Re: How to explain to lock validator: locking inodes in inode order

From: Joshua Hudson
Date: Tue Jul 18 2006 - 00:18:35 EST

On 7/17/06, Arjan van de Ven <arjan@xxxxxxxxxxxxx> wrote:
On Mon, 2006-07-17 at 19:24 -0700, Joshua Hudson wrote:
> Code does this:
> /* Lock two items. See locking.txt */
> static inline void kb0_lock2m(struct kb0_idata *m1, struct kb0_idata *m2)
> {
> if (m1->vi.i_ino > m2->vi.i_ino)
> mutex_lock(&m2->k_mutex);
> mutex_lock(&m1->k_mutex);
> if (m1->vi.i_ino < m2->vi.i_ino)
> mutex_lock(&m2->k_mutex);
> }
> Not sure how to explain to the lock validator that this code can never deadlock.

you're sure it can;t?
Yes. It never takes a lock on any higher inode without holding the lock on a
lower inode first. I have a full proof across the VFS+FS [see below].

all places in the kernel that take this mutex in that order only do it
in i_ino order, including all directory operations like cross directory

I had to disable the kernel's locking of multiple objects in namei.c using a
new FS_ flag because it would actually deadlock for this filesystem.

(which fs is this btw?)
Custom filesystem (kb0), not in any tree but mine. I am in progress
with syncing it up
to the latest kernel.

If anybody really wants it, it is 70-odd K of bzip2-compressed code of
filesystem + utilities. It's not ready for non-acedemic use though.
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