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SubjectRe: [PATCH] i2c: drop ancient protection against sysfs refcounting issues
On Mon, Jan 19, 2015 at 07:55:56PM +0100, Wolfram Sang wrote:
> Back in the days, sysfs seemed to have refcounting issues and subsystems
> needed a completion to be safe. This is not the case anymore, so I2C can
> get rid of this code. There is noone else besides I2C doing something
> like this currently (checked with the attached coccinelle script which
> checks if a release function exists and if it contains a completion).
>
> I have been digging through the history of linux.git and
> linux-history.git and found that e.g. w1 used to have such a mechanism
> and also simply removed it later.
>
> Some more info from Greg Kroah-Hartman:
> "Having that call "wait" for the other release call to happen is really
> old, as Jean points out, from 2003. We have "fixed" sysfs since then to
> detach the files from the devices easier, we used to have some nasy
> reference count issues in that area."
>
> And some testing from Jean Delvare which matches my results:
> "However I just tested unloading an i2c bus driver while its adapter's
> new_device attribute was opened and rmmod returned immediately. So it
> doesn't look like accessing sysfs attributes actually takes a reference
> to the underlying i2c_adapter."
>
> Let's get rid of this code before really nobody knows/understands
> anymore what this was for and if it has a subtle use.
>
> Reported-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
> Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Jean Delvare <jdelvare@suse.de>
> Cc: Julia Lawall <julia.lawall@lip6.fr>
> ---
>
> Of course, more testing is appreciated. Here is the coccinelle script:
>
> ===
>
> @has_type@
> identifier d_type, rel_f;
> @@
>
> struct device_type d_type = {
> .release = rel_f,
> };
>
> @has_device@
> struct device *d;
> identifier rel_f, p;
> @@
>
> (
> p->dev.release = &rel_f;
> |
> d->release = &rel_f;
> )
>
> @find_type depends on has_type@
> identifier has_type.rel_f, d;
> @@
>
> void rel_f(struct device *d)
> {
> ...
> * complete(...);
> ...
> }
>
> @find_device depends on has_device@
> identifier has_device.rel_f, d;
> @@
>
> void rel_f(struct device *d)
> {
> ...
> * complete(...);
> ...
> }
>
> ===
>
> drivers/i2c/i2c-core.c | 10 +---------
> include/linux/i2c.h | 1 -
> 2 files changed, 1 insertion(+), 10 deletions(-)
>
> diff --git a/drivers/i2c/i2c-core.c b/drivers/i2c/i2c-core.c
> index 39d25a8cb1ad..15cc5902cf89 100644
> --- a/drivers/i2c/i2c-core.c
> +++ b/drivers/i2c/i2c-core.c
> @@ -41,7 +41,6 @@
> #include <linux/of_device.h>
> #include <linux/of_irq.h>
> #include <linux/clk/clk-conf.h>
> -#include <linux/completion.h>
> #include <linux/hardirq.h>
> #include <linux/irqflags.h>
> #include <linux/rwsem.h>
> @@ -1184,8 +1183,7 @@ EXPORT_SYMBOL_GPL(i2c_new_dummy);
>
> static void i2c_adapter_dev_release(struct device *dev)
> {
> - struct i2c_adapter *adap = to_i2c_adapter(dev);
> - complete(&adap->dev_released);
> + /* empty, but the driver core insists we need a release function */

Yeah, it does, but I hate to see this in "real" code as something is
probably wrong with it if it happens.

Please move the rest of 'i2c_del_adapter' into the release function
(what was after the wait_for_completion() call), and then all should be
fine.

thanks,

greg k-h


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