Re: [PATCH rdma-rc] RDMA/mlx5: Release locks during notifier unregister

From: Leon Romanovsky <leon@kernel.org>
To: Doug Ledford <dledford@redhat.com>
Cc: Jason Gunthorpe <jgg@mellanox.com>,
	RDMA mailing list <linux-rdma@vger.kernel.org>,
	Saeed Mahameed <saeedm@mellanox.com>
Subject: Re: [PATCH rdma-rc] RDMA/mlx5: Release locks during notifier unregister
Date: Thu, 1 Aug 2019 19:23:56 +0300	[thread overview]
Message-ID: <20190801162356.GV4832@mtr-leonro.mtl.com> (raw)
In-Reply-To: <a0dc81b63fdef1b7e877d5172be13792dda763d2.camel@redhat.com>

On Thu, Aug 01, 2019 at 12:11:20PM -0400, Doug Ledford wrote:
> On Thu, 2019-08-01 at 18:59 +0300, Leon Romanovsky wrote:
> > > There's no need for a lockdep.  The removal of the notifier callback
> > > entry is re-entrant safe.  The core removal routines have their own
> > > spinlock they use to protect the actual notifier list.  If you call
> > > it
> > > more than once, the second and subsequent calls merely scan the
> > > list,
> > > find no matching entry, and return ENOENT.  The only reason this
> > > might
> > > need a lock and a lockdep entry is if you are protecting against a
> > > race
> > > with the *add* notifier code in the mlx5 driver specifically (the
> > > core
> > > add code won't have an issue, but since you only have a single place
> > > to
> > > store the notifier callback pointer, if it would be possible for you
> > > to
> > > add two callbacks and write over the first callback pointer with the
> > > second without removing the first, then you would leak a callback
> > > notifier in the core notifier list).
> >
> > atomic_notifier_chain_unregister() unconditionally calls to
> > syncronize_rcu() and I'm not so sure that it is best thing to do
> > for every port unbind.
> >
> > Actually, I'm completely lost here, we are all agree that the patch
> > fixes issue correctly, and it returns the code to be exactly as
> > it was before commit df097a278c75 ("IB/mlx5: Use the new mlx5 core
> > notifier
> > API"). Can we simply merge it and fix the kernel panic?
>
> As long as you are OK with me adding a comment to the patch so people
> coming back later won't scratch their head about how can it possible be
> right to do that sequence without a lock held, I'm fine merging the fix.
>
> Something like:
>
> /*
>  * The check/unregister/set-NULL sequence below does not need to be
>  * locked for correctness as it's only an optimization, and can't
>  * be under a lock or will throw a scheduling while atomic error.
>  */

I think that the best place will be in commit message for this explanation,
but I'm fine with the comment inside code as well.

Thanks a lot, I appreciate it.

>
> --
> Doug Ledford <dledford@redhat.com>
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