From: Peter Zijlstra Date: Thu, 10 Oct 2013 18:17:22 +0000 (+0200) Subject: sched: Fix race in migrate_swap_stop() X-Git-Tag: omap-for-v3.13/fixes-for-merge-window-take2~40^2~20 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=746023159c40c523b08a3bc3d213dac212385895;p=pandora-kernel.git sched: Fix race in migrate_swap_stop() There is a subtle race in migrate_swap, when task P, on CPU A, decides to swap places with task T, on CPU B. Task P: - call migrate_swap Task T: - go to sleep, removing itself from the runqueue Task P: - double lock the runqueues on CPU A & B Task T: - get woken up, place itself on the runqueue of CPU C Task P: - see that task T is on a runqueue, and pretend to remove it from the runqueue on CPU B Now CPUs B & C both have corrupted scheduler data structures. This patch fixes it, by holding the pi_lock for both of the tasks involved in the migrate swap. This prevents task T from waking up, and placing itself onto another runqueue, until after migrate_swap has released all locks. This means that, when migrate_swap checks, task T will be either on the runqueue where it was originally seen, or not on any runqueue at all. Migrate_swap deals correctly with of those cases. Tested-by: Joe Mario Acked-by: Mel Gorman Reviewed-by: Rik van Riel Signed-off-by: Peter Zijlstra Cc: hannes@cmpxchg.org Cc: aarcange@redhat.com Cc: srikar@linux.vnet.ibm.com Cc: tglx@linutronix.de Cc: hpa@zytor.com Link: http://lkml.kernel.org/r/20131010181722.GO13848@laptop.programming.kicks-ass.net Signed-off-by: Ingo Molnar --- Reading git-diff-tree failed