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Date: Wed, 8 Feb 2017 13:19:32 -0500 From: Tejun Heo <htejun@...il.com> To: Andrew Morton <akpm@...ux-foundation.org> Cc: kbuild test robot <lkp@...el.com>, Michal Hocko <mhocko@...nel.org>, kbuild-all@...org, Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>, Hillf Danton <hillf.zj@...baba-inc.com>, linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>, Michal Hocko <mhocko@...e.com> Subject: Re: [PATCH] mm-page_alloc-use-static-global-work_struct-for-draining-per-cpu-pages-fix Hello, Andrew. On Tue, Feb 07, 2017 at 02:14:20PM -0800, Andrew Morton wrote: > > extern __PCPU_DUMMY_ATTRS char __pcpu_unique_##name; \ > > ^ > > huh, yes. The DEFINE_PER_CPU() macro is broken. Yeah, that was the trade off I had to take with percpu vars to force s390 and alpha to generate long references (GOT based addressing) for percpu variables; otherwise, they generate memory deref which is too limited to access the special percpu addresses. It's explained in include/linux/percpu-defs.h. > If you do > > foo() > { > static DEFINE_PER_CPU(int, bar); > } > > then it won't compile, as described here. It should. > > And if you do > > static DEFINE_PER_CPU(int, bar); > > then you still get global symbols (__pcpu_unique_bar). > > The kernel does the above thing in, umm, 466 places and afaict they're > all broken. If two code sites ever use the same identifier, they'll > get linkage errors. So, we have CONFIG_DEBUG_FORCE_WEAK_PER_CPU to catch those cases on archs other than s390 or alpha. > huh. Seems hard to fix. This was the only way I could come up with to support alpha and s390. All the restrictions are there to ensure that. If we can do s390 and alpha w/o the global weak reference, neither restriction is necessary. Thanks. -- tejun
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