On 02/22/2017 12:38 AM, Michael Ellerman wrote:

Jason Baron [off-list ref] writes:

quoted

On 02/20/2017 10:05 PM, Sachin Sant wrote:

quoted

quoted

On 20-Feb-2017, at 8:27 PM, Jason Baron <jbaron@akamai.com
<mailto:jbaron@akamai.com>> wrote:

Hi,

On 02/19/2017 09:07 AM, Sachin Sant wrote:

quoted

While booting next-20170217 on a POWER8 LPAR following
warning is displayed.

Reverting the following commit helps boot cleanly.
commit 3821fd35b5 :  jump_label: Reduce the size of struct static_key

[   11.393008] ------------[ cut here ]------------
[   11.393031] WARNING: CPU: 5 PID: 2890 at kernel/jump_label.c:287
static_key_set_entries.isra.10+0x3c/0x50

Thanks for the report. So this is saying that the jump_entry table is
not at least 4-byte aligned. I wonder if this fixes it up?

Yes. With this patch the warning is gone.

Hi,

Thanks for testing. We probably need something like the following to 
make sure we don't hit this on other arches. Steve - I will send 4 
separate patches for this to get arch maintainers' acks for this?

What's the 4 byte alignment requirement from?

The 4 byte alignment is coming from this patch in linux-next:

https://lkml.org/lkml/2017/2/3/558

It reduces the size of 'struct static_key' by making use of the two
least significant bits of the static_key::entry pointer. Thus, the
jump_entry table needs to be 4 byte aligned to make it work. I added a
WARN_ON() to make sure the jump_entry table is in fact 4 byte aligned,
and that is what we hit here.

On 64-bit our JUMP_ENTRY_TYPE is 8 bytes, should we be aligning to 8
bytes?

4 bytes should be sufficient and apparently fixes the WARN_ON() that was
hit.

quoted

diff --git a/arch/powerpc/include/asm/jump_label.h 

b/arch/powerpc/include/asm/jump_label.h
index 9a287e0ac8b1..f870a85bac46 100644

--- a/arch/powerpc/include/asm/jump_label.h
+++ b/arch/powerpc/include/asm/jump_label.h

@@ -24,6 +24,7 @@ static __always_inline bool arch_static_branch(struct 

static_key *key, bool bran
         asm_volatile_goto("1:\n\t"
                  "nop # arch_static_branch\n\t"
                  ".pushsection __jump_table,  \"aw\"\n\t"
+                ".balign 4 \n\t"

Can you line those up vertically?

(That may just be an email artifact)

sure will fix.

Thanks,

-Jason

quoted

                  JUMP_ENTRY_TYPE "1b, %l[l_yes], %c0\n\t"
                  ".popsection \n\t"
                  : :  "i" (&((char *)key)[branch]) : : l_yes);

@@ -38,6 +39,7 @@ static __always_inline bool 

arch_static_branch_jump(struct static_key *key, bool
         asm_volatile_goto("1:\n\t"
                  "b %l[l_yes] # arch_static_branch_jump\n\t"
                  ".pushsection __jump_table,  \"aw\"\n\t"
+                ".balign 4 \n\t"
                  JUMP_ENTRY_TYPE "1b, %l[l_yes], %c0\n\t"
                  ".popsection \n\t"
                  : :  "i" (&((char *)key)[branch]) : : l_yes);

@@ -63,6 +65,7 @@ struct jump_entry {
  #define ARCH_STATIC_BRANCH(LABEL, KEY)         \
  1098:  nop;                                    \
         .pushsection __jump_table, "aw";        \
+       .balign 4;                              \
         FTR_ENTRY_LONG 1098b, LABEL, KEY;       \
         .popsection
  #endif

Otherwise that looks fine assuming 4 bytes is the correct alignment.

cheers