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

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

[Hillf Danton]

On Sat, 4 Feb 2023 10:40:52 -0500 Alan Stern <stern@rowland.harvard.edu>
> On Sun, Feb 05, 2023 at 12:30:07AM +0900, Tetsuo Handa wrote:
> > On 2023/02/05 0:12, Alan Stern wrote:
> > >>  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?
> > 
> > Since dev->mutex is hidden from lockdep checks, nobody can see lockdep warnings.
> > syzbot is reporting real deadlocks without lockdep warnings, for the fundamental
> > problem you mentioned in https://lkml.kernel.org/r/Pine.LNX.4.44L0.0804171117450.18040-100000@iolanthe.rowland.org
> > is remaining. I'm suggesting you that now is time to address this fundamental problem.
> 
> Maybe so.  But the place to address it is inside lockdep, not in the 
> driver core.
> 
> > >> (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.
> > 
> > What I'm suggesting is "Do not call driver callbacks with dev->mutex held,
> > by rewriting driver core code".
> 
> That cannot be done.  The only possible solution is to teach lockdep how 
> to handle recursive locking structures.

It works in dcache - see the slow path in dentry_kill() for instance.

Hillf

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

[Hillf Danton]

On Sun, 5 Feb 2023 11:46:06 -0500 Alan Stern <stern@rowland.harvard.edu>
> 
> And by the way, by disabling lockdep validation I am declaraing that 
> enabling it would cause an overwhelming number of false positives, 

Could you share 5 false positives you see upstream to help understand how
it is useless?

Hillf

> rendering lockdep useless (as you found out when you tried).  Not that 
> driver developers don't have to worry about dev->mutex.
> 
> Alan Stern

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

[Matthew Wilcox]

On Mon, Feb 06, 2023 at 10:56:29AM +0800, Hillf Danton wrote:
> On Sun, 5 Feb 2023 11:46:06 -0500 Alan Stern <stern@rowland.harvard.edu>
> > 
> > And by the way, by disabling lockdep validation I am declaraing that 
> > enabling it would cause an overwhelming number of false positives, 
> 
> Could you share 5 false positives you see upstream to help understand how
> it is useless?

I've asked you before to stop cc'ing linux-mm on things which aren't
about memory management.  Now I'm asking you publically.

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

[Greg Kroah-Hartman]

On Mon, Feb 06, 2023 at 10:56:29AM +0800, Hillf Danton wrote:
> On Sun, 5 Feb 2023 11:46:06 -0500 Alan Stern <stern@rowland.harvard.edu>
> > 
> > And by the way, by disabling lockdep validation I am declaraing that 
> > enabling it would cause an overwhelming number of false positives, 
> 
> Could you share 5 false positives you see upstream to help understand how
> it is useless?

Please see this other email in this thread:
	https://lore.kernel.org/r/Y98FLlr7jkiFlV0k@rowland.harvard.edu

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

[Hillf Danton]

On Mon, 6 Feb 2023 06:17:03 +0100 Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> On Mon, Feb 06, 2023 at 10:56:29AM +0800, Hillf Danton wrote:
> > On Sun, 5 Feb 2023 11:46:06 -0500 Alan Stern <stern@rowland.harvard.edu>
> > > 
> > > And by the way, by disabling lockdep validation I am declaraing that 
> > > enabling it would cause an overwhelming number of false positives, 
> > 
> > Could you share 5 false positives you see upstream to help understand how
> > it is useless?
> 
> Please see this other email in this thread:
> 	https://lore.kernel.org/r/Y98FLlr7jkiFlV0k@rowland.harvard.edu

What lockdep warnings?  Specific examples, please.

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

[Greg Kroah-Hartman]

On Mon, Feb 06, 2023 at 02:43:05PM +0800, Hillf Danton wrote:
> On Mon, 6 Feb 2023 06:17:03 +0100 Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > On Mon, Feb 06, 2023 at 10:56:29AM +0800, Hillf Danton wrote:
> > > On Sun, 5 Feb 2023 11:46:06 -0500 Alan Stern <stern@rowland.harvard.edu>
> > > > 
> > > > And by the way, by disabling lockdep validation I am declaraing that 
> > > > enabling it would cause an overwhelming number of false positives, 
> > > 
> > > Could you share 5 false positives you see upstream to help understand how
> > > it is useless?
> > 
> > Please see this other email in this thread:
> > 	https://lore.kernel.org/r/Y98FLlr7jkiFlV0k@rowland.harvard.edu
> 
> What lockdep warnings?  Specific examples, please.

Remove the one line of code, as per the patch in this thread, and boot
with lockdep enabled and see what happens if you wish to see them
yourself.

thanks,

greg k-h