linux-kernel - Re: [PATCH v2 2/7] acpi: utils: Add function to fetch dependent acpi

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Date:   Tue, 2 Feb 2021 13:27:36 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Daniel Scally <djrscally@...il.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        linux-gpio@...r.kernel.org, linux-i2c <linux-i2c@...r.kernel.org>,
        Platform Driver <platform-driver-x86@...r.kernel.org>,
        "open list:ACPI COMPONENT ARCHITECTURE (ACPICA)" <devel@...ica.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>, andy@...nel.org,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Wolfram Sang <wsa@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Mark Gross <mgross@...ux.intel.com>,
        Robert Moore <robert.moore@...el.com>,
        Erik Kaneda <erik.kaneda@...el.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Kieran Bingham <kieran.bingham@...asonboard.com>
Subject: Re: [PATCH v2 2/7] acpi: utils: Add function to fetch dependent
 acpi_devices

On Tue, Feb 02, 2021 at 09:58:17AM +0000, Daniel Scally wrote:
> On 21/01/2021 21:06, Daniel Scally wrote:
> > On 21/01/2021 18:08, Rafael J. Wysocki wrote:

...

> > No problem;  I'll tweak that then
> 
> Slightly walking back my "No problem" here; as I understand this there's
> kinda two options:
> 
> 1. Walk over the (locked) list, when a match is found unlock, run the
> callback and re-lock.
> 
> The problem with that idea is unless I'm mistaken there's no guarantee
> that the .next pointer is still valid then (even using the *_safe()
> methods) because either the next or the next + 1 entry could have been
> removed whilst the list was unlocked and the callback was being ran, so
> this seems a little unsafe.

It's easy to solve.
See an example in deferred_probe_work_func().

https://elixir.bootlin.com/linux/latest/source/drivers/base/dd.c#L75

> 2. Walk over the (locked) list twice, the first time counting matching
> entries and using that to allocate a temporary buffer, then walk again
> to store the matching entries into the buffer. Finally, run the callback
> for everything in the buffer, free it and return.
> 
> Obviously that's a lot less efficient than the current function, which
> isn't particularly palatable.
> 
> Apologies if I've missed a better option that would work fine; but
> failing that do you still want me to go ahead and change
> acpi_walk_dep_device_list() to do this (I'd choose #2 of the above), or
> fallback to using acpi_dev_get_next_dependent_dev() described above? If
> the latter, does acpi_walk_dep_device_list() maybe need re-naming to
> make clear it's not a generalised function?

-- 
With Best Regards,
Andy Shevchenko