Hi Johannes, On 12/05/14 15:38, Johannes Berg wrote: >> Unfortunately this breaks the build of today's linux-next for the Meta >> architecture (arch/metag), which happens to use a fairly old compiler >> (based on gcc 4.2.4) which I presume is the reason why. > > That's very odd. > > Unfortunately, I don't have most of arch/metag, it seems, where could I > get it? In particular no gup.c exists for metag in Linus's current tree. Hmm, mm/gup.c appears to be a new addition in linux-next from commit 3284cee59933 (mm: move get_user_pages()-related code to separate file) so probably wasn't the best example. My build output was from commit 0bed496ac091 (compiler.h: don't use temporary variable in __compiletime_assert()) which is the first bad commit according to a bisection of linux-next/stable..linux-next/master. >> A bunch of compile time asserts fail, even in code which should be >> optimised out. E.g. here's one which I analysed: >> >> mm/gup.c: In function ‘follow_page_mask’: >> mm/gup.c:208: error: size of array ‘type name’ is negative >> >> Line 208 uses HPAGE_PMD_NR which expands to a HPAGE_PMD_SHIFT, which >> expands to a BUILD_BUG(). However that line is inside an if block >> conditioned on pmd_trans_huge(*pmd) which include/asm-generic/pgtable.h >> defines inline to return 0, so the whole block should already be being >> optimised out. >> >> I don't understand why your patch should break things, I suspect it's >> related to the sparse behaviour you're trying to work around, but can we >> please drop this patch until a more portable workaround can be found? >> I'm happy to test further patches with metag if it helps. > > I don't really understand that either - if the compiler could prove that > the assignment to __cond was a constant, and remember that __cond is now > constant, I don't really see why it can't follow that through and > consider "!(condition)" a const?? > > I suppose the other option for the original problem is to ignore > _compiletime_assert() for sparse, like we do for BUG_ON(), but it'd > probably be good to analyse more why this particular code is broken now. The first one I analysed was strange too (the fixmap.h one). It appears that this particular assert was questionable anyway for metag which is why I didn't mention it, the case above is much more clear cut. Given an unsigned int idx argument the inline function fix_to_virt basically did: BUILD_BUG_ON(idx >= __end_of_fixed_addresses) where __end_of_fixed_addresses is an enum value which is 0 when CONFIG_HIGHMEM=n. In that case it took your patch for the compiler to apparently realise that an unsigned int is always >= 0, therefore the BUILD_BUG_ON will always fire, even though nothing actually called fix_to_virt from that source file so the code wasn't being used. I briefly attempted to reproduce this issue on other arches with newer compilers without success. Cheers James -- To unsubscribe from this list: send the line "unsubscribe linux-metag" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
