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

From: David Sterba <[email-masked]>
Date: Tue, 17 Dec 2019 16:25:11 +0100

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.