From: Greg KH <gregkh@linuxfoundation.org>
To: Dmitry Torokhov <dtor@vmware.com>
Cc: George Zhang <georgezhang@vmware.com>,
pv-drivers@vmware.com, linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org
Subject: Re: [Pv-drivers] [PATCH 01/12] VMCI: context implementation.
Date: Tue, 30 Oct 2012 08:46:52 -0700 [thread overview]
Message-ID: <20121030154652.GB14167@kroah.com> (raw)
In-Reply-To: <20121030040139.GA32055@dtor-ws.eng.vmware.com>
On Mon, Oct 29, 2012 at 09:01:40PM -0700, Dmitry Torokhov wrote:
> Hi Greg,
>
> On Mon, Oct 29, 2012 at 07:10:58PM -0700, Greg KH wrote:
> > On Mon, Oct 29, 2012 at 06:03:42PM -0700, George Zhang wrote:
> > > +/*
> > > + * Releases the VMCI context. If this is the last reference to
> > > + * the context it will be deallocated. A context is created with
> > > + * a reference count of one, and on destroy, it is removed from
> > > + * the context list before its reference count is
> > > + * decremented. Thus, if we reach zero, we are sure that nobody
> > > + * else are about to increment it (they need the entry in the
> > > + * context list for that). This function musn't be called with a
> > > + * lock held.
> > > + */
> > > +void vmci_ctx_release(struct vmci_ctx *context)
> > > +{
> > > + ASSERT(context);
> > > + kref_put(&context->kref, ctx_free_ctx);
> > > +}
> > > +
> >
> > Hm, are you _sure_ you should be calling this without a lock held?
> > That's usually kref-101, you MUST hold a lock when calling put,
> > otherwise you can race a kref_get() call, and all hell can break loose.
> >
> > Because of this, some saner people (like Al Viro), have suggested that I
> > force the kref_put() and kref_get() calls pass in a spinlock just to
> > enforce this.
> >
> > So, tell me what I'm missing here, and why you put the comment here
> > saying that it really is supposed to be called without a lock held? How
> > is that safe?
> >
>
> Contexts are created/registered in vmci_ctx_init_ctx() and unregistered in
> vmci_ctx_release_ctx() and these operations are protected by
> ctx_list.lock spinlock. Context lookup (vmci_ctx_get) also uses spinlock
> to traverse list of registered contexts and then grabs reference to the
> [valid] context. The use of kref_put() without additional locking in
> vmci_ctx_release() is fine as there is no chance of another thread
> bumping count from 0 to 1.
As I didn't see all callers of this holding that spinlock, it was
confusing. You should put this type of description somewhere so that
other reviewers don't have the same questions.
> I believe the comment should actually read that the function should not
> be called from atomic contexts.
That might be nice to document, but could it ever happen?
thanks,
greg k-h
next prev parent reply other threads:[~2012-10-30 15:46 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-30 1:03 [PATCH 00/12] VMCI for Linux upstreaming George Zhang
2012-10-30 1:03 ` [PATCH 01/12] VMCI: context implementation George Zhang
2012-10-30 2:07 ` Greg KH
2012-10-30 2:10 ` Greg KH
2012-10-30 4:01 ` [Pv-drivers] " Dmitry Torokhov
2012-10-30 15:46 ` Greg KH [this message]
2012-10-30 15:56 ` Dmitry Torokhov
2012-10-30 1:03 ` [PATCH 02/12] VMCI: datagram implementation George Zhang
2012-10-30 1:04 ` [PATCH 03/12] VMCI: doorbell implementation George Zhang
2012-10-30 1:04 ` [PATCH 04/12] VMCI: device driver implementaton George Zhang
2012-10-30 2:21 ` Greg KH
2012-10-30 2:23 ` Greg KH
2012-10-30 4:15 ` [Pv-drivers] " Dmitry Torokhov
2012-10-30 15:49 ` Greg KH
2012-10-30 1:04 ` [PATCH 05/12] VMCI: event handling implementation George Zhang
2012-10-30 2:24 ` Greg KH
2012-10-30 4:58 ` [Pv-drivers] " Dmitry Torokhov
2012-10-30 15:50 ` Greg KH
2012-10-30 2:26 ` Greg KH
2012-10-30 5:01 ` [Pv-drivers] " Dmitry Torokhov
2012-10-30 15:50 ` Greg KH
2012-10-30 15:59 ` Dmitry Torokhov
2012-10-30 1:04 ` [PATCH 06/12] VMCI: handle array implementation George Zhang
2012-10-30 1:04 ` [PATCH 07/12] VMCI: queue pairs implementation George Zhang
2012-10-30 1:04 ` [PATCH 08/12] VMCI: resource object implementation George Zhang
2012-10-30 2:29 ` Greg KH
2012-10-30 5:20 ` [Pv-drivers] " Dmitry Torokhov
2012-10-30 15:51 ` Greg KH
2012-10-30 16:11 ` Dmitry Torokhov
2012-10-30 2:29 ` Greg KH
2012-10-30 5:21 ` [Pv-drivers] " Dmitry Torokhov
2012-10-30 1:05 ` [PATCH 09/12] VMCI: routing implementation George Zhang
2012-10-30 1:05 ` [PATCH 10/12] VMCI: guest side driver implementation George Zhang
2012-10-30 1:05 ` [PATCH 11/12] VMCI: host " George Zhang
2012-10-30 1:05 ` [PATCH 12/12] VMCI: Some header and config files George Zhang
2012-10-30 2:32 ` Greg KH
2012-10-30 5:22 ` [Pv-drivers] " Dmitry Torokhov
2012-10-30 2:38 ` Greg KH
2012-10-30 2:19 ` [PATCH 00/12] VMCI for Linux upstreaming Greg KH
2012-10-30 4:07 ` [Pv-drivers] " Dmitry Torokhov
2012-10-30 15:48 ` Greg KH
2012-10-30 16:18 ` Dmitry Torokhov
2012-10-30 16:27 ` Greg KH
2012-10-30 19:43 ` Dmitry Torokhov
2012-10-30 19:59 ` Greg KH
2012-11-21 20:31 George Zhang
2012-11-21 20:31 ` [PATCH 01/12] VMCI: context implementation George Zhang
2012-11-21 21:04 ` Joe Perches
2012-11-21 21:10 ` [Pv-drivers] " Andy King
2012-11-21 21:29 ` Dmitry Torokhov
2012-11-21 21:35 ` Joe Perches
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