Messages in this thread Patch in this message | | | Date | Mon, 31 Aug 2020 15:10:07 -0700 | Subject | [PATCH v1] driver core: Fix device_pm_lock() locking for device links | From | Saravana Kannan <> |
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This commit fixes two issues:
1. The lockdep warning reported by Dong Aisheng <dongas86@gmail.com> [1].
It is a warning about a cycle (dpm_list_mtx --> kn->active#3 --> fw_lock) that was introduced when device-link devices were added to expose device link information in sysfs.
The patch that "introduced" this cycle can't be reverted because it's fixes a real SRCU issue and also ensures that the device-link device is deleted as soon as the device-link is deleted. This is important to avoid sysfs name collisions if the device-link is create again immediately (this can happen a lot with deferred probing).
2. device_link_drop_managed() is not grabbing device_pm_lock().
When device_link_del() calls __device_link_del() (device_link_del() -> device_link_put_kref() kref_put() -> __device_link_del()) it grabs the device_pm_lock().
However, when device_link_drop_managed() calls __device_link_del() (device_link_drop_managed() -> kref_put() -> __device_link_del()) it doesn't grab device_pm_lock(). There's nothing special about managed device-links that remove the need for grabbing device_pm_lock(). So, this patch makes sure device_pm_lock() is always held when deleting managed links.
And thanks to Stephen Boyd for helping me understand the lockdep splat.
Fixes: 843e600b8a2b ("driver core: Fix sleeping in invalid context during device link deletion") Fixes: 515db266a9da ("driver core: Remove device link creation limitation") [1] - https://lore.kernel.org/lkml/CAA+hA=S4eAreb7vo69LAXSk2t5=DEKNxHaiY1wSpk4xTp9urLg@mail.gmail.com/ Reported-by: Dong Aisheng <dongas86@gmail.com> Signed-off-by: Saravana Kannan <saravanak@google.com> ---
Rafael,
A bigger question I had is why we need to grab device_pm_lock() around device_link_del() in the first place. I understand the need to grab it during device_link_add() -- it's because we are checking the supplier is in the dpm_list and because we are reordering devices on the dpm_list.
But during deletion, we don't need to do either one of those. So, why do we even need to grab the device_pm_lock() in the first place. The device_links_write_lock() that we already grab before deleting a device link seems like it'd be sufficient. If you agree we don't need to grab device_pm_lock() during deletion, then I can change this patch to just delete that locking.
-Saravana
drivers/base/core.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/drivers/base/core.c b/drivers/base/core.c index f6f620aa9408..de1935e21d97 100644 --- a/drivers/base/core.c +++ b/drivers/base/core.c @@ -766,8 +766,10 @@ static void __device_link_del(struct kref *kref) if (link->flags & DL_FLAG_PM_RUNTIME) pm_runtime_drop_link(link->consumer); + device_pm_lock(); list_del_rcu(&link->s_node); list_del_rcu(&link->c_node); + device_pm_unlock(); device_unregister(&link->link_dev); } #else /* !CONFIG_SRCU */ @@ -781,8 +783,10 @@ static void __device_link_del(struct kref *kref) if (link->flags & DL_FLAG_PM_RUNTIME) pm_runtime_drop_link(link->consumer); + device_pm_lock(); list_del(&link->s_node); list_del(&link->c_node); + device_pm_unlock(); device_unregister(&link->link_dev); } #endif /* !CONFIG_SRCU */ @@ -807,9 +811,7 @@ static void device_link_put_kref(struct device_link *link) void device_link_del(struct device_link *link) { device_links_write_lock(); - device_pm_lock(); device_link_put_kref(link); - device_pm_unlock(); device_links_write_unlock(); } EXPORT_SYMBOL_GPL(device_link_del); @@ -830,7 +832,6 @@ void device_link_remove(void *consumer, struct device *supplier) return; device_links_write_lock(); - device_pm_lock(); list_for_each_entry(link, &supplier->links.consumers, s_node) { if (link->consumer == consumer) { @@ -839,7 +840,6 @@ void device_link_remove(void *consumer, struct device *supplier) } } - device_pm_unlock(); device_links_write_unlock(); } EXPORT_SYMBOL_GPL(device_link_remove); -- 2.28.0.402.g5ffc5be6b7-goog
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