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

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Message-ID: <20210824122504.GA3452187@bjorn-Precision-5520>
Date:   Tue, 24 Aug 2021 07:25:04 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     longli@...uxonhyperv.com
Cc:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org, Long Li <longli@...rosoft.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH] PCI: hv: Fix a bug on removing child devices on the bus

"Fix a bug ..." is not a very useful subject line.  It doesn't say
anything about what the patch *does*.  It doesn't hint at a locking
change.

On Tue, Aug 24, 2021 at 12:20:20AM -0700, longli@...uxonhyperv.com wrote:
> From: Long Li <longli@...rosoft.com>
> 
> In hv_pci_bus_exit, the code is holding a spinlock while calling
> pci_destroy_slot(), which takes a mutex.

It's unfortunate that slots are not better integrated into the PCI
core.  I'm sorry your driver even has to worry about this.
> 
> 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@...rosoft.com>
> Cc: Haiyang Zhang <haiyangz@...rosoft.com>
> Cc: Stephen Hemminger <sthemmin@...rosoft.com>
> Cc: Wei Liu <wei.liu@...nel.org>
> Cc: Dexuan Cui <decui@...rosoft.com>
> Cc: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
> Cc: Rob Herring <robh@...nel.org>
> Cc: "Krzysztof Wilczyński" <kw@...ux.com>
> Cc: Bjorn Helgaas <bhelgaas@...gle.com>
> Cc: Michael Kelley <mikelley@...rosoft.com>
> Cc: Dan Carpenter <dan.carpenter@...cle.com>
> Reported-by: Dan Carpenter <dan.carpenter@...cle.com>

A lore link to Dan's report would be useful here.

> Signed-off-by: Long Li <longli@...rosoft.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)
>  	struct hv_pci_dev *hpdev, *tmp;
>  	unsigned long flags;
>  	int ret;
> +	struct list_head removed;
>  
>  	/*
>  	 * 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)
>  		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) {
> +		list_for_each_entry_safe(hpdev, tmp, &hbus->children, list_entry)
> +			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_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)
>  			put_pcichild(hpdev);
>  			put_pcichild(hpdev);
>  		}
> -		spin_unlock_irqrestore(&hbus->device_list_lock, flags);
>  	}
>  
>  	ret = hv_send_resources_released(hdev);
> -- 
> 2.25.1
>