linux-kernel - Re: [PATCH 1/2] Revert "drivers: core: synchronize really_probe() and dev

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Message-ID: <9232C7B6-627B-43F9-AD5C-1EA4BB69E40D@gmail.com>
Date: Wed, 19 Feb 2025 23:22:25 -0800
From: Dmitry Torokhov <dmitry.torokhov@...il.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC: "Rafael J. Wysocki" <rafael@...nel.org>, Danilo Krummrich <dakr@...nel.org>,
 linux-kernel@...r.kernel.org,
 "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
 Dirk Behme <dirk.behme@...bosch.com>
Subject: Re: [PATCH 1/2] Revert "drivers: core: synchronize really_probe() and dev_uevent()"

On February 19, 2025 11:13:00 PM PST, Greg Kroah-Hartman <gregkh@...uxfoundation.org> wrote:
>On Wed, Feb 19, 2025 at 10:46:44PM -0800, Dmitry Torokhov wrote:
>> This reverts commit c0a40097f0bc81deafc15f9195d1fb54595cd6d0.
>> 
>> Probing a device can take arbitrary long time. In the field we observed
>> that, for example, probing a bad micro-SD cards in an external USB card
>> reader (or maybe cards were good but cables were flaky) sometimes takes
>> longer than 2 minutes due to multiple retries at various levels of the
>> stack. We can not block uevent_show() method for that long because udev
>> is reading that attribute very often and that blocks udev and interferes
>> with booting of the system.
>> 
>> The change that introduced locking was concerned with dev_uevent()
>> racing with unbinding the driver. However we can handle it without
>> locking (which will be done in subsequent patch).
>
>So shouldn't we take the second patch first to prevent any issues here?

I think the potential for the NULL dereference is extremely small, we lived with it for many years. But if you prefer the patches can be swapped.

>
>> There was also claim that synchronization with probe() is needed to
>> properly load USB drivers, however this is a red herring: the change
>> adding the lock was introduced in May of last year and USB loading and
>> probing worked properly for many years before that.
>> 
>> Revert the harmful locking.
>> 
>> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
>
>No Fixes: or cc: stable for this?

I did not think we need "fixes" for a revert... Do we?

As far as stable goes: you're the maintainer so IMO it's for you to decide, but yes, I'd like it to land in stable.

Thanks. 
Hi Greg, 
-- 
Dmitry