Backport rcu-tasks fix to stable kernel version 6.6.y

Backport rcu-tasks fix to stable kernel version 6.6.y

[Tahera Fahimi]

Dear Stable Kernel Maintainers,

I am writing to request the backport of the following patches to stable kernel versions 6.6.y,
addressing a deadlock issue in RCU Tasks related to do_exit() on preemptible systems.

Issue Description:
The kernel may experience deadlocks due to shared locking between exit_tasks_rcu_start() and
rcu_tasks_postscan() via synchronize_srcu(), when multiple processes exit concurrently. The problem
manifests as stalls in the RCU tasks grace period. This issue manifest as a deadlock in WSL kernel
which uses the stable kernel 6.6.87 (Please see the issue on https://github.com/microsoft/WSL/issues/13480)

Patches to Apply:
Patch 1:
	Subject: rcu-tasks: Maintain lists to eliminate RCU-tasks/do_exit() deadlocks
	Commit ID: 6b70399f9ef3809f6e308fd99dd78b072c1bd05c
	Justification: Introduces per-CPU lists for exiting tasks, replacing SRCU-based waits and
	eliminating deadlocks during concurrent exits.
Patch 2:
	Subject: rcu-tasks: Eliminate deadlocks involving do_exit() and RCU tasks
	Commit ID: 1612160b91272f5b1596f499584d6064bf5be794
	Justification: Ensures all exiting tasks are properly gathered and synchronized, preventing
	missed synchronizations and further deadlocks.
Patch 3:
	Subject: rcu-tasks: Maintain real-time response in rcu_tasks_postscan()
	Commit ID: 0bb11a372fc8d7006b4d0f42a2882939747bdbff
	Justification: Periodically enables interrupts during per-CPU list traversal to maintain
	system responsiveness, especially when many tasks are present.

Kernel version: latest kernel version in 6.6.y branch

Thank you for your consideration.

Best regards,
Tahera Fahimi

Re: Backport rcu-tasks fix to stable kernel version 6.6.y

[Greg KH]

On Thu, Sep 11, 2025 at 01:35:33PM -0700, Tahera Fahimi wrote:
> Dear Stable Kernel Maintainers,
> 
> I am writing to request the backport of the following patches to stable kernel versions 6.6.y,
> addressing a deadlock issue in RCU Tasks related to do_exit() on preemptible systems.
> 
> Issue Description:
> The kernel may experience deadlocks due to shared locking between exit_tasks_rcu_start() and
> rcu_tasks_postscan() via synchronize_srcu(), when multiple processes exit concurrently. The problem
> manifests as stalls in the RCU tasks grace period. This issue manifest as a deadlock in WSL kernel
> which uses the stable kernel 6.6.87 (Please see the issue on https://github.com/microsoft/WSL/issues/13480)
> 
> Patches to Apply:
> Patch 1:
> 	Subject: rcu-tasks: Maintain lists to eliminate RCU-tasks/do_exit() deadlocks
> 	Commit ID: 6b70399f9ef3809f6e308fd99dd78b072c1bd05c
> 	Justification: Introduces per-CPU lists for exiting tasks, replacing SRCU-based waits and
> 	eliminating deadlocks during concurrent exits.
> Patch 2:
> 	Subject: rcu-tasks: Eliminate deadlocks involving do_exit() and RCU tasks
> 	Commit ID: 1612160b91272f5b1596f499584d6064bf5be794
> 	Justification: Ensures all exiting tasks are properly gathered and synchronized, preventing
> 	missed synchronizations and further deadlocks.
> Patch 3:
> 	Subject: rcu-tasks: Maintain real-time response in rcu_tasks_postscan()
> 	Commit ID: 0bb11a372fc8d7006b4d0f42a2882939747bdbff
> 	Justification: Periodically enables interrupts during per-CPU list traversal to maintain
> 	system responsiveness, especially when many tasks are present.
> 
> Kernel version: latest kernel version in 6.6.y branch

Now queued up, thanks.

greg k-h