Re: next/master boot bisection: next-20190215 on beaglebone-black

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Apr 16, 2019, at 3:17 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:

> ----- On Apr 16, 2019, at 2:54 PM, Dan Williams dan.j.williams@intel.com wrote:
> 
>> On Thu, Apr 11, 2019 at 1:54 PM Guenter Roeck <groeck@google.com> wrote:
>> [..]
>>> > > Boot tests report
>>> > >
>>> > > Qemu test results:
>>> > >     total: 345 pass: 345 fail: 0
>>> > >
>>> > > This is on top of next-20190410 with CONFIG_SHUFFLE_PAGE_ALLOCATOR=y
>>> > > and the known crashes fixed.
>>> >
>>> > In addition to CONFIG_SHUFFLE_PAGE_ALLOCATOR=y you also need the
>>> > kernel command line option "page_alloc.shuffle=1"
>>> >
>>> > ...so I doubt you are running with shuffling enabled. Another way to
>>> > double check is:
>>> >
>>> >    cat /sys/module/page_alloc/parameters/shuffle
>>>
>>> Yes, you are right. Because, with it enabled, I see:
>>>
>>> Kernel command line: rdinit=/sbin/init page_alloc.shuffle=1 panic=-1
>>> console=ttyAMA0,115200 page_alloc.shuffle=1
>>> ------------[ cut here ]------------
>>> WARNING: CPU: 0 PID: 0 at ./include/linux/jump_label.h:303
>>> page_alloc_shuffle+0x12c/0x1ac
>>> static_key_enable(): static key 'page_alloc_shuffle_key+0x0/0x4' used
>>> before call to jump_label_init()
>> 
>> This looks to be specific to ARM never having had to deal with
>> DEFINE_STATIC_KEY_TRUE in the past.
>> 
>> I am able to avoid this warning by simply not enabling JUMP_LABEL
>> support in my build.
> 
> How large is your kernel image in memory ? Is it larger than 32MB
> by any chance ?
> 
> On arm, the arch_static_branch() uses a "nop" instruction, which seems
> fine. However, I have a concern wrt arch_static_branch_jump():
> 
> arch/arm/include/asm/jump_label.h defines:
> 
> static __always_inline bool arch_static_branch_jump(struct static_key *key, bool
> branch)
> {
>        asm_volatile_goto("1:\n\t"
>                 WASM(b) " %l[l_yes]\n\t"
>                 ".pushsection __jump_table,  \"aw\"\n\t"
>                 ".word 1b, %l[l_yes], %c0\n\t"
>                 ".popsection\n\t"
>                 : :  "i" (&((char *)key)[branch]) :  : l_yes);
> 
>        return false;
> l_yes:
>        return true;
> }
> 
> Which should work fine as long as the branch target is within +/-32MB range of
> the branch instruction. However, based on
> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0489e/Cihfddaf.html
> :
> 
> "Extending branch ranges
> 
> Machine-level B and BL instructions have restricted ranges from the address of
> the current instruction. However, you can use these instructions even if label
> is out of range. Often you do not know where the linker places label. When
> necessary, the linker adds code to enable longer branches. The added code is
> called a veneer."
> 
> So if by an odd chance this branch is turned into a longer branch by the linker,
> then
> the code pattern would be completely unexpected by arch/arm/kernel/jump_label.c.
> 
> Can you try with the following (untested) patch ?

The logic in my previous patch was bogus. Here is an updated version (untested):

diff --git a/arch/arm/include/asm/jump_label.h b/arch/arm/include/asm/jump_label.h
index e12d7d096fc0..7c35f57b72c5 100644
--- a/arch/arm/include/asm/jump_label.h
+++ b/arch/arm/include/asm/jump_label.h
@@ -23,12 +23,21 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
        return true;
 }
 
+/*
+ * The linker adds veneer code if target of the branch is beyond +/-32MB
+ * range, so ensure we never patch a branch instruction which target is
+ * outside of the inline asm.
+ */
 static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)
 {
        asm_volatile_goto("1:\n\t"
+                WASM(nop) "\n\t"
+                WASM(b) "2f\n\t"
+               "3:\n\t"
                 WASM(b) " %l[l_yes]\n\t"
+               "2:\n\t"
                 ".pushsection __jump_table,  \"aw\"\n\t"
-                ".word 1b, %l[l_yes], %c0\n\t"
+                ".word 1b, 3b, %c0\n\t"
                 ".popsection\n\t"
                 : :  "i" (&((char *)key)[branch]) :  : l_yes);

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com