| Subject | Re: [PATCH -tip 03/32] sched/fair: Fix pick_task_fair crashes due to empty rbtree | From | "Singh, Balbir" <> | Date | Fri, 20 Nov 2020 21:15:28 +1100 |
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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.
Balbir Singh
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