Re: [PATCH] sched: Restore the normalize_rt_tasks() cpuset_mutex exemption

[Juri Lelli <juri.lelli@redhat.com>]

Hello,

On 22/09/26 12:07, Donggeun Yoo wrote:
> sysrq-n runs normalize_rt_tasks(), which walks the task list under
> read_lock(&tasklist_lock) and hands every user RT or deadline task to
> __sched_setscheduler() with pi == false.  __sched_setscheduler() takes
> cpuset_mutex whenever the old or the new policy is deadline, and
> cpuset_lock() is a plain mutex_lock(), so a single SCHED_DEADLINE user
> task makes the sysrq handler sleep in atomic context:
> 
>   sysrq: Nice All RT Tasks
>   BUG: sleeping function called from invalid context at kernel/locking/mutex.c:623
>   in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 1, name: init
>   preempt_count: 1, expected: 0
>   RCU nest depth: 1, expected: 0
>   locks held by init/1: 3, last CPU#1:
>    #0: ffff9fa2c34e1470 (sb_writers#3){.+.+}-{0:0}, at: ksys_write+0x74/0xf0
>    #1: ffffffffb8f66540 (rcu_read_lock){....}-{1:3}, at: __handle_sysrq+0x3a/0x110
>    #2: ffffffffb8e060d8 (tasklist_lock){.+.+}-{3:3}, at: normalize_rt_tasks+0x34/0x150
>   Call Trace:
>    __might_resched.cold+0xe3/0xf5
>    __mutex_lock+0x8a/0x11c0
>    __sched_setscheduler+0x35f/0x9c0
>    normalize_rt_tasks+0xe6/0x150
>    __handle_sysrq.cold+0x9b/0xde
>    write_sysrq_trigger+0x65/0x90
> 
> Test pi along with the policy. The only pi == false caller is
> normalize_rt_tasks(), where the lock is given up since the sysrq
> emergency already voids deadline guarantees.
> 
> Fixes: 111cd11bbc54 ("sched/cpuset: Bring back cpuset_mutex")
> Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
> Cc: stable@vger.kernel.org
> ---

Ah, yes indeed.

Acked-by: Juri Lelli <juri.lelli@redhat.com>

Thanks!
Juri