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Subject[RFC PATCH v2] pciehp: fix a race between pciehp and removing operations by sysfs
Date
From: Xiongfeng Wang <xiongfeng.wang@linaro.com>

When I run a stress test about pcie hotplug and removing operations by
sysfs, I got a hange task, and the following call trace is printed.

INFO: task kworker/0:2:4413 blocked for more than 120 seconds.
Tainted: P W O 4.12.0-rc1 #1
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/0:2 D 0 4413 2 0x00000000
Workqueue: pciehp-0 pciehp_power_thread
Call trace:
[<ffff0000080861d4>] __switch_to+0x94/0xa8
[<ffff000008bea9c0>] __schedule+0x1b0/0x708
[<ffff000008beaf58>] schedule+0x40/0xa4
[<ffff000008beb33c>] schedule_preempt_disabled+0x28/0x40
[<ffff000008bec1dc>] __mutex_lock.isra.8+0x148/0x50c
[<ffff000008bec5c4>] __mutex_lock_slowpath+0x24/0x30
[<ffff000008bec618>] mutex_lock+0x48/0x54
[<ffff0000084d8188>] pci_lock_rescan_remove+0x20/0x28
[<ffff0000084f87c0>] pciehp_unconfigure_device+0x54/0x1cc
[<ffff0000084f8260>] pciehp_disable_slot+0x4c/0xbc
[<ffff0000084f8370>] pciehp_power_thread+0xa0/0xb8
[<ffff0000080e9ce8>] process_one_work+0x13c/0x3f8
[<ffff0000080ea004>] worker_thread+0x60/0x3e4
[<ffff0000080f0814>] kthread+0x10c/0x138
[<ffff0000080836c0>] ret_from_fork+0x10/0x50
INFO: task bash:31732 blocked for more than 120 seconds.
Tainted: P W O 4.12.0-rc1 #1
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
bash D 0 31732 1 0x00000009
Call trace:
[<ffff0000080861d4>] __switch_to+0x94/0xa8
[<ffff000008bea9c0>] __schedule+0x1b0/0x708
[<ffff000008beaf58>] schedule+0x40/0xa4
[<ffff000008bee7b4>] schedule_timeout+0x1a0/0x340
[<ffff000008bebb88>] wait_for_common+0x108/0x1bc
[<ffff000008bebc64>] wait_for_completion+0x28/0x34
[<ffff0000080e7594>] flush_workqueue+0x130/0x488
[<ffff0000080e79b0>] drain_workqueue+0xc4/0x164
[<ffff0000080ec3cc>] destroy_workqueue+0x28/0x1f4
[<ffff0000084fa094>] pciehp_release_ctrl+0x34/0xe0
[<ffff0000084f75b0>] pciehp_remove+0x30/0x3c
[<ffff0000084f24d8>] pcie_port_remove_service+0x3c/0x54
[<ffff00000876b1e4>] device_release_driver_internal+0x150/0x1d0
[<ffff00000876b28c>] device_release_driver+0x28/0x34
[<ffff00000876a018>] bus_remove_device+0xe0/0x11c
[<ffff000008766348>] device_del+0x200/0x304
[<ffff00000876646c>] device_unregister+0x20/0x38
[<ffff0000084f2560>] remove_iter+0x44/0x54
[<ffff000008765230>] device_for_each_child+0x4c/0x90
[<ffff0000084f2c98>] pcie_port_device_remove+0x2c/0x48
[<ffff0000084f2f48>] pcie_portdrv_remove+0x60/0x6c
[<ffff0000084e3de4>] pci_device_remove+0x48/0x110
[<ffff00000876b1e4>] device_release_driver_internal+0x150/0x1d0
[<ffff00000876b28c>] device_release_driver+0x28/0x34
[<ffff0000084db028>] pci_stop_bus_device+0x9c/0xac
[<ffff0000084db190>] pci_stop_and_remove_bus_device_locked+0x24/0x3c
[<ffff0000084e5eb0>] remove_store+0x74/0x80
[<ffff000008764680>] dev_attr_store+0x44/0x5c
[<ffff0000082e7e1c>] sysfs_kf_write+0x5c/0x74
[<ffff0000082e7014>] kernfs_fop_write+0xcc/0x1dc
[<ffff0000082602e0>] __vfs_write+0x48/0x13c
[<ffff00000826174c>] vfs_write+0xa8/0x198
[<ffff000008262ce8>] SyS_write+0x54/0xb0
[<ffff000008083730>] el0_svc_naked+0x24/0x28

There is a race condition between these two kinds of operations.
When the Attention button on a PCIE slot is pressed, 5 seconds later,
pciehp_power_thread() will be scheduled on slot->wq. This function will
call pciehp_unconfigure_device(), which will try to get a global mutex
lock 'pci_rescan_remove_lock'.

At the same time, we remove the pcie port by sysfs, which results in
pci_stop_and_remove_bus_device_locked() called. This function will get
the global mutex lock 'pci_rescan_remove_lock', and then release the
struct 'ctrl', which will wait until the work_struct on slot->wq is
finished.

If pci_stop_and_remove_bus_device_locked() got the mutex lock, and
before it drains workqueue slot->wq, pciehp_power_thread() is scheduled
on slot->wq and tries to get the mutex lock but failed, so it will just
wait. Then pci_stop_and_remove_bus_device_locked() tries to drain workqueue
slot->wq and wait until work struct 'pciehp_power_thread()' is finished.
Then a hung_task occurs.

So this two kinds of operation, removing through attention buttion and
removing through /sys/devices/pci***/remove, should not be excuted at the
same time. This patch add a global variable to mark that one of these
operations is under processing. When this variable is set, if another
operation is requested, it will be rejected.

At first, I want to add a flag for each pci slot to record whether a
removing operation is under processing. When a bridge is being removed,
the flags of all the slots below the bridge need to be checked. But it
is hard for us to guarantee the atomic access. So I just use a global
flag.

This workaround method uses a global flag, which is not good for the code
framework and can't fix the race condition fully. But I can't figure out
a better way. I think we may need to reconstruct the code framework a lot
to fix this issue nicely. There are so many work struct created. Before
that, maybe we can use this patch as a temporary fix.

Signed-off-by: Xiongfeng Wang <xiongfeng.wang@linaro.org>
---
drivers/pci/hotplug/pciehp_ctrl.c | 7 +++++++
drivers/pci/hotplug/pciehp_hpc.c | 12 +++++++++++-
drivers/pci/pci-sysfs.c | 11 +++++++++--
drivers/pci/remove.c | 6 ++++++
include/linux/pci.h | 3 +++
5 files changed, 36 insertions(+), 3 deletions(-)

diff --git a/drivers/pci/hotplug/pciehp_ctrl.c b/drivers/pci/hotplug/pciehp_ctrl.c
index 83f3d4a..5680439 100644
--- a/drivers/pci/hotplug/pciehp_ctrl.c
+++ b/drivers/pci/hotplug/pciehp_ctrl.c
@@ -44,6 +44,7 @@ void pciehp_queue_interrupt_event(struct slot *p_slot, u32 event_type)
info = kmalloc(sizeof(*info), GFP_ATOMIC);
if (!info) {
ctrl_err(p_slot->ctrl, "dropped event %d (ENOMEM)\n", event_type);
+ slot_being_removed_rescanned = 0;
return;
}

@@ -188,6 +189,7 @@ static void pciehp_power_thread(struct work_struct *work)
mutex_lock(&p_slot->lock);
p_slot->state = STATIC_STATE;
mutex_unlock(&p_slot->lock);
+ slot_being_removed_rescanned = 0;
break;
case ENABLE_REQ:
mutex_lock(&p_slot->hotplug_lock);
@@ -198,6 +200,7 @@ static void pciehp_power_thread(struct work_struct *work)
mutex_lock(&p_slot->lock);
p_slot->state = STATIC_STATE;
mutex_unlock(&p_slot->lock);
+ slot_being_removed_rescanned = 0;
break;
default:
break;
@@ -216,6 +219,7 @@ static void pciehp_queue_power_work(struct slot *p_slot, int req)
if (!info) {
ctrl_err(p_slot->ctrl, "no memory to queue %s request\n",
(req == ENABLE_REQ) ? "poweron" : "poweroff");
+ slot_being_removed_rescanned = 0;
return;
}
info->p_slot = p_slot;
@@ -284,6 +288,7 @@ static void handle_button_press_event(struct slot *p_slot)
ctrl_info(ctrl, "Slot(%s): Action canceled due to button press\n",
slot_name(p_slot));
p_slot->state = STATIC_STATE;
+ slot_being_removed_rescanned = 0;
break;
case POWEROFF_STATE:
case POWERON_STATE:
@@ -294,10 +299,12 @@ static void handle_button_press_event(struct slot *p_slot)
*/
ctrl_info(ctrl, "Slot(%s): Button ignored\n",
slot_name(p_slot));
+ slot_being_removed_rescanned = 0;
break;
default:
ctrl_err(ctrl, "Slot(%s): Ignoring invalid state %#x\n",
slot_name(p_slot), p_slot->state);
+ slot_being_removed_rescanned = 0;
break;
}
}
diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c
index 7bab060..7d1b559 100644
--- a/drivers/pci/hotplug/pciehp_hpc.c
+++ b/drivers/pci/hotplug/pciehp_hpc.c
@@ -622,7 +622,17 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
if (events & PCI_EXP_SLTSTA_ABP) {
ctrl_info(ctrl, "Slot(%s): Attention button pressed\n",
slot_name(slot));
- pciehp_queue_interrupt_event(slot, INT_BUTTON_PRESS);
+
+ if (!test_and_set_bit(0, &slot_being_removed_rescanned))
+ pciehp_queue_interrupt_event(slot, INT_BUTTON_PRESS);
+ else {
+ if (slot->state == BLINKINGOFF_STATE || slot->state == BLINKINGON_STATE)
+ pciehp_queue_interrupt_event(slot, INT_BUTTON_PRESS);
+ else
+ ctrl_info(ctrl, "Slot(%s): Slot operation failed because a remove or"
+ " rescan operation is under processing, please try later!\n",
+ slot_name(slot));
+ }
}

/*
diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
index 06c7f0b..206162f 100644
--- a/drivers/pci/pci-sysfs.c
+++ b/drivers/pci/pci-sysfs.c
@@ -489,8 +489,15 @@ static ssize_t remove_store(struct device *dev, struct device_attribute *attr,
if (kstrtoul(buf, 0, &val) < 0)
return -EINVAL;

- if (val && device_remove_file_self(dev, attr))
- pci_stop_and_remove_bus_device_locked(to_pci_dev(dev));
+ if (val && device_remove_file_self(dev, attr)) {
+ if (!test_and_set_bit(0, &slot_being_removed_rescanned)) {
+ pci_stop_and_remove_bus_device_locked(to_pci_dev(dev));
+ slot_being_removed_rescanned = 0;
+ } else {
+ pr_info("Slot is being removed or rescanned, please try later!\n");
+ return -EPERM;
+ }
+ }
return count;
}
static struct device_attribute dev_remove_attr = __ATTR(remove,
diff --git a/drivers/pci/remove.c b/drivers/pci/remove.c
index 2fa0dbd..c7c3e37 100644
--- a/drivers/pci/remove.c
+++ b/drivers/pci/remove.c
@@ -3,6 +3,12 @@
#include <linux/pci-aspm.h>
#include "pci.h"

+/*
+ * When a slot is being hotplug through Attention Button or being
+ * removed/rescanned through sysfs, this flag is set.
+ */
+unsigned long slot_being_removed_rescanned;
+
static void pci_free_resources(struct pci_dev *dev)
{
int i;
diff --git a/include/linux/pci.h b/include/linux/pci.h
index c170c92..85a6e75 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -854,6 +854,9 @@ enum pcie_bus_config_types {
/* Do NOT directly access these two variables, unless you are arch-specific PCI
* code, or PCI core code. */
extern struct list_head pci_root_buses; /* list of all known PCI buses */
+
+extern unsigned long slot_being_removed_rescanned;
+
/* Some device drivers need know if PCI is initiated */
int no_pci_devices(void);

--
1.7.12.4
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