From: Halil Pasic <pasic@linux.ibm.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Tony Krowiak <akrowiak@linux.ibm.com>,
linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
borntraeger@de.ibm.com, cohuck@redhat.com,
pasic@linux.vnet.ibm.com, jjherne@linux.ibm.com,
alex.williamson@redhat.com, kwankhede@nvidia.com,
frankja@linux.ibm.com, david@redhat.com, imbrenda@linux.ibm.com,
hca@linux.ibm.com
Subject: Re: [PATCH] s390/vfio-ap: do not open code locks for VFIO_GROUP_NOTIFY_SET_KVM notification
Date: Thu, 15 Jul 2021 13:31:36 +0200 [thread overview]
Message-ID: <20210715133136.420c40b0.pasic@linux.ibm.com> (raw)
In-Reply-To: <20210713170533.GF136586@nvidia.com>
On Tue, 13 Jul 2021 14:05:33 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:
> On Tue, Jul 13, 2021 at 06:45:17PM +0200, Halil Pasic wrote:
>
> > Jason may give it another try to convince us that 0cc00c8d4050 only
> > silenced lockdep, but vfio_ap remained prone to deadlocks. To my best
> > knowledge using condition variable and a mutex is one of the well known
> > ways to implement an rwlock.
>
> The well known pattern is to use a rwsem.
I think you are missing the point. We are discussing whether
this qualifies for stable, i.e. if 0cc00c8d4050 is really broken
like the patch description says.
Using a readers-writers lock (as a primitive) to implement a
a readers-writers lock is a fallacy, so I guess you wanted to
say that when a readers-writers lock is needed in the kernel the
obvious choices are rw_semaphore and/or rwlock_t (depending on the
spin).
What I wanted to say is using a condition variable and a mutex is
not per-see wrong, because one can even implement an readers-writers
lock with it. For reference see https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock
>
> This:
> wait_event_cmd(matrix_mdev->wait_for_kvm,
> !matrix_mdev->kvm_busy,
> mutex_unlock(&matrix_dev->lock),
> mutex_lock(&matrix_dev->lock));
>
>
> Is not really a rwsem, and is invsible to lockdep.
>
I agree. But this is not a proof of a problem that qualifies to be fixed
using the stable process as documented in
https://github.com/torvalds/linux/blob/master/Documentation/process/stable-kernel-rules.rst
I'm in favor of rewriting this to use rw_semaphore. I'm not in favor
of proclaiming this a fix for stable, because for that you first have
to prove that you fix a real problem.
I hope we are on the same page.
Regards,
Halil
next prev parent reply other threads:[~2021-07-15 11:31 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-07 15:41 [PATCH] s390/vfio-ap: do not open code locks for VFIO_GROUP_NOTIFY_SET_KVM notification Tony Krowiak
2021-07-12 13:42 ` Tony Krowiak
2021-07-12 23:38 ` Halil Pasic
2021-07-13 13:48 ` Tony Krowiak
2021-07-13 16:45 ` Halil Pasic
2021-07-13 17:05 ` Jason Gunthorpe
2021-07-13 19:04 ` Tony Krowiak
2021-07-13 19:21 ` Jason Gunthorpe
2021-07-14 13:25 ` Tony Krowiak
2021-07-14 17:56 ` Christian Borntraeger
2021-07-14 18:43 ` Jason Gunthorpe
2021-07-14 19:02 ` Alex Williamson
2021-07-15 11:31 ` Halil Pasic [this message]
2021-07-15 14:57 ` Jason Gunthorpe
2021-07-13 18:47 ` Tony Krowiak
2021-07-15 13:44 ` Halil Pasic
2021-07-15 14:38 ` Tony Krowiak
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