Re: linux-next: Tree for Dec 6 (objtool, lots in btrfs)

[Marco Elver]

On Fri, 17 Jan 2020 at 18:26, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>
> On Tue, Dec 17, 2019 at 04:25:11PM +0100, David Sterba wrote:
> > On Fri, Dec 13, 2019 at 11:05:18PM -0800, Randy Dunlap wrote:
> > > OK, that fixes most of them, but still leaves these 2:
> > >
> > > btrfs006.out:fs/btrfs/extent_io.o: warning: objtool: __set_extent_bit()+0x536: unreachable instruction
> >
> > Hard to read from the assembly what C statement is it referring to. I
> > think there are also several functions inlined, I don't see anything
> > suspicious inside __set_extent_bit itself.
> >
> > > btrfs006.out:fs/btrfs/relocation.o: warning: objtool: add_tree_block()+0x501: unreachable instruction
> >
> > Probably also heavily inlined, the function has like 50 lines, a few
> > non-trivial function calls but the offset in the warning suggests a
> > larger size.
> >
> > While browsing the callees I noticed that both have in common a function
> > that is supposed to print and stop at fatal errors. They're
> > extent_io_tree_panic (extent_io.c) and backref_tree_panic
> > (relocation.c). Both call btrfs_panic which is a macro:
> >
> > 3239 #define btrfs_panic(fs_info, errno, fmt, args...)                       \
> > 3240 do {                                                                    \
> > 3241         __btrfs_panic(fs_info, __func__, __LINE__, errno, fmt, ##args); \
> > 3242         BUG();                                                          \
> > 3243 } while (0)
> >
> > There are no conditionals and BUG has the __noreturn annotation
> > (unreachable()) so all is in place and I don't have better ideas what's
> > causing the reports.
>
> I think KCSAN is somehow disabling GCC's detection of implicit noreturn
> functions -- or at least some calls to them.  So GCC is inserting dead
> code after the calls.  BUG() uses __builtin_unreachable(), so GCC should
> know better.
>
> If this is specific to KCSAN then I might just disable these warnings
> for KCSAN configs.

I noticed that this is also a CC_OPTIMIZE_FOR_SIZE config. I recently
sent some patches to turn some inlines into __always_inlines because
CC_OPTIMIZE_FOR_SIZE decides to not inline functions that should
always be inlined.

I noticed that 'assfail' is a 'static inline' function and you
mentioned earlier that GCC seems to not be able to determine if it
returns or not. If CC_OPTIMIZE_FOR_SIZE decides to not inline, then
maybe this could be a problem?  It could also be the compiler having
some trouble here with the CC_OPTIMIZE_FOR_SIZE + KCSAN combination.

Thanks,
-- Marco