Re: [llvmlinux] percpu | bitmap issue? (Cannot boot on bare metal due to a kernel NULL pointer dereference)

[Sedat Dilek]

On Wed, Sep 9, 2015 at 5:25 AM, Baoquan He <bhe@xxxxxxxxxx> wrote:
> Hi Sedat,
>
> On 09/09/15 at 04:51am, Sedat Dilek wrote:
>> On Wed, Sep 9, 2015 at 4:29 AM, Baoquan He <bhe@xxxxxxxxxx> wrote:
>> commit 1a1d48a4a8fde49aedc045d894efe67173d59fe0
>> "linux/bitmap: Force inlining of bitmap weight functions"
>>
>> ...on top of Linux v4.2.
>>
>> This resulted in the same call-trace in QEMU.
>>
>> I hacked around to only re-build mm/percpu.c with GCC (rest with
>> CLANG) with some guidance from Linus (compiler warapper-script) etc.
>
> Sorry, from below log message and code flow I didn't get what's wrong
> with it. I am working on another issue which gives me much headache,
> don't hvae time to look into the the disassembling code now. But if GCC
> built code works, it should be related to compiler issues. You can try
> more tests, e.g build percpu.c bitmap.c with GCC. Maybe other people
> can give suggestions.
>
> Sorry again, Sedat.
>

Hi Baoquan,

OK, I understand your situation and kernel folks are within
merge-window or you have to concentrate on your daily work etc.

I have found some more playground

The article "Force a function to be inline in Clang/LLVM" from
<http://stackoverflow.com> in [1] says...

"There is a good solution if compiling with C99 which is Clang's
default. Its simply using inline attribute.

inline void foo() {}

[...]

So in order to guarantee that the function is inlined:

1. Donât use static inline.
2. Donât add another implementation for the function that doesn't have
inline attribute.
3. You must use optimization. But even if there isn't optimization the
compilation will fail which is good.
4. Make sure not to compile with GNU89."

So I added to Linux main Makefile...

$ git diff
diff --git a/Makefile b/Makefile
index 0e333fd142a1..e1723367ece4 100644
--- a/Makefile
+++ b/Makefile
@@ -702,6 +702,14 @@ KBUILD_CFLAGS += $(call cc-disable-warning,
tautological-compare)
 # See modpost pattern 2
 KBUILD_CFLAGS += $(call cc-option, -mno-global-merge,)

+# Force a function to be inline in Clang/LLVM
+# http://stackoverflow.com/questions/25602813/force-a-function-to-be-inline-in-clang-llvm
+# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66122
+# http://marc.info/?t=144156156700001&r=1&w=2
+ifdef CONFIG_OPTIMIZE_INLINING
+KBUILD_CFLAGS += $(call cc-option,-always-inline)
+endif
+
 else

 KBUILD_CFLAGS += $(call cc-option,-fno-delete-null-pointer-checks,)

Some comments below...

Thanks in advance!

> setup_percpu: NR_CPUS:256 nr_cpumask_bits:256 nr_cpu_ids:1 nr_node_ids:1
>
> arch/x86/kernel/setup_percpu.c:setup_per_cpu_areas() ->
> mm/percpu.c : pcpu_embed_first_chunk() ->
> mm/percpu.c:pcpu_build_alloc_info() ->
> include/linux/cpumask.h:
> #define num_possible_cpus()     cpumask_weight(cpu_possible_mask)
> static inline unsigned int cpumask_weight(const struct cpumask *srcp)
> {
>         return bitmap_weight(cpumask_bits(srcp), nr_cpumask_bits);
> }
>

s/inline/__always_inline/ ???

> include/linux/bitmap.h:
> static inline int bitmap_weight(const unsigned long *src, unsigned int nbits)
> {
>         if (small_const_nbits(nbits))
>                 return hweight_long(*src & BITMAP_LAST_WORD_MASK(nbits));
>         return __bitmap_weight(src, nbits);
> }
>

s/inline/__always_inline/ ???

Already realized in "linux/bitmap: Force inlining of bitmap weight
functions" upstream (see [3], but not available for Linux v4.2).

> lib/bitmap.c:
> int __bitmap_weight(const unsigned long *bitmap, unsigned int bits)
> {
>         unsigned int k, lim = bits/BITS_PER_LONG;
>         int w = 0;
>
>         for (k = 0; k < lim; k++)
>                 w += hweight_long(bitmap[k]);
>
>         if (bits % BITS_PER_LONG)
>                 w += hweight_long(bitmap[k] & BITMAP_LAST_WORD_MASK(bits));
>
>         return w;
> }
> EXPORT_SYMBOL(__bitmap_weight);
>

Thanks again.

- Sedat -

[1] http://stackoverflow.com/questions/25602813/force-a-function-to-be-inline-in-clang-llvm
[2] http://git.linuxfoundation.org/?p=llvmlinux.git;a=blob;f=arch/all/patches/compiler-gcc.patch
[3] http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=1a1d48a4a8fde49aedc045d894efe67173d59fe0
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