[PATCH net v2] net/mlx5: Avoid deadlock between PCI error recovery and health reporter

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

Try to block further PCI config accesses just once when trying to acquire
the VSC GW lock. PCI error recovery on s390 may be blocking accesses
while trying to acquire the devlink lock that mlx5_crdump_collect is
holding already. In effect, this sacrifices the crdump if there is
contention with other tasks about PCI config accesses.

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>

Extensive tests with the same setup that showed the original dead-lock
didn't reproduce, nor did the second deadlock situation hit with
this patch applied.

Fixes: b25bbc2f24dc ("net/mlx5: Add Vendor Specific Capability access gateway")
Signed-off-by: Gerd Bayer <gbayer@linux.ibm.com>

---

v1:
Attempted to fix this differently, but still had potential for deadlocks
and the second inject reproduced it quite regularly, still
https://lore.kernel.org/netdev/20250807131130.4056349-1-gbayer@linux.ibm.com/
---
 drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c b/drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c
index 432c98f2626d..f668237b6bb0 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c
@@ -73,7 +73,9 @@ int mlx5_vsc_gw_lock(struct mlx5_core_dev *dev)
 	u32 lock_val;
 	int ret;
 
-	pci_cfg_access_lock(dev->pdev);
+	if (!pci_cfg_access_trylock(dev->pdev))
+		return -EBUSY;
+
 	do {
 		if (retries > VSC_MAX_RETRIES) {
 			ret = -EBUSY;
-- 
2.48.1