Subject: Re: context switch within RCU read-side critical section in next-20260518+ with PREEMPT_RT

From: Bert Karwatzki <[email-masked]>
Date: Thu, 21 May 2026 12:21:52 +0200

Am Donnerstag, dem 21.05.2026 um 12:17 +0200 schrieb Thomas Gleixner:
> On Thu, May 21 2026 at 11:20, Bert Karwatzki wrote:
> > Am Donnerstag, dem 21.05.2026 um 11:09 +0200 schrieb Mateusz Guzik:
> > 
> > with next-20260519 (no RT, no LOCKDEP) and got no crash so far (4 boots only though (next-20260619
> > crashed in 2 out of 3 boots without RT)) but I get this warning on every boot:
> > 
> > [    2.793416] [    T331] ------------[ cut here ]------------
> > [    2.793433] [    T331] DEBUG_LOCKS_WARN_ON(lock->magic != lock)
> > [    2.793434] [    T331] WARNING: kernel/locking/mutex.c:625 at __mutex_lock+0x586/0x10c0, CPU#17: (udev-worker)/331
> 
> So either the mutex is corrupted or was never initialized.
> 
> > [    2.793463] [    T331] Modules linked in: amdgpu(+) hid_generic usbhid drm_client_lib i2c_algo_bit drm_buddy hid drm_ttm_helper ttm drm_exec
> > drm_suballoc_helper mfd_core drm_panel_backlight_quirks gpu_sched amdxcp drm_display_helper drm_kms_helper ahci libahci xhci_pci libata xhci_hcd drm nvme
> > scsi_mod igc usbcore nvme_core scsi_common video nvme_keyring i2c_piix4 cec nvme_auth usb_common crc16 i2c_smbus wmi gpio_amdpt gpio_generic
> > [    2.793518] [    T331] CPU: 17 UID: 0 PID: 331 Comm: (udev-worker) Not tainted 7.1.0-rc4-next-20260519-rcunortlockdep-dirty #465 PREEMPT 
> > [    2.793534] [    T331] Hardware name: ASUS System Product Name/ROG STRIX B850-F GAMING WIFI, BIOS 1627 02/05/2026
> > [    2.793547] [    T331] RIP: 0010:__mutex_lock+0x58d/0x10c0
> > [    2.793555] [    T331] Code: 4c 8b 4d 88 85 c0 0f 84 f8 fa ff ff 44 8b 15 ca 9b 81 00 45 85 d2 0f 85 e8 fa ff ff 48 8d 3d 1a 57 82 00 48 c7 c6 a6 51 9e 83
> > <67> 48 0f b9 3a 4c 8b 4d 88 e9 cc fa ff ff 48 8b bd 78 ff ff ff e8
> > [    2.793579] [    T331] RSP: 0018:ffffa497016c3510 EFLAGS: 00010246
> > [    2.793588] [    T331] RAX: 0000000000000001 RBX: ffff88c33a4c2ad8 RCX: 0000000000000000
> > [    2.793598] [    T331] RDX: 0000000000000001 RSI: ffffffff839e51a6 RDI: ffffffff83de3c00
> > [    2.793609] [    T331] RBP: ffffa497016c35c0 R08: ffffffffc0a55d92 R09: 0000000000000000
> > [    2.793619] [    T331] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> > [    2.793629] [    T331] R13: 0000000000000002 R14: ffffa497016c3550 R15: 0000000000268000
> > [    2.793641] [    T331] FS:  00007f1f32e5b9c0(0000) GS:ffff88d23b2ca000(0000) knlGS:0000000000000000
> > [    2.793653] [    T331] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [    2.793662] [    T331] CR2: 000055cdfa28f588 CR3: 0000000112e73000 CR4: 0000000000f50ef0
> > [    2.793673] [    T331] PKRU: 55555554
> > [    2.793678] [    T331] Call Trace:
> > [    2.793683] [    T331]  <TASK>
> > [    2.793687] [    T331]  ? lock_acquire+0xbe/0x2d0
> > [    2.793696] [    T331]  ? init_mqd+0x122/0x190 [amdgpu]
> > [    2.793809] [    T331]  ? lock_release+0xc6/0x2a0
> > [    2.793816] [    T331]  ? init_mqd+0x122/0x190 [amdgpu]
> > [    2.793902] [    T331]  init_mqd+0x122/0x190 [amdgpu]
> > [    2.793961] [    T331]  init_mqd_hiq+0xd/0x20 [amdgpu]
> > [    2.794015] [    T331]  kq_initialize.constprop.0+0x2b8/0x370 [amdgpu]
> > [    2.794071] [    T331]  kernel_queue_init+0x3f/0x60 [amdgpu]
> > [    2.794125] [    T331]  pm_init+0x6b/0x100 [amdgpu]
> > [    2.794178] [    T331]  start_cpsch+0x1d6/0x270 [amdgpu]
> > [    2.794234] [    T331]  kgd2kfd_device_init.cold+0x7b9/0xa1a [amdgpu]
> > [    2.794365] [    T331]  amdgpu_amdkfd_device_init+0x190/0x260 [amdgpu]
> 
> amdgpu_amdkfd_device_init()
>   kgd2kfd_device_init() {
>       ....
>         init_mqd()
>           mutex_lock(... profiler_lock); <- FAIL
> 
>       mutex_init(...profiler_lock);
>   }
> 
> Seems the famous graphics CI failed to catch this...
> 
> Thanks,
> 
>         tglx
> ---
> --- a/drivers/gpu/drm/amd/amdkfd/kfd_device.c
> +++ b/drivers/gpu/drm/amd/amdkfd/kfd_device.c
> @@ -744,6 +744,9 @@ bool kgd2kfd_device_init(struct kfd_dev
>  			KGD_ENGINE_SDMA1);
>  	kfd->shared_resources = *gpu_resources;
>  
> +	kfd->profiler_process = NULL;
> +	mutex_init(&kfd->profiler_lock);
> +
>  	kfd->num_nodes = amdgpu_xcp_get_num_xcp(kfd->adev->xcp_mgr);
>  
>  	if (kfd->num_nodes == 0) {
> @@ -936,9 +939,6 @@ bool kgd2kfd_device_init(struct kfd_dev
>  
>  	svm_range_set_max_pages(kfd->adev);
>  
> -	kfd->profiler_process = NULL;
> -	mutex_init(&kfd->profiler_lock);
> -
>  	kfd->init_complete = true;
>  	dev_info(kfd_device, "added device %x:%x\n", kfd->adev->pdev->vendor,
>  		 kfd->adev->pdev->device);

Actually, when I test next-20260519 with the improved fix, I do not see
the warning from amdgpu.

diff --git a/fs/filesystems.c b/fs/filesystems.c
index 771fc31a69b8..712316a1e3e0 100644
--- a/fs/filesystems.c
+++ b/fs/filesystems.c
@@ -269,7 +269,7 @@ static __cold noinline int regen_filesystems_string(void)
 	hlist_for_each_entry_rcu(p, &file_systems, list) {
 		if (!(p->fs_flags & FS_REQUIRES_DEV))
 			newlen += strlen("nodev");
-		newlen += strlen("\t") + strlen(p->name) +  strlen("\n");
+		newlen += strlen("\t") + strlen(p->name) + strlen("\n");
 	}
 	spin_unlock(&file_systems_lock);
 
@@ -289,6 +289,7 @@ static __cold noinline int regen_filesystems_string(void)
 	 * Did someone beat us to it?
 	 */
 	if (old && old->gen == file_systems_gen) {
+		spin_unlock(&file_systems_lock);
 		kfree(new);
 		return 0;
 	}
@@ -297,6 +298,7 @@ static __cold noinline int regen_filesystems_string(void)
 	 * Did the list change in the meantime?
 	 */
 	if (gen != file_systems_gen) {
+		spin_unlock(&file_systems_lock);
 		kfree(new);
 		goto retry;
 	}
@@ -321,13 +323,12 @@ static __cold noinline int regen_filesystems_string(void)
 		 * generation above and messes it up.
 		 */
 		spin_unlock(&file_systems_lock);
-		if (old)
-			kfree_rcu(old, rcu);
+		kfree(new);
 		return -EINVAL;
 	}
 
 	/*
-	 * Paired with consume fence in READ_ONCE() in filesystems_proc_show()
+	 * Paired with consume fence in rcu_dereference() in filesystems_proc_show()
 	 */
 	smp_store_release(&file_systems_string, new);
 	spin_unlock(&file_systems_lock);

Bert Karwatzki