Subject: RE: [PATCH] PCI: hv: Fix a bug on removing child devices on the bus

From: Long Li <longli@microsoft.com> Sent: Tuesday, August 24, 2021 10:28
AM

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Subject: Re: [PATCH] PCI: hv: Fix a bug on removing child devices on
the bus

On Tue, Aug 24, 2021 at 12:20:20AM -0700, longli@linuxonhyperv.com

wrote:

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From: Long Li <longli@microsoft.com>

In hv_pci_bus_exit, the code is holding a spinlock while calling
pci_destroy_slot(), which takes a mutex.

This is not safe for spinlock. Fix this by moving the children to
be deleted to a list on the stack, and removing them after
spinlock is released.

Fixes: 94d22763207a ("PCI: hv: Fix a race condition when removing
the
device")

Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Stephen Hemminger <redacted>
Cc: Wei Liu <wei.liu@kernel.org>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Lorenzo Pieralisi <redacted>
Cc: Rob Herring <robh@kernel.org>
Cc: "Krzysztof Wilczyński" <redacted>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Michael Kelley <redacted>
Cc: Dan Carpenter <redacted>
Reported-by: Dan Carpenter <redacted>
Signed-off-by: Long Li <longli@microsoft.com>
---
 drivers/pci/controller/pci-hyperv.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/pci/controller/pci-hyperv.c

b/drivers/pci/controller/pci-hyperv.c
index a53bd8728d0d..d4f3cce18957 100644

--- a/drivers/pci/controller/pci-hyperv.c
+++ b/drivers/pci/controller/pci-hyperv.c

@@ -3220,6 +3220,7 @@ static int hv_pci_bus_exit(struct hv_device

*hdev,

bool keep_devs)

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 	struct hv_pci_dev *hpdev, *tmp;
 	unsigned long flags;
 	int ret;
+	struct list_head removed;

This can be moved to where it is needed -- the if(!keep_dev) branch
-- to limit its scope.

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 	/*
 	 * After the host sends the RESCIND_CHANNEL message, it doesn't
@@
-3229,9 +3230,18 @@ static int hv_pci_bus_exit(struct hv_device
*hdev, bool

keep_devs)

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 		return 0;

 	if (!keep_devs) {
-		/* Delete any children which might still exist. */
+		INIT_LIST_HEAD(&removed);
+
+		/* Move all present children to the list on stack */
 		spin_lock_irqsave(&hbus->device_list_lock, flags);
-		list_for_each_entry_safe(hpdev, tmp, &hbus->children,

list_entry) {

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+		list_for_each_entry_safe(hpdev, tmp, &hbus->children,

list_entry)

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+			list_move_tail(&hpdev->list_entry, &removed);
+		spin_unlock_irqrestore(&hbus->device_list_lock, flags);
+
+		/* Remove all children in the list */
+		while (!list_empty(&removed)) {
+			hpdev = list_first_entry(&removed, struct hv_pci_dev,
+						 list_entry);

list_for_each_entry_safe can also be used here, right?

Wei.

I will address your comments.

Long

I thought list_for_each_entry_safe() is for use when list manipulation is *not*
protected by a lock and you want to safely walk the list even if an entry gets
removed.  If the list is protected by a lock or not subject to contention (as is the
case here), then
list_for_each_entry() is the simpler implementation.  The original
implementation didn't need to use the _safe version because of the spin lock.

Or do I have it backwards?

Michael

I think we need list_for_each_entry_safe() because we delete the list elements while going through them:

Here is the comment on list_for_each_entry_safe():
/**
 * Loop through the list, keeping a backup pointer to the element. This
 * macro allows for the deletion of a list element while looping through the
 * list.
 *
 * See list_for_each_entry for more details.
 */

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 			list_del(&hpdev->list_entry);
 			if (hpdev->pci_slot)
 				pci_destroy_slot(hpdev->pci_slot);

@@ -3239,7 +3249,6 @@ static int hv_pci_bus_exit(struct hv_device

*hdev,

bool keep_devs)

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 			put_pcichild(hpdev);
 			put_pcichild(hpdev);
 		}
-		spin_unlock_irqrestore(&hbus->device_list_lock, flags);
 	}

 	ret = hv_send_resources_released(hdev);
--
2.25.1