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SubjectRe: [PATCH 3/3] pciehp: Fix race condition handling surprise link-down
On Sat, Nov 19, 2016 at 12:32:47AM -0800, Ashok Raj wrote:
> A surprise link down may retrain very quickly, causing the same slot to
> generate a link up event before handling the link down completes.
>
> Since the link is active, the power off work queued from the first link
> down will cause a second down event when the power is disabled. The second
> down event should be ignored because the slot is already powering off;
> however, the "link up" event sets the slot state to POWERON before the
> event to handle this is enqueued, making the second down event believe
> it needs to do something. This creates a constant link up and down
> event cycle.
>
> This patch fixes that by setting the slot state only when the work to
> handle the power event is executing, protected by the hot plug mutex.

Please mention the mutex specifically by name, e.g.,
"p_slot->hotplug_lock".

> Cc: linux-kernel@vger.kernel.org
> Cc: stable@vger.kernel.org
>
> Signed-off-by: Ashok Raj <ashok.raj@intel.com>
> Reviewed-by: Keith Busch <keith.busch@intel.com>
> ---
> drivers/pci/hotplug/pciehp_ctrl.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/pci/hotplug/pciehp_ctrl.c b/drivers/pci/hotplug/pciehp_ctrl.c
> index ec0b4c1..7ae068c 100644
> --- a/drivers/pci/hotplug/pciehp_ctrl.c
> +++ b/drivers/pci/hotplug/pciehp_ctrl.c
> @@ -182,6 +182,7 @@ static void pciehp_power_thread(struct work_struct *work)
> switch (info->req) {
> case DISABLE_REQ:
> mutex_lock(&p_slot->hotplug_lock);
> + p_slot->state = POWEROFF_STATE;

It sounds right that p_slot->state should be protected.

It looks like handle_button_press_event() and
pciehp_sysfs_enable_slot() hold p_slot->lock while updating
p_slot->state.

You're setting "state = POWEROFF_STATE" while holding
p_slot->hotplug_lock (not p_slot->lock). Four lines down, we set
"state = STATIC_STATE", but this time we're holding p_slot->lock.

What is the difference between the p_slot->lock and
p_slot->hotplug_lock? Do we need both? How do we know which one to
use?

I'm very confused.

> pciehp_disable_slot(p_slot);
> mutex_unlock(&p_slot->hotplug_lock);
> mutex_lock(&p_slot->lock);
> @@ -190,6 +191,7 @@ static void pciehp_power_thread(struct work_struct *work)
> break;
> case ENABLE_REQ:
> mutex_lock(&p_slot->hotplug_lock);
> + p_slot->state = POWERON_STATE;
> ret = pciehp_enable_slot(p_slot);
> mutex_unlock(&p_slot->hotplug_lock);
> if (ret)
> @@ -209,8 +211,6 @@ static void pciehp_queue_power_work(struct slot *p_slot, int req)
> {
> struct power_work_info *info;
>
> - p_slot->state = (req == ENABLE_REQ) ? POWERON_STATE : POWEROFF_STATE;
> -
> info = kmalloc(sizeof(*info), GFP_KERNEL);
> if (!info) {
> ctrl_err(p_slot->ctrl, "no memory to queue %s request\n",
> --
> 2.7.4
>
> --
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