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    SubjectRe: [PATCH] vfs: Speed up deactivate_super for non-modular filesystems
    On 8 May 2012 11:07, Eric W. Biederman <> wrote:
    > "Paul E. McKenney" <> writes:
    >> On Mon, May 07, 2012 at 11:17:06PM +0100, Al Viro wrote:
    >>> On Mon, May 07, 2012 at 02:51:08PM -0700, Eric W. Biederman wrote:
    >>> > /proc and similar non-modular filesystems do not need a rcu_barrier
    >>> > in deactivate_locked_super.  Being non-modular there is no danger
    >>> > of the rcu callback running after the module is unloaded.
    >>> There's more than just a module unload there, though - actual freeing
    >>>  struct super_block also happens past that rcu_barrier()...
    > Al.  I have not closely audited the entire code path but at a quick
    > sample I see no evidence that anything depends on inode->i_sb being
    > rcu safe.  Do you know of any such location?
    > It has only been a year and a half since Nick added this code which
    > isn't very much time to have grown strange dependencies like that.

    No, it has always depended on this.

    Look at ncp_compare_dentry(), for example.

    >> Is there anything in there for which synchronous operation is required?
    >> If not, one approach would be to drop the rcu_barrier() calls to a
    >> workqueue or something similar.
    > We need to drain all of the rcu callbacks before we free the slab
    > and unload the module.
    > This actually makes deactivate_locked_super the totally wrong place
    > for the rcu_barrier.  We want the rcu_barrier in the module exit
    > routine where we destroy the inode cache.
    > What I see as the real need is the filesystem modules need to do:
    >        rcu_barrier()
    >        kmem_cache_destroy(cache);
    > Perhaps we can add some helpers to make it easy.  But I think
    > I would be happy today with simply moving the rcu_barrier into
    > every filesystems module exit path, just before the file system
    > module destoryed it's inode cache.

    No, because that's not the only requirement for the rcu_barrier.

    Making it asynchronous is not something I wanted to do, because
    then we potentially have a process exiting from kernel space after
    releasing last reference on a mount, but the mount does not go
    away until "some time" later. Which is crazy.

    However. We are holding vfsmount_lock for read at the point
    where we ever actually do anything with an "rcu-referenced"
    dentry/inode. I wonder if we could use this to get i_sb pinned.
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      Last update: 2012-05-09 10:21    [W:0.024 / U:4.488 seconds]
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