate map
for patch_instruction()")
Reported-by: Meelis Roos <redacted>
Cc: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Christophe Leroy <redacted>
---
  v2: Added missing asm/setup.h

  arch/powerpc/lib/code-patching.c | 6 ++----
  1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-p=

atching.c

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index c9de03e0c1f1..d469224c4ada 100644

--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c

@@ -21,6 +21,7 @@
  #include <asm/tlbflush.h>
  #include <asm/page.h>
  #include <asm/code-patching.h>
+#include <asm/setup.h>

  static int __patch_instruction(unsigned int *addr, unsigned int instr)
  {

@@ -146,11 +147,8 @@ int patch_instruction(unsigned int *addr, unsigned=

 int instr)

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          * During early early boot patch_instruction is called
          * when text_poke_area is not ready, but we still need
          * to allow patching. We just do the plain old patching
-        * We use slab_is_available and per cpu read * via this_cpu_read
-        * of text_poke_area. Per-CPU areas might not be up early
-        * this can create problems with just using this_cpu_read()
          */
-       if (!slab_is_available() || !this_cpu_read(text_poke_area))
+       if (!this_cpu_read(*PTRRELOC(&text_poke_area)))
                 return __patch_instruction(addr, instr);

=20
On ppc64, we call apply_feature_fixups() in early_setup() after we've
relocated ourselves. Sorry for missing the ppc32 case. I would like to
avoid PTRRELOC when unnecessary.

What do you suggest then ?

Some #ifdef PPC32 around that ?

No I don't think that improves anything.

I think the comment about per-cpu not being up is wrong, you'll just get
the static version of text_poke_area, which should be NULL. So we don't
need the slab_available() check anyway.

So I'll take this as-is.

Having said that I absolutely hate PTRRELOC, so if it starts spreading
we will have to come up with something less bug prone.

cheers