linux-kernel - Re: [PATCH 01/12] VMCI: context implementation.

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Message-ID: <20121030021058.GB1920@kroah.com>
Date:	Mon, 29 Oct 2012 19:10:58 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	George Zhang <georgezhang@...are.com>
Cc:	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, pv-drivers@...are.com
Subject: Re: [PATCH 01/12] VMCI: context implementation.

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?

confused,

greg k-h
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