linux-kernel - Re: [PATCH V3] firmware: google: Test spinlock on panic path to avoid lockups

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Message-ID: <YxDhiSDs4YcUrqV5@kroah.com>
Date:   Thu, 1 Sep 2022 18:44:57 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>
Cc:     evgreen@...omium.org, arnd@...db.de, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel@...ccoli.net, ardb@...nel.org,
        davidgow@...gle.com, jwerner@...omium.org,
        Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH V3] firmware: google: Test spinlock on panic path to
 avoid lockups

On Thu, Sep 01, 2022 at 01:24:46PM -0300, Guilherme G. Piccoli wrote:
> On 01/09/2022 13:04, Greg KH wrote:
> > [...]
> >>> What happens if the lock is grabbed right after testing for it?
> >>> Shouldn't you use lockdep_assert_held() instead as the documentation
> >>> says to?
> >>
> >> How, if in this point only a single CPU (this one, executing the code)
> >> is running?
> > 
> > How are we supposed to know this here?
> > 
> 
> Reading the code?
> Or you mean, in the commit description this should be mentioned?

Yes, and in the comment as this type of call is very rare and should
almost never be used.

> >> other CPUs, except this one executing the code. So, either the lock was
> >> taken (and we bail), or it wasn't and it's safe to continue.
> > 
> > Then who else could have taken the lock?  And if all other CPUs are
> > stopped, who cares about the lock at all?  Just don't grab it (you
> > should check for that when you want to grab it) and then you can work
> > properly at that point in time.
> > 
> 
> I don't think it is so simple - we are in the panic path.

Great, then the lock doesn't matter :)

> So, imagine the lock was taken in CPU0, where GSMI is doing some
> operation. During that operation, CPU1 panics!
> 
> When that happens, panic() executes in CPU1, disabling CPU0 through
> "strong" mechanisms (NMI). So, CPU0 had the lock, it is now off, and
> when CPU1 goes through the panic notifiers, it'll eventually wait
> forever for this lock in the GSMI handler, unless we have this patch
> that would prevent the handler to run in such case.
> Makes sense?

I'm trying to say "if you are in panic, never grab the lock in the first
place".  So change the place when you grab the lock, not here.

thanks,

greg k-h