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Date:   Tue, 24 Nov 2020 09:31:56 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Singh, Balbir" <bsingharora@gmail.com>
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
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        Aubrey Li <aubrey.intel@gmail.com>, tglx@linutronix.de,
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        Tim Chen <tim.c.chen@intel.com>
Subject: Re: [PATCH -tip 03/32] sched/fair: Fix pick_task_fair crashes due to
 empty rbtree
Message-ID: <20201124083156.GU3021@hirez.programming.kicks-ass.net>
References: <20201117232003.3580179-1-joel@joelfernandes.org>
 <20201117232003.3580179-4-joel@joelfernandes.org>
 <cab6918c-7b52-923c-4274-f92e9f0a5cd2@gmail.com>
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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.