Re: [PATCH] soc/tegra: Fix bad of_node_put() in powergate init

[Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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On Sat, Jul 29, 2017 at 02:58:43AM +0300, Tuomas Tynkkynen wrote:
> The for_each_child_of_node macro itself maintains the correct reference
> count of the nodes so the explicit of_node_put() call causes a warning:
> 
> [    0.098960] OF: ERROR: Bad of_node_put() on /pmc@7000e400/powergates/xusba
> [    0.098981] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.11.3 #1-NixOS
> [    0.098996] Hardware name: NVIDIA Jetson TX1 Developer Kit (DT)
> [    0.099011] Call trace:
> [    0.099034] [<ffff00000808a048>] dump_backtrace+0x0/0x2a0
> [    0.099051] [<ffff00000808a30c>] show_stack+0x24/0x30
> [    0.099069] [<ffff0000084a6494>] dump_stack+0x9c/0xc0
> [    0.099090] [<ffff000008992214>] of_node_release+0xa4/0xa8
> [    0.099107] [<ffff0000084a9270>] kobject_put+0x90/0x1f8
> [    0.099124] [<ffff0000089914ac>] of_node_put+0x24/0x30
> [    0.099140] [<ffff00000898cec4>] __of_get_next_child+0x4c/0x70
> [    0.099155] [<ffff00000898cf28>] of_get_next_child+0x40/0x68
> [    0.099173] [<ffff0000090a099c>] tegra_pmc_early_init+0x4e8/0x5ac
> [    0.099189] [<ffff00000808399c>] do_one_initcall+0x5c/0x168
> [    0.099206] [<ffff000009050c98>] kernel_init_freeable+0xd4/0x240
> [    0.099224] [<ffff000008b2d658>] kernel_init+0x18/0x108
> [    0.099238] [<ffff0000080836c0>] ret_from_fork+0x10/0x50
> 
> (It's not very apparent from the OF documentation that of_node_put() is
> not needed; the macro itself has no docstring and of_get_next_child()
> used in the implementation begins with "Returns a node pointer with
> refcount incremented" but then only at the very end of the docstring
> the crucial part "Decrements the refcount of prev" is mentioned.)

Yeah, this kind of mistake has been going around for a while. There's
even a semantic patch that fixes this kind of error (see
(scripts/coccinelle/iterators/device_node_continue.cocci). I wonder if
adding kerneldoc to for_each_child_of_node() may help avoid this kind of
mistake in the future.

> Fixes: a38045121bf42 ("soc/tegra: pmc: Add generic PM domain support")
> Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen-X3B1VOXEql0@public.gmane.org>
> ---
> Compile tested only (backtrace received via a distro bug report).
> ---
>  drivers/soc/tegra/pmc.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)

Applied, thanks.

Thierry

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