On Wed, Jan 28, 2015 at 02:46:19PM +0000, Jan Beulich wrote: > >>> On 28.01.15 at 15:29, <tip...@zytor.com> wrote: > > Commit-ID: 81907478c4311a679849216abf723999184ab984 > > Gitweb: > > http://git.kernel.org/tip/81907478c4311a679849216abf723999184ab984 > > Author: Jan Beulich <jbeul...@suse.com> > > AuthorDate: Fri, 23 Jan 2015 08:25:38 +0000 > > Committer: Ingo Molnar <mi...@kernel.org> > > CommitDate: Wed, 28 Jan 2015 13:14:12 +0100 > > > > sched/fair: Avoid using uninitialized variable in preferred_group_nid() > > > > At least some gcc versions - validly afaict - warn about potentially > > using max_group uninitialized: There's no way the compiler can prove > > that the body of the conditional where it and max_faults get set/ > > updated gets executed; in fact, without knowing all the details of > > other scheduler code, I can't prove this either. > > > > Generally the necessary change would appear to be to clear max_group > > prior to entering the inner loop, and break out of the outer loop when > > it ends up being all clear after the inner one. This, however, seems > > inefficient, and afaict the same effect can be achieved by exiting the > > outer loop when max_faults is still zero after the inner loop. > > > > [ mingo: changed the solution to zero initialization: uninitialized_var() > > needs to die, as it's an actively dangerous construct: if in the future > > a known-proven-good piece of code is changed to have a true, buggy > > uninitialized variable, the compiler warning is then supressed... > > But you went farther than that: You also dropped the breaking > out of the outer loop. Yet that has - beyond the fixing of the bug > here - the desirable effect of not continuing for perhaps many > iterations when nothing new can ever be found anymore.
That break is indeed desired. The 'problem' it fixes is that when group_faults() returns 0, faults will be 0, which will not > max_faults, and therefore we will not set max_group. Without that break, we'll now set nodes to NODE_MASK_NONE, which will mean the for_each_node(a, nodes) loop will NOP and our dist loop will iterate pointlessly. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
