linux-kernel - Re: [PATCH] sysrq: Auto release device node using _

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Date: Sat, 13 Apr 2024 21:15:25 +0200 (CEST)
From: Julia Lawall <julia.lawall@...ia.fr>
To: Greg KH <gregkh@...uxfoundation.org>
cc: Julia Lawall <julia.lawall@...ia.fr>, Rob Herring <robh@...nel.org>, 
    Roman Storozhenko <romeusmeister@...il.com>, jirislaby@...nel.org, 
    skhan@...uxfoundation.org, javier.carrasco.cruz@...il.com, 
    linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH] sysrq: Auto release device node using __free attribute



On Fri, 12 Apr 2024, Greg KH wrote:

> On Thu, Apr 11, 2024 at 08:17:07PM +0200, Julia Lawall wrote:
> >
> >
> > On Thu, 11 Apr 2024, Greg KH wrote:
> >
> > > On Thu, Apr 11, 2024 at 08:02:56PM +0200, Roman Storozhenko wrote:
> > > > Add a cleanup function attribute '__free(device_node)' to the device node
> > > > pointer initialization statement and remove the pairing cleanup function
> > > > call of 'of_node_put' at the end of the function.
> > > > The '_free()' attrubute is introduced by scope-based resource management
> > > > in-kernel framework implemented in 'cleanup.h'. A pointer marked with
> > > > '__free()' attribute makes a compiler insert a cleanup function call
> > > > to the places where the pointer goes out of the scope. This feature
> > > > allows to get rid of manual cleanup function calls.
> > > >
> > > > Suggested-by: Julia.Lawall <Julia.Lawall@...ia.fr>
> > > > Signed-off-by: Roman Storozhenko <romeusmeister@...il.com>
> > > > ---
> > > > This patch targets the next tree:
> > > > tree: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
> > > > tag: next-20240411
> > > > ---
> > > >  drivers/tty/sysrq.c | 7 +++----
> > > >  1 file changed, 3 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> > > > index 02217e3c916b..1d1261f618c0 100644
> > > > --- a/drivers/tty/sysrq.c
> > > > +++ b/drivers/tty/sysrq.c
> > > > @@ -758,11 +758,12 @@ static void sysrq_detect_reset_sequence(struct sysrq_state *state,
> > > >  static void sysrq_of_get_keyreset_config(void)
> > > >  {
> > > >  	u32 key;
> > > > -	struct device_node *np;
> > > >  	struct property *prop;
> > > >  	const __be32 *p;
> > > >
> > > > -	np = of_find_node_by_path("/chosen/linux,sysrq-reset-seq");
> > > > +	struct device_node *np __free(device_node) =
> > > > +		of_find_node_by_path("/chosen/linux,sysrq-reset-seq");
> > > > +
> > > >  	if (!np) {
> > > >  		pr_debug("No sysrq node found");
> > > >  		return;
> > > > @@ -781,8 +782,6 @@ static void sysrq_of_get_keyreset_config(void)
> > > >
> > > >  	/* Get reset timeout if any. */
> > > >  	of_property_read_u32(np, "timeout-ms", &sysrq_reset_downtime_ms);
> > > > -
> > > > -	of_node_put(np);
> > > >  }
> > > >  #else
> > > >  static void sysrq_of_get_keyreset_config(void)
> > >
> > > Also, this change really makes no sense at all, the pointer never goes
> > > out of scope except when the function is over, at the bottom.  So why
> > > make this complex change at all for no benefit?
> > >
> > > In other words, properly understand the change you are making and only
> > > make it if it actually makes sense.  It does not make any sense here,
> > > right?
> >
> > Maybe it would be nice to get rid of of_node_puts in the general case?
>
> That's a call for the of maintainer to make, and then if so, to do it
> across the whole tree, right?
>
> > Even though this one is not very annoying, there are some other functions
> > where there are many of_node_puts, and convoluted error handling code to
> > incorporate them on both the success and failure path.  So maybe it would
> > be better to avoid the situation of having them sometimes and not having
> > them other times?  But this is an opinion, and if the general consensus is
> > to only get rid of the cases that currently add complexity, then that is
> > possible too.
>
> Let's keep things simple until it has to be complex please.

Jonathan Cameron pointed me to the following series from Rob Herring:

https://lore.kernel.org/linux-devicetree/20240409-dt-cleanup-free-v2-0-5b419a4af38d@kernel.org/

The patch for of_node_put is:

https://lore.kernel.org/linux-devicetree/20240409-dt-cleanup-free-v2-3-5b419a4af38d@kernel.org/

It uses __free directy. The cases in the file drivers/of/property.c have
quite simple structure, with for each get just one put at the end of the
scope in most cases.

julia


>
> thanks,
>
> greg k-h
>