linux-kernel - Re: [PATCH] xen-blkback: allow module to be cleanly unloaded

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Date:   Fri, 29 Nov 2019 12:56:15 +0100
From:   Jan Beulich <jbeulich@...e.com>
To:     Paul Durrant <pdurrant@...zon.com>
Cc:     xen-devel@...ts.xenproject.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Roger Pau Monné <roger.pau@...rix.com>,
        Jens Axboe <axboe@...nel.dk>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH] xen-blkback: allow module to be cleanly unloaded

On 29.11.2019 12:31, Paul Durrant wrote:
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -173,6 +173,8 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
>  	init_completion(&blkif->drain_complete);
>  	INIT_WORK(&blkif->free_work, xen_blkif_deferred_free);
>  
> +	__module_get(THIS_MODULE);
> +
>  	return blkif;
>  }
>  
> @@ -320,6 +322,8 @@ static void xen_blkif_free(struct xen_blkif *blkif)
>  
>  	/* Make sure everything is drained before shutting down */
>  	kmem_cache_free(xen_blkif_cachep, blkif);
> +
> +	module_put(THIS_MODULE);
>  }

I realize there are various example of this in the tree, but
isn't this a flawed approach? __module_get() (nor even
try_module_get()) will prevent an unload attempt ahead of it
getting invoked, while execution is already in this module's
.text section. I think the xenbus driver should do this
before calling ->probe(), in case of its failure, and after
a successful call to ->remove().

Jan