Re: Converting dev->mutex into dev->spinlock ?

[Tetsuo Handa]

On 2023/02/04 22:47, Greg Kroah-Hartman wrote:
> 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".
> 
> The marking of a lock does not cause a deadlock problem, so what do you
> mean exactly by this?  Where is the actual deadlock?

A few of examples:

  https://syzkaller.appspot.com/bug?extid=2d6ac90723742279e101
  https://syzkaller.appspot.com/bug?extid=2e39bc6569d281acbcfb
  https://syzkaller.appspot.com/bug?extid=9ef743bba3a17c756174

> 
>> 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].
> 
> Any reason why you didn't cc: us on these patches?

We can't apply this "drivers/core: Remove lockdep_set_novalidate_class() usage" patch
until we fix all lockdep warnings that happen during the boot stage; otherwise syzbot
testing can't work which is more painful than applying this patch now.

Therefore, I locally tested this patch (in order not to be applied now). And I got
a lockdep warning on the perf_event code. I got next lockdep warning on the driver core
code when I tried a fix for the perf_event code suggested by Peter Zijlstra. Since Peter
confirmed that this is a problem that led to commit 1704f47b50b5 ("lockdep: Add novalidate
class for dev->mutex conversion"), this time I'm reporting this problem to you (so that
you can propose a fix for the driver core code).