Re: [PATCH] rust: devres: fix race condition due to nesting

["Danilo Krummrich" <dakr@kernel.org>]

On Thu Feb 5, 2026 at 11:25 PM CET, Danilo Krummrich wrote:
> Commit f5d3ef25d238 ("rust: devres: get rid of Devres' inner Arc") did
> attempt to optimize away the internal reference count of Devres.
>
> However, without an internal reference count, we can't support cases
> where Devres is indirectly nested, resulting into a deadlock.
>
> Such indirect nesting easily happens in the following way:
>
> A registration object (which is guarded by devres) hold a reference
> count of an object that holds a device resource guarded by devres
> itself.
>
> For instance a drm::Registration holds a reference of a drm::Device. The
> drm::Device itself holds a device resource in its private data.
>
> When the drm::Registration is dropped by devres, and it happens that it
> did hold the last reference count of the drm::Device, it also drops the
> device resource, which is guarded by devres itself.
>
> Thus, resulting into a deadlock in the Devres destructor of the device
> resource, as in the following backtrace.
>
> 	sysrq: Show Blocked State
> 	task:rmmod           state:D stack:0     pid:1331  tgid:1331  ppid:1330   task_flags:0x400100 flags:0x00000010
> 	Call trace:
> 	 __switch_to+0x190/0x294 (T)
> 	 __schedule+0x878/0xf10
> 	 schedule+0x4c/0xcc
> 	 schedule_timeout+0x44/0x118
> 	 wait_for_common+0xc0/0x18c
> 	 wait_for_completion+0x18/0x24
> 	 _RINvNtCs4gKlGRWyJ5S_4core3ptr13drop_in_placeINtNtNtCsgzhNYVB7wSz_6kernel4sync3arc3ArcINtNtBN_6devres6DevresmEEECsRdyc7Hyps3_15rust_driver_pci+0x68/0xe8 [rust_driver_pci]
> 	 _RINvNvNtCsgzhNYVB7wSz_6kernel6devres16register_foreign8callbackINtNtCs4gKlGRWyJ5S_4core3pin3PinINtNtNtB6_5alloc4kbox3BoxINtNtNtB6_4sync3arc3ArcINtB4_6DevresmEENtNtB1A_9allocator7KmallocEEECsRdyc7Hyps3_15rust_driver_pci+0x34/0xc8 [rust_driver_pci]
> 	 devm_action_release+0x14/0x20
> 	 devres_release_all+0xb8/0x118
> 	 device_release_driver_internal+0x1c4/0x28c
> 	 driver_detach+0x94/0xd4
> 	 bus_remove_driver+0xdc/0x11c
> 	 driver_unregister+0x34/0x58
> 	 pci_unregister_driver+0x20/0x80
> 	 __arm64_sys_delete_module+0x1d8/0x254
> 	 invoke_syscall+0x40/0xcc
> 	 el0_svc_common+0x8c/0xd8
> 	 do_el0_svc+0x1c/0x28
> 	 el0_svc+0x54/0x1d4
> 	 el0t_64_sync_handler+0x84/0x12c
> 	 el0t_64_sync+0x198/0x19c
>
> In order to fix this, re-introduce the internal reference count.
>
> Reported-by: Boris Brezillon <boris.brezillon@collabora.com>
> Closes: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/.E2.9C.94.20Deadlock.20caused.20by.20nested.20Devres/with/571242651
> Reported-by: Markus Probst <markus.probst@posteo.de>
> Closes: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/.E2.9C.94.20Devres.20inside.20Devres.20stuck.20on.20cleanup/with/571239721
> Reported-by: Alice Ryhl <aliceryhl@google.com>
> Closes: https://gitlab.freedesktop.org/panfrost/linux/-/merge_requests/56#note_3282757
> Fixes: f5d3ef25d238 ("rust: devres: get rid of Devres' inner Arc")
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Applied to driver-core-testing, thanks!

    [ Call clone() prior to devm_add_action(). - Danilo ]