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

    --
    Josh

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