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Date: Fri, 17 Jan 2020 15:26:49 -0600 From: Josh Poimboeuf <jpoimboe@...hat.com> To: Marco Elver <elver@...gle.com> Cc: David Sterba <dsterba@...e.cz>, Randy Dunlap <rdunlap@...radead.org>, Stephen Rothwell <sfr@...b.auug.org.au>, Linux Next Mailing List <linux-next@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Linux Btrfs <linux-btrfs@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org> Subject: Re: linux-next: Tree for Dec 6 (objtool, lots in btrfs) On Fri, Jan 17, 2020 at 09:28:27PM +0100, Marco Elver wrote: > On Fri, 17 Jan 2020 at 18:26, Josh Poimboeuf <jpoimboe@...hat.com> 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. Even for a non-inlined static function, GCC typically detects when it's implicitly "noreturn", and optimizes the call sites accordingly. And that has also been true even for CC_OPTIMIZE_FOR_SIZE in the past. So something changed apparently. (KCSAN was just a guess.) -- Josh
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