From: Alan Stern <stern@rowland.harvard.edu>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
"Rafael J. Wysocki" <rafael@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
USB list <linux-usb@vger.kernel.org>
Subject: Re: Converting dev->mutex into dev->spinlock ?
Date: Sat, 4 Feb 2023 10:12:15 -0500 [thread overview]
Message-ID: <Y951z0Au9MlP1GxX@rowland.harvard.edu> (raw)
In-Reply-To: <28a82f50-39d5-a45f-7c7a-57a66cec0741@I-love.SAKURA.ne.jp>
On Sat, Feb 04, 2023 at 10:32:11PM +0900, Tetsuo Handa wrote:
> Hello.
>
> There is a long-standing deadlock problem in driver core code caused by
> "struct device"->mutex being marked as "do not apply lockdep checks".
What exactly is the deadlock problem? Furthermore, how can skipping
some lockdep checks _cause_ a problem?
> We can make this deadlock visible by applying [1], and we can confirm that
> there is a deadlock problem that I think needs to be addressed in core code [2].
I don't understand why you think there is a deadlock problem. The splat
in [2] says "WARNING: possible recursive locking detected". This is
only a warning; it doesn't mean there really is a problem.
> Also, since driver developers are taking it for granted that driver callback
> functions can behave as if dev->mutex is not held (because possibility of deadlock
> was never reported),
What? I have no idea what developers you're talking about. I have
never heard of anyone taking this for granted. Certainly developers
working on the USB subsystem's core are well aware of dev->mutex
locking.
> it would solve many deadlocks in driver code if you can update
What deadlocks? If there are so many deadlocks floating around in
driver code, why haven't we heard about them before now?
> driver core code to avoid calling driver callback functions with dev->mutex held
We most definitely cannot do that. The driver core relies on mutual
exclusion.
> (by e.g. replacing dev->mutex with dev->spinlock and dev->atomic_flags).
> But I'm not familiar enough to propose such change...
Such a change cannot be made. Consider this: Driver callbacks often
need to sleep. But when a thread holds a spinlock, it is not allowed to
sleep. Therefore driver callbacks must not be invoked while a spinlock
is held.
Besides, how will replacing a mutex with a spinlock prevent any deadlock
problems? If the new locks get held at the same time as the old mutexes
were held, won't the same deadlocks occur?
Alan Stern
> [1] https://lkml.kernel.org/r/8c3fc3d1-8fed-be22-e0e7-ef1e1ea723ce@I-love.SAKURA.ne.jp
> [2] https://lkml.kernel.org/r/b7bc63c8-bb28-d21d-7c3f-97e4e79a9292@I-love.SAKURA.ne.jp
next prev parent reply other threads:[~2023-02-04 15:12 UTC|newest]
Thread overview: 75+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-04 13:32 Converting dev->mutex into dev->spinlock ? Tetsuo Handa
2023-02-04 13:47 ` Greg Kroah-Hartman
2023-02-04 14:21 ` Tetsuo Handa
2023-02-04 14:34 ` Greg Kroah-Hartman
2023-02-04 15:16 ` Tetsuo Handa
2023-02-04 15:34 ` Alan Stern
2023-02-04 16:12 ` Tetsuo Handa
2023-02-04 16:27 ` Alan Stern
2023-02-04 17:09 ` Tetsuo Handa
2023-02-04 20:01 ` Alan Stern
2023-02-04 20:14 ` Linus Torvalds
2023-02-05 1:23 ` Alan Stern
2023-02-06 14:13 ` Tetsuo Handa
2023-02-06 15:45 ` Alan Stern
2023-02-07 13:07 ` Tetsuo Handa
2023-02-07 17:46 ` Alan Stern
2023-02-07 22:17 ` Tetsuo Handa
2023-02-08 0:34 ` Alan Stern
[not found] ` <20230208080739.1649-1-hdanton@sina.com>
2023-02-08 10:30 ` [PATCH] drivers/core: Replace lockdep_set_novalidate_class() with unique class keys Tetsuo Handa
2023-02-08 15:07 ` Alan Stern
2023-02-09 0:22 ` Tetsuo Handa
2023-02-09 0:46 ` Linus Torvalds
2023-02-09 1:50 ` Tetsuo Handa
2023-02-09 2:26 ` Alan Stern
2023-02-11 2:04 ` Tetsuo Handa
2023-02-11 21:41 ` [PATCH RFC] " Alan Stern
2023-02-11 21:51 ` Linus Torvalds
2023-02-11 23:06 ` Kent Overstreet
2023-02-11 23:08 ` Kent Overstreet
2023-02-11 23:24 ` Kent Overstreet
2023-02-12 2:40 ` Alan Stern
2023-02-12 2:46 ` Kent Overstreet
2023-02-12 3:03 ` Alan Stern
2023-02-12 3:10 ` Kent Overstreet
2023-02-12 15:23 ` Alan Stern
2023-02-12 19:14 ` Kent Overstreet
2023-02-12 20:19 ` Alan Stern
2023-02-12 20:51 ` Kent Overstreet
2023-02-13 1:23 ` Alan Stern
2023-02-13 2:21 ` Kent Overstreet
2023-02-13 15:25 ` Alan Stern
2023-02-13 9:29 ` Peter Zijlstra
2023-02-13 9:27 ` Peter Zijlstra
2023-02-13 15:28 ` Alan Stern
2023-02-13 16:36 ` Peter Zijlstra
2023-02-13 9:24 ` Peter Zijlstra
2023-02-13 15:25 ` Alan Stern
2023-02-13 16:29 ` Peter Zijlstra
2023-02-14 1:51 ` Boqun Feng
2023-02-14 1:53 ` Boqun Feng
2023-02-14 2:03 ` Alan Stern
2023-02-14 2:09 ` Boqun Feng
[not found] ` <20230214052733.3354-1-hdanton@sina.com>
2023-02-14 5:55 ` Boqun Feng
2023-02-14 10:48 ` Peter Zijlstra
2023-02-14 16:22 ` Boqun Feng
2023-02-15 10:45 ` Peter Zijlstra
2023-02-20 17:32 ` Boqun Feng
2023-02-13 18:46 ` Kent Overstreet
2023-02-14 11:05 ` Peter Zijlstra
2023-02-14 20:05 ` Alan Stern
2023-02-15 10:33 ` Peter Zijlstra
2023-02-14 20:16 ` Kent Overstreet
[not found] ` <20230212013220.2678-1-hdanton@sina.com>
2023-02-12 1:52 ` Kent Overstreet
2023-02-13 9:49 ` Peter Zijlstra
2023-02-13 16:18 ` Alan Stern
2023-02-13 17:51 ` Greg Kroah-Hartman
2023-02-13 18:05 ` Alan Stern
2023-02-05 1:31 ` Converting dev->mutex into dev->spinlock ? Tetsuo Handa
2023-02-05 16:46 ` Alan Stern
[not found] ` <20230206025629.1786-1-hdanton@sina.com>
2023-02-06 4:44 ` Matthew Wilcox
2023-02-06 5:17 ` Greg Kroah-Hartman
[not found] ` <20230206064305.1838-1-hdanton@sina.com>
2023-02-06 6:48 ` Greg Kroah-Hartman
2023-02-04 15:12 ` Alan Stern [this message]
2023-02-04 15:30 ` Tetsuo Handa
2023-02-04 15:40 ` Alan Stern
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