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

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>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	Vineeth Pillai <viremana@linux.microsoft.com>,
	Aaron Lu <aaron.lwe@gmail.com>,
	Aubrey Li <aubrey.intel@gmail.com>,
	tglx@linutronix.de, linux-kernel@vger.kernel.org,
	mingo@kernel.org, torvalds@linux-foundation.org,
	fweisbec@gmail.com, keescook@chromium.org, kerrnel@google.com,
	Phil Auld <pauld@redhat.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Mel Gorman <mgorman@techsingularity.net>,
	Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	vineeth@bitbyteword.org, Chen Yu <yu.c.chen@intel.com>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Agata Gruza <agata.gruza@intel.com>,
	Antonio Gomez Iglesias <antonio.gomez.iglesias@intel.com>,
	graf@amazon.com, konrad.wilk@oracle.com, dfaggioli@suse.com,
	pjt@google.com, rostedt@goodmis.org, derkling@google.com,
	benbjiang@tencent.com,
	Alexandre Chartre <alexandre.chartre@oracle.com>,
	James.Bottomley@hansenpartnership.com, OWeisse@umich.edu,
	Dhaval Giani <dhaval.giani@oracle.com>,
	Junaid Shahid <junaids@google.com>,
	jsbarnes@google.com, chris.hyser@oracle.com,
	Ben Segall <bsegall@google.com>, Josh Don <joshdon@google.com>,
	Hao Luo <haoluo@google.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Aubrey Li <aubrey.li@linux.intel.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Tim Chen <tim.c.chen@intel.com>
Subject: Re: [PATCH -tip 03/32] sched/fair: Fix pick_task_fair crashes due to empty rbtree
Date: Tue, 24 Nov 2020 09:31:56 +0100	[thread overview]
Message-ID: <20201124083156.GU3021@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <cab6918c-7b52-923c-4274-f92e9f0a5cd2@gmail.com>

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.