On Fri, Mar 30, 2018 at 1:46 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> On Thu, 2018-03-29 at 17:08 +0800, yangerkun wrote:
>> Split sys_flock(), add flock_setlk() routine to help kernel set
>> flock easily.
>>
>> Signed-off-by: yangerkun <yangerkun@xxxxxxxxxx>
>> ---
>> fs/locks.c | 29 +++++++++++++++++------------
>> include/linux/fs.h | 1 +
>> 2 files changed, 18 insertions(+), 12 deletions(-)
>>
>> diff --git a/fs/locks.c b/fs/locks.c
>> index d6ff4be..aeef6cf 100644
>> --- a/fs/locks.c
>> +++ b/fs/locks.c
>> @@ -1974,6 +1974,22 @@ int locks_lock_inode_wait(struct inode *inode, struct file_lock *fl)
>> }
>> EXPORT_SYMBOL(locks_lock_inode_wait);
>>
>> +int flock_setlk(struct file *filp, struct file_lock *lock)
>> +{
>> + int err;
>> +
>> + err = security_file_lock(filp, lock->fl_type);
>> + if (err)
>> + return err;
>> +
>> + if (filp->f_op->flock && is_remote_lock(filp))
>> + return filp->f_op->flock(filp,
>> + (lock->fl_flags & FL_SLEEP) ? F_SETLKW : F_SETLK,
>> + lock);
>> + else
>> + return locks_lock_file_wait(filp, lock);
>> +}
>> +
>> /**
>> * sys_flock: - flock() system call.
>> * @fd: the file descriptor to lock.
>> @@ -2019,18 +2035,7 @@ int locks_lock_inode_wait(struct inode *inode, struct file_lock *fl)
>> if (can_sleep)
>> lock->fl_flags |= FL_SLEEP;
>>
>> - error = security_file_lock(f.file, lock->fl_type);
>> - if (error)
>> - goto out_free;
>> -
>> - if (f.file->f_op->flock && is_remote_lock(f.file))
>> - error = f.file->f_op->flock(f.file,
>> - (can_sleep) ? F_SETLKW : F_SETLK,
>> - lock);
>> - else
>> - error = locks_lock_file_wait(f.file, lock);
>> -
>> - out_free:
>> + error = flock_setlk(f.file, lock);
>> locks_free_lock(lock);
>>
>> out_putf:
>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>> index 2a81556..3dab90c 100644
>> --- a/include/linux/fs.h
>> +++ b/include/linux/fs.h
>> @@ -1090,6 +1090,7 @@ extern int fcntl_setlk64(unsigned int, struct file *, unsigned int,
>> extern int vfs_lock_file(struct file *, unsigned int, struct file_lock *, struct file_lock *);
>> extern int vfs_cancel_lock(struct file *filp, struct file_lock *fl);
>> extern int locks_lock_inode_wait(struct inode *inode, struct file_lock *fl);
>> +extern int flock_setlk(struct file *filp, struct file_lock *fl);
>> extern int __break_lease(struct inode *inode, unsigned int flags, unsigned int type);
>> extern void lease_get_mtime(struct inode *, struct timespec *time);
>> extern int generic_setlease(struct file *, long, struct file_lock **, void **priv);
>
> Looks reasonable, but I'm not sure I understand the overall rationale
> for wanting to do this: What problem are you trying to fix?
For blockdev filesystems, the usermode tools open the blockdev with
O_EXCL to prevent mutual access to blockdev by usermode tools
and kernel mount. This is a (non POSIX?) feature for blockdev described in
man open(2).
We would like to have the same useful feature to mutually exclude
access to overlay layers by fsck.overaly and overlayfs mount.
I had initially suggested to interpret the (undefined) behavior of
O_EXCL without O_CREATE for directories similar to that of a blockdev.
Overlayfs already introduces an inode flag I_OVL_INUSE to mutually
exclude mounting several overlay with the same upper dir.
open of directory with O_EXCL could test and set I_OVL_INUSE.
Then someone suggested to use file locks instead.
This patch is an RFC for that implementation.
I am yet uncertain if using file locks for this purpose is a good use of
proven technology or a mismatch to the requirements.
What do you think?
The way that flock is implemented on NFS server side (as well as CIFS)
is not a big issue IMO. We mostly want to serialize access to those
directories by tools and overlay mount on the client machine if layer
are on NFS.
Thanks,
Amir.
--
To unsubscribe from this list: send the line "unsubscribe linux-unionfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
