On 2022-10-26 at 13:43:00 +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?
>
> ---
> 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
Should we also add p->on_cpu check to return 0 in __set_task_frozen()?
Otherwise it might still warn that p is holding the lock?
thanks,
Chenyu
