Question about synchronization (nfssvc, vfs_busy)

[Konstantin Belousov]

On Mon, Jun 05, 2006 at 02:01:36PM +0300, Andrey Simonenko wrote:
> 2.
> 
> If vfs_busy() is called without LK_NOWAIT flag, then it can sleep
> if a filesystem is being unmounted.  At some point unmount() will
If vfs_busy() is called without LK_NOWAIT and fs is being unmounted,
then vfs_busy returns with ENOENT error, isn't it ?

> reach vfs_mount_destroy() and since there is one ref from vfs_busy()
> it will sleep 3 seconds and will notice MNTK_MWAIT flag and wake up
> a process, which is sleeping in vfs_busy().  How woken up process
> can work with mount structure in vfs_busy() after wakeup(), which
> could be already deallocated in vfs_mount_destroy()?
vfs_busy() internally increases the ref count for mount point, so, it cannot
be taken from under it (look for MNT_REF/MNT_REL). Simultameous entrance
into the code in question in vfs_busy/vfs_mount_destroy is protected
by mnt_mtx (MNT_ILOCK/MNT_IUNLOCK).


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