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

On Wed, Aug 25, 2021 at 2:11 PM Michael Kelley [off-list ref] wrote:

From: Long Li <redacted> Sent: Tuesday, August 24, 2021 10:28 AM

quoted

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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 <redacted>

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" <redacted>
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 <redacted>
---
 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?

"_safe" only means "safe against removal of list entry" as the
kerneldoc says. But that means removal within the loop iteration, not
any writer. A lock is needed in either case if there's another writer.

Don't ask me about the RCU variant though...

Rob