On Wed, Jan 14, 2015 at 10:07:13PM +0000, Hans Petter Selasky wrote:
> Author: hselasky
> Date: Wed Jan 14 22:07:13 2015
> New Revision: 277199
> URL: https://svnweb.freebsd.org/changeset/base/277199
>
> Log:
> Avoid race with "dev_rel()" when using the recently added
> "delist_dev()" function. Make sure the character device structure
> doesn't go away until the end of the "destroy_dev()" function due to
> concurrently running cleanup code inside "devfs_populate()".
>
> MFC after: 1 week
> Reported by: dchagin@
>
> Modified:
> head/sys/fs/devfs/devfs_devs.c
> head/sys/kern/kern_conf.c
>
> Modified: head/sys/fs/devfs/devfs_devs.c
> ==============================================================================
> --- head/sys/fs/devfs/devfs_devs.c Wed Jan 14 22:05:57 2015
> (r277198)
> +++ head/sys/fs/devfs/devfs_devs.c Wed Jan 14 22:07:13 2015
> (r277199)
> @@ -137,6 +137,12 @@ devfs_alloc(int flags)
> vfs_timestamp(&ts);
> cdev->si_atime = cdev->si_mtime = cdev->si_ctime = ts;
> cdev->si_cred = NULL;
> + /*
> + * Avoid race with dev_rel() by setting the initial
> + * reference count to 1. This last reference is taken
> + * by the destroy_dev() function.
> + */
> + cdev->si_refcount = 1;
This is wrong. Not all devices are destroyed with destroy_dev().
dev_rel() must be allowed to clean up allocated device.
That said, I do not understand what race you are trying to solve.
Freeing of the accessible cdev memory cannot happen in parallel while
dev_mtx is owned.
Please do not commit (to devfs) without seeking for the review first.
>
> return (cdev);
> }
>
> Modified: head/sys/kern/kern_conf.c
> ==============================================================================
> --- head/sys/kern/kern_conf.c Wed Jan 14 22:05:57 2015 (r277198)
> +++ head/sys/kern/kern_conf.c Wed Jan 14 22:07:13 2015 (r277199)
> @@ -1048,8 +1048,6 @@ destroy_devl(struct cdev *dev)
> /* Remove name marking */
> dev->si_flags &= ~SI_NAMED;
>
> - dev->si_refcount++; /* Avoid race with dev_rel() */
> -
> /* If we are a child, remove us from the parents list */
> if (dev->si_flags & SI_CHILD) {
> LIST_REMOVE(dev, si_siblings);
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