Re: [PATCH v3 6/6] freezer,sched: Rewrite core freezer logic

Ville Syrjälä
2022-10-27T13:09:25Z

On Wed, Oct 26, 2022 at 01:43:00PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 26, 2022 at 01:32:31PM +0300, Ville Syrjälä wrote:
> > Short form looks to be this:
> > <4>[ 355.437846] 1 lock held by rs:main Q:Reg/359:
> > <4>[ 355.438418] #0: ffff88844693b758 (&rq->__lock){-.-.}-{2:2}, at: raw_spin_rq_lock_nested+0x1b/0x30
> > <4>[ 355.438432] rs:main Q:Reg/359 holding locks while freezing
>
> > <4>[ 355.438429] ------------[ cut here ]------------
> > <4>[ 355.438432] rs:main Q:Reg/359 holding locks while freezing
> > <4>[ 355.438439] WARNING: CPU: 0 PID: 6211 at kernel/freezer.c:134 __set_task_frozen+0x86/0xb0
> > <4>[ 355.438447] Modules linked in: snd_hda_intel i915 mei_hdcp mei_pxp drm_display_helper drm_kms_helper vgem drm_shmem_helper snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic ledtrig_audio snd_intel_dspcfg snd_hda_codec snd_hwdep snd_hda_core snd_pcm prime_numbers ttm drm_buddy syscopyarea sysfillrect sysimgblt fb_sys_fops fuse x86_pkg_temp_thermal coretemp kvm_intel btusb btrtl btbcm btintel kvm irqbypass bluetooth crct10dif_pclmul crc32_pclmul ecdh_generic ghash_clmulni_intel ecc e1000e mei_me i2c_i801 ptp mei i2c_smbus pps_core lpc_ich video wmi [last unloaded: drm_kms_helper]
> > <4>[ 355.438521] CPU: 0 PID: 6211 Comm: rtcwake Tainted: G U 6.1.0-rc2-CI_DRM_12295-g3844a56a0922+ #1
> > <4>[ 355.438526] Hardware name: /NUC5i7RYB, BIOS RYBDWi35.86A.0385.2020.0519.1558 05/19/2020
> > <4>[ 355.438530] RIP: 0010:__set_task_frozen+0x86/0xb0
> > <4>[ 355.438536] Code: 83 60 09 00 00 85 c0 74 2a 48 89 df e8 ac 02 9b 00 8b 93 38 05 00 00 48 8d b3 48 07 00 00 48 c7 c7 a0 62 2b 82 e8 ee c1 9a 00 <0f> 0b c6 05 51 75 e3 02 01 c7 43 18 00 80 00 00 b8 00 80 00 00 5b
> > <4>[ 355.438541] RSP: 0018:ffffc900012cbcf0 EFLAGS: 00010086
> > <4>[ 355.438546] RAX: 0000000000000000 RBX: ffff88810d090040 RCX: 0000000000000004
> > <4>[ 355.438550] RDX: 0000000000000004 RSI: 00000000fffff5de RDI: 00000000ffffffff
> > <4>[ 355.438553] RBP: 0000000000000000 R08: 0000000000000000 R09: c0000000fffff5de
> > <4>[ 355.438557] R10: 00000000002335f8 R11: ffffc900012cbb88 R12: 0000000000000246
> > <4>[ 355.438561] R13: ffffffff81165430 R14: 0000000000000000 R15: ffff88810d090040
> > <4>[ 355.438565] FS: 00007fcfa43c7740(0000) GS:ffff888446800000(0000) knlGS:0000000000000000
> > <4>[ 355.438569] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > <4>[ 355.438582] CR2: 00007fceb380f6b8 CR3: 0000000117c5c004 CR4: 00000000003706f0
> > <4>[ 355.438586] Call Trace:
> > <4>[ 355.438589] <TASK>
> > <4>[ 355.438592] task_call_func+0xc4/0xe0
> > <4>[ 355.438600] freeze_task+0x84/0xe0
> > <4>[ 355.438607] try_to_freeze_tasks+0xac/0x260
> > <4>[ 355.438616] freeze_processes+0x56/0xb0
> > <4>[ 355.438622] pm_suspend.cold.7+0x1d9/0x31c
> > <4>[ 355.438629] state_store+0x7b/0xe0
> > <4>[ 355.438637] kernfs_fop_write_iter+0x124/0x1c0
> > <4>[ 355.438644] vfs_write+0x34f/0x4e0
> > <4>[ 355.438655] ksys_write+0x57/0xd0
> > <4>[ 355.438663] do_syscall_64+0x3a/0x90
> > <4>[ 355.438670] entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
> Oh I think I see what's going on.
>
> It's a very narrow race between schedule() and task_call_func().
>
> CPU0 CPU1
>
> __schedule()
> rq_lock();
> prev_state = READ_ONCE(prev->__state);
> if (... && prev_state) {
> deactivate_tasl(rq, prev, ...)
> prev->on_rq = 0;
>
> task_call_func()
> raw_spin_lock_irqsave(p->pi_lock);
> state = READ_ONCE(p->__state);
> smp_rmb();
> if (... || p->on_rq) // false!!!
> rq = __task_rq_lock()
>
> ret = func();
>
> next = pick_next_task();
> rq = context_switch(prev, next)
> prepare_lock_switch()
> spin_release(&__rq_lockp(rq)->dep_map...)
>
>
>
> So while the task is on it's way out, it still holds rq->lock for a
> little while, and right then task_call_func() comes in and figures it
> doesn't need rq->lock anymore (because the task is already dequeued --
> but still running there) and then the __set_task_frozen() thing observes
> it's holding rq->lock and yells murder.
>
> Could you please give the below a spin?

Thanks. I've added this to our CI branch. I'll try to keep and eye
on it in the coming days and let you know if anything still trips.
And I'll report back maybe ~middle of next week if we haven't caught
anything by then.

>
> ---
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index cb2aa2b54c7a..f519f44cd4c7 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4200,6 +4200,37 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> return success;
> }
>
> +static bool __task_needs_rq_lock(struct task_struct *p)
> +{
> + unsigned int state = READ_ONCE(p->__state);
> +
> + /*
> + * Since pi->lock blocks try_to_wake_up(), we don't need rq->lock when
> + * the task is blocked. Make sure to check @state since ttwu() can drop
> + * locks at the end, see ttwu_queue_wakelist().
> + */
> + if (state == TASK_RUNNING || state == TASK_WAKING)
> + return true;
> +
> + /*
> + * Ensure we load p->on_rq after p->__state, otherwise it would be
> + * possible to, falsely, observe p->on_rq == 0.
> + *
> + * See try_to_wake_up() for a longer comment.
> + */
> + smp_rmb();
> + if (p->on_rq)
> + return true;
> +
> +#ifdef CONFIG_SMP
> + smp_rmb();
> + if (p->on_cpu)
> + return true;
> +#endif
> +
> + return false;
> +}
> +
> /**
> * task_call_func - Invoke a function on task in fixed state
> * @p: Process for which the function is to be invoked, can be @current.
> @@ -4217,28 +4248,12 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> int task_call_func(struct task_struct *p, task_call_f func, void *arg)
> {
> struct rq *rq = NULL;
> - unsigned int state;
> struct rq_flags rf;
> int ret;
>
> raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
>
> - state = READ_ONCE(p->__state);
> -
> - /*
> - * Ensure we load p->on_rq after p->__state, otherwise it would be
> - * possible to, falsely, observe p->on_rq == 0.
> - *
> - * See try_to_wake_up() for a longer comment.
> - */
> - smp_rmb();
> -
> - /*
> - * Since pi->lock blocks try_to_wake_up(), we don't need rq->lock when
> - * the task is blocked. Make sure to check @state since ttwu() can drop
> - * locks at the end, see ttwu_queue_wakelist().
> - */
> - if (state == TASK_RUNNING || state == TASK_WAKING || p->on_rq)
> + if (__task_needs_rq_lock(p))
> rq = __task_rq_lock(p, &rf);
>
> /*

--
Ville Syrjälä
Intel