Messages in this thread | | | From | (Eric W. Biederman) | Date | Wed, 16 Jul 2008 14:09:55 -0700 | Subject | Re: [PATCH 12/15] driver core: Implement tagged directory support for device classes. |
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Tejun Heo <htejun@gmail.com> writes: >> To do that I believe we would need to ensure sysfs does not use >> the inode->i_mutex lock except to keep the VFS layer out. Allowing us >> to safely change the directory structure, without holding it. > > I don't think sysfs is depending on i_mutex anymore but I need to go > through the code to make sure.
The vfs still does. So at least for directory tree manipulation we need to hold i_mutex before we grab sysfs_mutex.
I think that means we need to unscramble the whole set of locking order issues.
In lookup we have: local_vfs_lock -> fs_global_lock
In modifications we have: fs_global_lock -> local_vfs_lock
Which is the definition of a lock ordering problem.
Currently we play jump through some significant hoops to keep things in local_vfs_lock -> fs_global_lock order.
If we also take the rename_mutex on directory adds and deletes we may be able to keep jumping through those hoops. However I expect we would be in a much better situation if we could figure out how to avoid the problem.
It looks like the easy way to handle this is to make the sysfs_dirent list rcu protected. Which means we can fix our lock ordering problem without VFS modifications. Allowing the locking to always be: sysfs_mutex ... i_mutex.
After that it would be safe and a good idea to have unshared inodes between superblocks, just so we don't surprise anyone making generic VFS assumptions.
Eric
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