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Date: Tue, 2 Apr 2024 21:25:11 +0300
From: Aaro Koskinen <aaro.koskinen@iki.fi>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: stable@vger.kernel.org, Waiman Long <longman@redhat.com>,
	Mukesh Ojha <quic_mojha@quicinc.com>,
	Peter Zijlstra <peterz@infradead.org>, mingo@redhat.com,
	will@kernel.org, boqun.feng@gmail.com, linux-kernel@vger.kernel.org,
	Gokul krishna Krishnakumar <quic_gokukris@quicinc.com>
Subject: Re: [PATCH v2] locking/rwsem: Disable preemption while trying for
 rwsem lock
Message-ID: <20240402182511.GD91663@darkstar.musicnaut.iki.fi>
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In-Reply-To: <2024040242-babbling-dreamy-6336@gregkh>

Hi,

On Tue, Apr 02, 2024 at 07:42:03PM +0200, Greg KH wrote:
> On Tue, Apr 02, 2024 at 08:30:32PM +0300, Aaro Koskinen wrote:
> > Stable team,
> > 
> > Please cherry-pick this patch into v5.15 stable:
> > 
> > 	locking/rwsem: Disable preemption while trying for rwsem lock
> > 
> > 	commit 48dfb5d2560d36fb16c7d430c229d1604ea7d185
> > 
> > It fixes the following bug present in v5.15:
> > 
> > > > > From: Gokul krishna Krishnakumar <quic_gokukris@quicinc.com>
> > > > > We observe RT task is hogging CPU when trying to acquire rwsem lock
> > > > > which was acquired by a kworker task but before the rwsem owner was set.
> > > > > 
> > > > > Here is the scenario:
> > > > > 1. CFS task (affined to a particular CPU) takes rwsem lock.
> > > > > 
> > > > > 2. CFS task gets preempted by a RT task before setting owner.
> > > > > 
> > > > > 3. RT task (FIFO) is trying to acquire the lock, but spinning until
> > > > > RT throttling happens for the lock as the lock was taken by CFS task.
> > 
> > If the RT throttling is disabled, the RT task will remain looping forever
> > in the kernel. If the system is UP, it will lock up completely.
> > 
> > The issue can be easily reproduced by running RT task and normal task which
> > are affined to the same CPU core.
> 
> Does this only affect the -rt patchset, or is this an issue with a
> "clean" kernel release?

It's with a normal "clean" kernel.

A.