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Date:   Tue, 25 Jan 2022 06:23:23 -0600
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        Theodore Tso <tytso@mit.edu>,
        Hans de Goede <hdegoede@redhat.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Lukas Wunner <lukas@wunner.de>
Subject: Re: [PATCH 5.16 0873/1039] PCI: pciehp: Use
 down_read/write_nested(reset_lock) to fix lockdep errors
Message-ID: <20220125122323.GA1597465@bhelgaas>
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On Mon, Jan 24, 2022 at 07:44:22PM +0100, Greg Kroah-Hartman wrote:
> From: Hans de Goede <hdegoede@redhat.com>
> 
> commit 085a9f43433f30cbe8a1ade62d9d7827c3217f4d upstream.

I would hold off on backporting the pciehp changes until we resolve
this regression in v5.17-rc1:

  https://bugzilla.kernel.org/show_bug.cgi?id=215525

> Use down_read_nested() and down_write_nested() when taking the
> ctrl->reset_lock rw-sem, passing the number of PCIe hotplug controllers in
> the path to the PCI root bus as lock subclass parameter.
> 
> This fixes the following false-positive lockdep report when unplugging a
> Lenovo X1C8 from a Lenovo 2nd gen TB3 dock:
> 
>   pcieport 0000:06:01.0: pciehp: Slot(1): Link Down
>   pcieport 0000:06:01.0: pciehp: Slot(1): Card not present
>   ============================================
>   WARNING: possible recursive locking detected
>   5.16.0-rc2+ #621 Not tainted
>   --------------------------------------------
>   irq/124-pciehp/86 is trying to acquire lock:
>   ffff8e5ac4299ef8 (&ctrl->reset_lock){.+.+}-{3:3}, at: pciehp_check_presence+0x23/0x80
> 
>   but task is already holding lock:
>   ffff8e5ac4298af8 (&ctrl->reset_lock){.+.+}-{3:3}, at: pciehp_ist+0xf3/0x180
> 
>    other info that might help us debug this:
>    Possible unsafe locking scenario:
> 
> 	 CPU0
> 	 ----
>     lock(&ctrl->reset_lock);
>     lock(&ctrl->reset_lock);
> 
>    *** DEADLOCK ***
> 
>    May be due to missing lock nesting notation
> 
>   3 locks held by irq/124-pciehp/86:
>    #0: ffff8e5ac4298af8 (&ctrl->reset_lock){.+.+}-{3:3}, at: pciehp_ist+0xf3/0x180
>    #1: ffffffffa3b024e8 (pci_rescan_remove_lock){+.+.}-{3:3}, at: pciehp_unconfigure_device+0x31/0x110
>    #2: ffff8e5ac1ee2248 (&dev->mutex){....}-{3:3}, at: device_release_driver+0x1c/0x40
> 
>   stack backtrace:
>   CPU: 4 PID: 86 Comm: irq/124-pciehp Not tainted 5.16.0-rc2+ #621
>   Hardware name: LENOVO 20U90SIT19/20U90SIT19, BIOS N2WET30W (1.20 ) 08/26/2021
>   Call Trace:
>    <TASK>
>    dump_stack_lvl+0x59/0x73
>    __lock_acquire.cold+0xc5/0x2c6
>    lock_acquire+0xb5/0x2b0
>    down_read+0x3e/0x50
>    pciehp_check_presence+0x23/0x80
>    pciehp_runtime_resume+0x5c/0xa0
>    device_for_each_child+0x45/0x70
>    pcie_port_device_runtime_resume+0x20/0x30
>    pci_pm_runtime_resume+0xa7/0xc0
>    __rpm_callback+0x41/0x110
>    rpm_callback+0x59/0x70
>    rpm_resume+0x512/0x7b0
>    __pm_runtime_resume+0x4a/0x90
>    __device_release_driver+0x28/0x240
>    device_release_driver+0x26/0x40
>    pci_stop_bus_device+0x68/0x90
>    pci_stop_bus_device+0x2c/0x90
>    pci_stop_and_remove_bus_device+0xe/0x20
>    pciehp_unconfigure_device+0x6c/0x110
>    pciehp_disable_slot+0x5b/0xe0
>    pciehp_handle_presence_or_link_change+0xc3/0x2f0
>    pciehp_ist+0x179/0x180
> 
> This lockdep warning is triggered because with Thunderbolt, hotplug ports
> are nested. When removing multiple devices in a daisy-chain, each hotplug
> port's reset_lock may be acquired recursively. It's never the same lock, so
> the lockdep splat is a false positive.
> 
> Because locks at the same hierarchy level are never acquired recursively, a
> per-level lockdep class is sufficient to fix the lockdep warning.
> 
> The choice to use one lockdep subclass per pcie-hotplug controller in the
> path to the root-bus was made to conserve class keys because their number
> is limited and the complexity grows quadratically with number of keys
> according to Documentation/locking/lockdep-design.rst.
> 
> Link: https://lore.kernel.org/linux-pci/20190402021933.GA2966@mit.edu/
> Link: https://lore.kernel.org/linux-pci/de684a28-9038-8fc6-27ca-3f6f2f6400d7@redhat.com/
> Link: https://lore.kernel.org/r/20211217141709.379663-1-hdegoede@redhat.com
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=208855
> Reported-by: "Theodore Ts'o" <tytso@mit.edu>
> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
> Reviewed-by: Lukas Wunner <lukas@wunner.de>
> Cc: stable@vger.kernel.org
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
>  drivers/pci/hotplug/pciehp.h      |    3 +++
>  drivers/pci/hotplug/pciehp_core.c |    2 +-
>  drivers/pci/hotplug/pciehp_hpc.c  |   21 ++++++++++++++++++---
>  3 files changed, 22 insertions(+), 4 deletions(-)
> 
> --- a/drivers/pci/hotplug/pciehp.h
> +++ b/drivers/pci/hotplug/pciehp.h
> @@ -75,6 +75,8 @@ extern int pciehp_poll_time;
>   * @reset_lock: prevents access to the Data Link Layer Link Active bit in the
>   *	Link Status register and to the Presence Detect State bit in the Slot
>   *	Status register during a slot reset which may cause them to flap
> + * @depth: Number of additional hotplug ports in the path to the root bus,
> + *	used as lock subclass for @reset_lock
>   * @ist_running: flag to keep user request waiting while IRQ thread is running
>   * @request_result: result of last user request submitted to the IRQ thread
>   * @requester: wait queue to wake up on completion of user request,
> @@ -106,6 +108,7 @@ struct controller {
>  
>  	struct hotplug_slot hotplug_slot;	/* hotplug core interface */
>  	struct rw_semaphore reset_lock;
> +	unsigned int depth;
>  	unsigned int ist_running;
>  	int request_result;
>  	wait_queue_head_t requester;
> --- a/drivers/pci/hotplug/pciehp_core.c
> +++ b/drivers/pci/hotplug/pciehp_core.c
> @@ -166,7 +166,7 @@ static void pciehp_check_presence(struct
>  {
>  	int occupied;
>  
> -	down_read(&ctrl->reset_lock);
> +	down_read_nested(&ctrl->reset_lock, ctrl->depth);
>  	mutex_lock(&ctrl->state_lock);
>  
>  	occupied = pciehp_card_present_or_link_active(ctrl);
> --- a/drivers/pci/hotplug/pciehp_hpc.c
> +++ b/drivers/pci/hotplug/pciehp_hpc.c
> @@ -583,7 +583,7 @@ static void pciehp_ignore_dpc_link_chang
>  	 * the corresponding link change may have been ignored above.
>  	 * Synthesize it to ensure that it is acted on.
>  	 */
> -	down_read(&ctrl->reset_lock);
> +	down_read_nested(&ctrl->reset_lock, ctrl->depth);
>  	if (!pciehp_check_link_active(ctrl))
>  		pciehp_request(ctrl, PCI_EXP_SLTSTA_DLLSC);
>  	up_read(&ctrl->reset_lock);
> @@ -746,7 +746,7 @@ static irqreturn_t pciehp_ist(int irq, v
>  	 * Disable requests have higher priority than Presence Detect Changed
>  	 * or Data Link Layer State Changed events.
>  	 */
> -	down_read(&ctrl->reset_lock);
> +	down_read_nested(&ctrl->reset_lock, ctrl->depth);
>  	if (events & DISABLE_SLOT)
>  		pciehp_handle_disable_request(ctrl);
>  	else if (events & (PCI_EXP_SLTSTA_PDC | PCI_EXP_SLTSTA_DLLSC))
> @@ -906,7 +906,7 @@ int pciehp_reset_slot(struct hotplug_slo
>  	if (probe)
>  		return 0;
>  
> -	down_write(&ctrl->reset_lock);
> +	down_write_nested(&ctrl->reset_lock, ctrl->depth);
>  
>  	if (!ATTN_BUTTN(ctrl)) {
>  		ctrl_mask |= PCI_EXP_SLTCTL_PDCE;
> @@ -962,6 +962,20 @@ static inline void dbg_ctrl(struct contr
>  
>  #define FLAG(x, y)	(((x) & (y)) ? '+' : '-')
>  
> +static inline int pcie_hotplug_depth(struct pci_dev *dev)
> +{
> +	struct pci_bus *bus = dev->bus;
> +	int depth = 0;
> +
> +	while (bus->parent) {
> +		bus = bus->parent;
> +		if (bus->self && bus->self->is_hotplug_bridge)
> +			depth++;
> +	}
> +
> +	return depth;
> +}
> +
>  struct controller *pcie_init(struct pcie_device *dev)
>  {
>  	struct controller *ctrl;
> @@ -975,6 +989,7 @@ struct controller *pcie_init(struct pcie
>  		return NULL;
>  
>  	ctrl->pcie = dev;
> +	ctrl->depth = pcie_hotplug_depth(dev->port);
>  	pcie_capability_read_dword(pdev, PCI_EXP_SLTCAP, &slot_cap);
>  
>  	if (pdev->hotplug_user_indicators)
> 
>