linux-kernel - Re: [PATCH 05/30] misc/pvpanic: Convert regular spinlock into trylock on panic path

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Message-ID: <YoN/x2fpdDU4+nSB@alley>
Date:   Tue, 17 May 2022 12:58:15 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>
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Subject: Re: [PATCH 05/30] misc/pvpanic: Convert regular spinlock into
 trylock on panic path

On Tue 2022-05-10 10:00:58, Guilherme G. Piccoli wrote:
> On 10/05/2022 09:14, Petr Mladek wrote:
> > [...]
> >> With that said, it's dangerous to use regular spinlocks in such path,
> >> as introduced by commit b3c0f8774668 ("misc/pvpanic: probe multiple instances").
> >> This patch fixes that by replacing regular spinlocks with the trylock
> >> safer approach.
> > 
> > It seems that the lock is used just to manipulating a list. A super
> > safe solution would be to use the rcu API: rcu_add_rcu() and
> > list_del_rcu() under rcu_read_lock(). The spin lock will not be
> > needed and the list will always be valid.
> > 
> > The advantage would be that it will always call members that
> > were successfully added earlier. That said, I am not familiar
> > with pvpanic and am not sure if it is worth it.
> > 
> >> It also fixes an old comment (about a long gone framebuffer code) and
> >> the notifier priority - we should execute hypervisor notifiers early,
> >> deferring this way the panic action to the hypervisor, as expected by
> >> the users that are setting up pvpanic.
> > 
> > This should be done in a separate patch. It changes the behavior.
> > Also there might be a discussion whether it really should be
> > the maximal priority.
> > 
> > Best Regards,
> > Petr
> 
> Thanks for the review Petr. Patch was already merged - my goal was to be
> concise, i.e., a patch per driver / module, so the patch kinda fixes
> whatever I think is wrong with the driver with regards panic handling.
> 
> Do you think it worth to remove this patch from Greg's branch just to
> split it in 2? Personally I think it's not worth, but opinions are welcome.

No problem. It is not worth the effort.


> About the RCU part, this one really could be a new patch, a good
> improvement patch - it makes sense to me, we can think about that after
> the fixes I guess.

Yup.

Best Regards,
Petr