* Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > On Sat, Apr 16, 2016 at 11:03:32AM +0200, Ingo Molnar wrote: > > > > * Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > > > > > > I don't think we know yet if there's a reliable way to turn the bug off. > > > > > > > > Also, according to the gcc guys, this bug won't always result in a > > > > truncated function, and may sometimes just make some inline function > > > > call sites disappear: > > > > > > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646#c14 > > > > > > > > though I haven't been able to confirm that experimentally. But if it's > > > > true, that means that objtool won't be able to detect all cases of the > > > > bug and some function calls may just silently disappear! > > > > > > > > There's a lot of activity in the bug now, so hopefully they'll be able > > > > to tell us soon if there's a reliable way to avoid it and/or detect it. > > > > > > > > BTW, Denys posted a workaround patch for the qla2xxxx code: > > > > > > > > https://lkml.kernel.org/r/1460716583-15673-1-git-send-email-dvlasenk@xxxxxxxxxx > > > > > > Martin Jambor wrote a succinct summary of the conditions needed for this > > > bug: > > > > > > "This bug can occur when an inlineable function containing a call to > > > __builtin_constant_p, which checks a parameter or a value it > > > references and a (possibly indirect) caller of the function actually > > > passes a constant, but stores it using a type of a different size." > > > > > > So to prevent it from happening elsewhere in the kernel, it sounds like > > > we'd have to either remove all uses of __builtin_constant_p() or disable > > > inlining completely. > > > > > > There's also no reliable way to detect the bug has occurred, though > > > objtool will detect it in cases when the function gets truncated. > > > > So it appears to me that due to the hard to detect nature of the GCC bug the fix > > will probably be backported by them, so I think we should be fine with relying on > > objtool to detect weird code sequences in the kernel, and should work around > > specific instances of the bug. > > I agree. So how should we work around the bug in this case? There have > been several suggestions: > > - change wwn_to_u64() to __always_inline > > - change qla2x00_get_host_fabric_name() to skip the unnecessary call to > wwn_to_u64() > > - revert one of the two commits: > bc27fb68aaad ("include/uapi/linux/byteorder, swab: force inlining of some byteswap operations") > ef3fb2422ffe ("scsi: fc: use get/put_unaligned64 for wwn access") The first option sounds like the best one by far: it does a change that is related to the GCC bug (tweaks inlining), has near zero impact and does not revert other useful progress. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
