Messages in this thread |  | | Date | Tue, 24 Nov 2020 09:31:56 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH -tip 03/32] sched/fair: Fix pick_task_fair crashes due to empty rbtree |
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On Fri, Nov 20, 2020 at 09:15:28PM +1100, Singh, Balbir wrote: > On 18/11/20 10:19 am, Joel Fernandes (Google) wrote: > > From: Peter Zijlstra <peterz@infradead.org> > > > > pick_next_entity() is passed curr == NULL during core-scheduling. Due to > > this, if the rbtree is empty, the 'left' variable is set to NULL within > > the function. This can cause crashes within the function. > > > > This is not an issue if put_prev_task() is invoked on the currently > > running task before calling pick_next_entity(). However, in core > > scheduling, it is possible that a sibling CPU picks for another RQ in > > the core, via pick_task_fair(). This remote sibling would not get any > > opportunities to do a put_prev_task(). > > > > Fix it by refactoring pick_task_fair() such that pick_next_entity() is > > called with the cfs_rq->curr. This will prevent pick_next_entity() from > > crashing if its rbtree is empty. > > > > Also this fixes another possible bug where update_curr() would not be > > called on the cfs_rq hierarchy if the rbtree is empty. This could effect > > cross-cpu comparison of vruntime. > > > > It is not clear from the changelog as to what does put_prev_task() do to prevent > the crash from occuring? Why did we pass NULL as curr in the first place to > pick_next_entity? > > The patch looks functionally correct as in, it passes curr as the reference > to pick_next_entity() for left and entity_before comparisons.
This patch should not exist; it should be smashed into the previous patch. There is no point in preserving a crash.
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