[PATCH] s390/pci: Avoid deadlock between PCI error recovery and mlx5 crdump

[Gerd Bayer <gbayer@linux.ibm.com>]

Do not block PCI config accesses through pci_cfg_access_lock() when
executing the s390 variant of PCI error recovery: Acquire just
device_lock() instead of pci_dev_lock() as powerpc's EEH and
generig PCI AER processing do.

During error recovery testing a pair of tasks was reported to be hung:

[10144.859042] mlx5_core 0000:00:00.1: mlx5_health_try_recover:338:(pid 5553): health recovery flow aborted, PCI reads still not working
[10320.359160] INFO: task kmcheck:72 blocked for more than 122 seconds.
[10320.359169]       Not tainted 5.14.0-570.12.1.bringup7.el9.s390x #1
[10320.359171] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[10320.359172] task:kmcheck         state:D stack:0     pid:72    tgid:72    ppid:2      flags:0x00000000
[10320.359176] Call Trace:
[10320.359178]  [<000000065256f030>] __schedule+0x2a0/0x590
[10320.359187]  [<000000065256f356>] schedule+0x36/0xe0
[10320.359189]  [<000000065256f572>] schedule_preempt_disabled+0x22/0x30
[10320.359192]  [<0000000652570a94>] __mutex_lock.constprop.0+0x484/0x8a8
[10320.359194]  [<000003ff800673a4>] mlx5_unload_one+0x34/0x58 [mlx5_core]
[10320.359360]  [<000003ff8006745c>] mlx5_pci_err_detected+0x94/0x140 [mlx5_core]
[10320.359400]  [<0000000652556c5a>] zpci_event_attempt_error_recovery+0xf2/0x398
[10320.359406]  [<0000000651b9184a>] __zpci_event_error+0x23a/0x2c0
[10320.359411]  [<00000006522b3958>] chsc_process_event_information.constprop.0+0x1c8/0x1e8
[10320.359416]  [<00000006522baf1a>] crw_collect_info+0x272/0x338
[10320.359418]  [<0000000651bc9de0>] kthread+0x108/0x110
[10320.359422]  [<0000000651b42ea4>] __ret_from_fork+0x3c/0x58
[10320.359425]  [<0000000652576642>] ret_from_fork+0xa/0x30
[10320.359440] INFO: task kworker/u1664:6:1514 blocked for more than 122 seconds.
[10320.359441]       Not tainted 5.14.0-570.12.1.bringup7.el9.s390x #1
[10320.359442] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[10320.359443] task:kworker/u1664:6 state:D stack:0     pid:1514  tgid:1514  ppid:2      flags:0x00000000
[10320.359447] Workqueue: mlx5_health0000:00:00.0 mlx5_fw_fatal_reporter_err_work [mlx5_core]
[10320.359492] Call Trace:
[10320.359521]  [<000000065256f030>] __schedule+0x2a0/0x590
[10320.359524]  [<000000065256f356>] schedule+0x36/0xe0
[10320.359526]  [<0000000652172e28>] pci_wait_cfg+0x80/0xe8
[10320.359532]  [<0000000652172f94>] pci_cfg_access_lock+0x74/0x88
[10320.359534]  [<000003ff800916b6>] mlx5_vsc_gw_lock+0x36/0x178 [mlx5_core]
[10320.359585]  [<000003ff80098824>] mlx5_crdump_collect+0x34/0x1c8 [mlx5_core]
[10320.359637]  [<000003ff80074b62>] mlx5_fw_fatal_reporter_dump+0x6a/0xe8 [mlx5_core]
[10320.359680]  [<0000000652512242>] devlink_health_do_dump.part.0+0x82/0x168
[10320.359683]  [<0000000652513212>] devlink_health_report+0x19a/0x230
[10320.359685]  [<000003ff80075a12>] mlx5_fw_fatal_reporter_err_work+0xba/0x1b0 [mlx5_core]
[10320.359728]  [<0000000651bbf852>] process_one_work+0x1c2/0x458
[10320.359733]  [<0000000651bc073e>] worker_thread+0x3ce/0x528
[10320.359735]  [<0000000651bc9de0>] kthread+0x108/0x110
[10320.359737]  [<0000000651b42ea4>] __ret_from_fork+0x3c/0x58
[10320.359739]  [<0000000652576642>] ret_from_fork+0xa/0x30

No kernel log of the exact same error with an upstream kernel is
available - but the very same deadlock situation can be constructed there,
too:

- task: kmcheck
  mlx5_unload_one() tries to acquire devlink lock while the PCI error
  recovery code has set pdev->block_cfg_access by way of
  pci_cfg_access_lock()
- task: kworker
  mlx5_crdump_collect() tries to set block_cfg_access through
  pci_cfg_access_lock() while devlink_health_report() had acquired
  the devlink lock.

A similar deadlock situation can be reproduced by requesting a
crdump with
  > devlink health dump show pci/<BDF> reporter fw_fatal

while PCI error recovery is executed on the same <BDF> physical function
by mlx5_core's pci_error_handlers. On s390 this can be injected with
  > zpcictl --reset-fw <BDF>

Tests with this patch failed to reproduce that second deadlock situation,
the devlink command is rejected with "kernel answers: Permission denied" -
and we get a kernel log message of:

Oct 14 13:32:39 b46lp03.lnxne.boe kernel: mlx5_core 1ed0:00:00.1: mlx5_crdump_collect:50:(pid 254382): crdump: failed to lock vsc gw err -5

because the config read of VSC_SEMAPHORE is rejected by the underlying
hardware.

Two prior attempts to address this issue have been discussed and
ultimately rejected [1], with the primary argument that s390's
implementation of PCI error recovery is imposing restriction that
neither powerpc's EEH nor PCI AER handling need. Tests show that PCI
error recovery on s390 is running to completion even without blocking
access to PCI config space.

Link[1]: https://lore.kernel.org/all/20251007144826.2825134-1-gbayer@linux.ibm.com/

Fixes: 4cdf2f4e24ff ("s390/pci: implement minimal PCI error recovery")
Signed-off-by: Gerd Bayer <gbayer@linux.ibm.com>

---

Hi Niklas, Shay, Jason,

by now I believe fixing this in s390/pci is the right way to go, since
the other PCI error recovery implementations apparently don't require
this strict blocking of accesses to the PCI config space.

Hi Alexander, Vasily, Heiko,

while I sent this to netdev since prior versions were discussed there,
I assume this patch will go through the s390 tree, right?

Thanks,
Gerd
---
 arch/s390/pci/pci_event.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/s390/pci/pci_event.c b/arch/s390/pci/pci_event.c
index d930416d4c90..da0de34d2e5c 100644
--- a/arch/s390/pci/pci_event.c
+++ b/arch/s390/pci/pci_event.c
@@ -187,7 +187,7 @@ static pci_ers_result_t zpci_event_attempt_error_recovery(struct pci_dev *pdev)
 	 * is unbound or probed and that userspace can't access its
 	 * configuration space while we perform recovery.
 	 */
-	pci_dev_lock(pdev);
+	device_lock(&pdev->dev);
 	if (pdev->error_state == pci_channel_io_perm_failure) {
 		ers_res = PCI_ERS_RESULT_DISCONNECT;
 		goto out_unlock;
@@ -254,7 +254,7 @@ static pci_ers_result_t zpci_event_attempt_error_recovery(struct pci_dev *pdev)
 	if (driver->err_handler->resume)
 		driver->err_handler->resume(pdev);
 out_unlock:
-	pci_dev_unlock(pdev);
+	device_unlock(&pdev->dev);
 	zpci_report_status(zdev, "recovery", status_str);
 
 	return ers_res;
-- 
2.48.1