From: Doug Ledford <dledford@redhat.com>
To: Leon Romanovsky <leon@kernel.org>
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, 01 Aug 2019 16:09:27 -0400 [thread overview]
Message-ID: <7b87cbbdfc4455fd7b265449f2f3f2d4a38e7441.camel@redhat.com> (raw)
In-Reply-To: <20190801173320.GZ4832@mtr-leonro.mtl.com>
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On Thu, 2019-08-01 at 20:33 +0300, Leon Romanovsky wrote:
> On Thu, Aug 01, 2019 at 12:42:43PM -0400, Doug Ledford wrote:
> > On Thu, 2019-08-01 at 19:23 +0300, Leon Romanovsky wrote:
> > > 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.
> >
> > Patch (unmodified) is applied to for-rc, thanks.
>
> Thanks Doug, I'll prepare patch with lockdep for Jason and
> will submit it to -next later on after passing verification.
Perfect, thanks Leon.
--
Doug Ledford <dledford@redhat.com>
GPG KeyID: B826A3330E572FDD
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prev parent reply other threads:[~2019-08-01 20:09 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-31 8:38 [PATCH rdma-rc] RDMA/mlx5: Release locks during notifier unregister Leon Romanovsky
2019-07-31 16:22 ` Doug Ledford
2019-07-31 17:00 ` Jason Gunthorpe
2019-07-31 17:09 ` Leon Romanovsky
2019-07-31 17:22 ` Jason Gunthorpe
2019-07-31 18:01 ` Leon Romanovsky
2019-07-31 18:51 ` Doug Ledford
2019-08-01 8:22 ` Leon Romanovsky
2019-07-31 19:55 ` Jason Gunthorpe
2019-08-01 8:27 ` Leon Romanovsky
2019-08-01 12:00 ` Jason Gunthorpe
2019-08-01 12:08 ` Leon Romanovsky
2019-08-01 14:16 ` Doug Ledford
2019-08-01 15:59 ` Leon Romanovsky
2019-08-01 16:11 ` Doug Ledford
2019-08-01 16:20 ` Jason Gunthorpe
2019-08-01 16:40 ` Doug Ledford
2019-08-01 16:43 ` Jason Gunthorpe
2019-08-01 16:50 ` Doug Ledford
2019-08-01 17:31 ` Leon Romanovsky
2019-08-01 16:23 ` Leon Romanovsky
2019-08-01 16:42 ` Doug Ledford
2019-08-01 17:33 ` Leon Romanovsky
2019-08-01 20:09 ` Doug Ledford [this message]
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