Re: [PATCH -tip 03/32] sched/fair: Fix pick_task_fair crashes due to empty rbtree

[Singh, Balbir]

On 18/11/20 10:19 am, Joel Fernandes (Google) wrote:
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> 
> 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