On Mon, 17 Jun 2013 11:46:09 -0400
Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> On Mon, 17 Jun 2013 11:13:49 -0400
> Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>
> > Having a global lock that protects all of this code is a clear
> > scalability problem. Instead of doing that, move most of the code to be
> > protected by the i_lock instead. The exceptions are the global lists
> > that the ->fl_link sits on, and the ->fl_block list.
> >
> > ->fl_link is what connects these structures to the
> > global lists, so we must ensure that we hold those locks when iterating
> > over or updating these lists.
> >
> > Furthermore, sound deadlock detection requires that we hold the
> > blocked_list state steady while checking for loops. We also must ensure
> > that the search and update to the list are atomic.
> >
> > For the checking and insertion side of the blocked_list, push the
> > acquisition of the global lock into __posix_lock_file and ensure that
> > checking and update of the blocked_list is done without dropping the
> > lock in between.
> >
> > On the removal side, when waking up blocked lock waiters, take the
> > global lock before walking the blocked list and dequeue the waiters from
> > the global list prior to removal from the fl_block list.
> >
> > With this, deadlock detection should be race free while we minimize
> > excessive file_lock_lock thrashing.
> >
> > Finally, in order to avoid a lock inversion problem when handling
> > /proc/locks output we must ensure that manipulations of the fl_block
> > list are also protected by the file_lock_lock.
> >
> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > ---
> > Documentation/filesystems/Locking | 21 ++++--
> > fs/afs/flock.c | 5 +-
> > fs/ceph/locks.c | 2 +-
> > fs/ceph/mds_client.c | 8 +-
> > fs/cifs/cifsfs.c | 2 +-
> > fs/cifs/file.c | 13 ++--
> > fs/gfs2/file.c | 2 +-
> > fs/lockd/svcsubs.c | 12 ++--
> > fs/locks.c | 151 ++++++++++++++++++++++---------------
> > fs/nfs/delegation.c | 10 +-
> > fs/nfs/nfs4state.c | 8 +-
> > fs/nfsd/nfs4state.c | 8 +-
> > include/linux/fs.h | 11 ---
> > 13 files changed, 140 insertions(+), 113 deletions(-)
> >
>
> [...]
>
> > @@ -1231,7 +1254,7 @@ int __break_lease(struct inode *inode, unsigned int mode)
> > if (IS_ERR(new_fl))
> > return PTR_ERR(new_fl);
> >
> > - lock_flocks();
> > + spin_lock(&inode->i_lock);
> >
> > time_out_leases(inode);
> >
> > @@ -1281,11 +1304,11 @@ restart:
> > break_time++;
> > }
> > locks_insert_block(flock, new_fl);
> > - unlock_flocks();
> > + spin_unlock(&inode->i_lock);
> > error = wait_event_interruptible_timeout(new_fl->fl_wait,
> > !new_fl->fl_next, break_time);
> > - lock_flocks();
> > - __locks_delete_block(new_fl);
> > + spin_lock(&inode->i_lock);
> > + locks_delete_block(new_fl);
>
> Doh -- bug here. This should not have been changed to
> locks_delete_block(). My apologies.
>
> > if (error >= 0) {
> > if (error == 0)
> > time_out_leases(inode);
>
> [...]
>
> > posix_unblock_lock(struct file *filp, struct file_lock *waiter)
> > {
> > + struct inode *inode = file_inode(filp);
> > int status = 0;
> >
> > - lock_flocks();
> > + spin_lock(&inode->i_lock);
> > if (waiter->fl_next)
> > - __locks_delete_block(waiter);
> > + locks_delete_block(waiter);
>
>
> Ditto here...
>
> > else
> > status = -ENOENT;
> > - unlock_flocks();
> > + spin_unlock(&inode->i_lock);
> > return status;
> > }
> >
>
Bah, scratch that -- this patch is actually fine. We hold the i_lock
here and locks_delete_block takes the file_lock_lock, which is correct.
There is a potential race in patch 7 though. I'll reply to that patch
to point it out in a minute.
--
Jeff Layton <jlayton@xxxxxxxxxx>
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