md/raid1: fix read balance when a drive is write-mostly.
When a drive is marked write-mostly it should only be the target of reads if there is no other option. This behaviour was broken by commit 9dedf603 md/raid1: read balance chooses idlest disk for SSD which causes a write-mostly device to be *preferred* is some cases. Restore correct behaviour by checking and setting best_dist_disk and best_pending_disk rather than best_disk. We only need to test one of these as they are both changed from -1 or >=0 at the same time. As we leave min_pending and best_dist unchanged, any non-write-mostly device will appear better than the write-mostly device. Reported-by: NTomáš Hodek <tomas.hodek@volny.cz> Reported-by: NDark Penguin <darkpenguin@yandex.ru> Signed-off-by: NNeilBrown <neilb@suse.de> Link: http://marc.info/?l=linux-raid&m=135982797322422 Fixes: 9dedf603 Cc: stable@vger.kernel.org (3.6+)
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