(un)lock_kernel() ?

(un)lock_kernel() ?

[John Anthony Kazos Jr.]

Why does ext4_fill_super release the BKL on entry and take it on both 
normal and abnormal exit? As far as I can see, ext4_fill_super is called 
by get_sb_bdev, which calls the ->get_sb method without the BKL, and 
ext4_get_sb calls get_sb_bdev without the BKL. And the ext2 code does not 
touch the BKL in ext2_fill_super.

Is the VFS code going to be changed somewhere in the future and it's being 
anticipated, or is this a bug?

Re: (un)lock_kernel() ?

[Dave Kleikamp]

On Wed, 2007-04-04 at 08:57 -0400, John Anthony Kazos Jr. wrote:
> Why does ext4_fill_super release the BKL on entry and take it on both 
> normal and abnormal exit? As far as I can see, ext4_fill_super is called 
> by get_sb_bdev, which calls the ->get_sb method without the BKL, and 
> ext4_get_sb calls get_sb_bdev without the BKL. And the ext2 code does not 
> touch the BKL in ext2_fill_super.
> 
> Is the VFS code going to be changed somewhere in the future and it's being 
> anticipated, or is this a bug?

According to Documentation/filesystems/Locking, ->get_sb() is called
with the BKL held, but looking through the code, I'm not able to find
where it is being taken.

Shaggy
-- 
David Kleikamp
IBM Linux Technology Center

Re: (un)lock_kernel() ?

[John Anthony Kazos Jr.]

> According to Documentation/filesystems/Locking, ->get_sb() is called
> with the BKL held, but looking through the code, I'm not able to find
> where it is being taken.

I noticed that too. Unless I'm just dumb and can't see it, I'm not able to 
find any BKL references during filesystem mounting until you get into 
FS-specific code. I looked through everything from sys_mount through to 
vfs_kern_mount. Documentation/filesystems/porting talks about several 
situations where the VFS code was modified to not take the BKL, and BLK 
calls were added by FS non-maintainers for safety until each FS could be 
audited independently, but that wouldn't be the case, would it?

The ext2 code takes the BKL in three places: ext2_update_inode, 
write_super, and ext2_compat_ioctl. Starting with ext3, it's in 
ext3_compat_ioctl and ext3_fill_super, and the same with ext4.

I suppose the BKL does have to be held, somehow, somewhere, during 
mounting, or anybody using ext3 on a multiprocessor box would lock their 
system from unmatched locking calls. Unless the first unlock_kernel() 
would make the count -1 and the lock would bring it back to zero?

Re: (un)lock_kernel() ?

[Ming Zhang]

On Wed, 2007-04-04 at 12:52 -0400, John Anthony Kazos Jr. wrote:
> > According to Documentation/filesystems/Locking, ->get_sb() is called
> > with the BKL held, but looking through the code, I'm not able to find
> > where it is being taken.
> 
> I noticed that too. Unless I'm just dumb and can't see it, I'm not able to 
> find any BKL references during filesystem mounting until you get into 
> FS-specific code. I looked through everything from sys_mount through to 
> vfs_kern_mount. Documentation/filesystems/porting talks about several 
> situations where the VFS code was modified to not take the BKL, and BLK 
> calls were added by FS non-maintainers for safety until each FS could be 
> audited independently, but that wouldn't be the case, would it?


sys_mount->do_mount->do_new_mount->do_kern_mount path

part of sys_mount()

1570                 goto out3;
1571 
1572         lock_kernel();
1573         retval = do_mount((char *)dev_page, dir_page, (char *)type_page,
1574                           flags, (void *)data_page);
1575         unlock_kernel();
1576         free_page(data_page);
1577 
1578 out3:
1579         free_page(dev_page)

;
> 
> The ext2 code takes the BKL in three places: ext2_update_inode, 
> write_super, and ext2_compat_ioctl. Starting with ext3, it's in 
> ext3_compat_ioctl and ext3_fill_super, and the same with ext4.
> 
> I suppose the BKL does have to be held, somehow, somewhere, during 
> mounting, or anybody using ext3 on a multiprocessor box would lock their 
> system from unmatched locking calls. Unless the first unlock_kernel() 
> would make the count -1 and the lock would bring it back to zero?
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Re: (un)lock_kernel() ?

[Dave Kleikamp]

On Tue, 2007-04-10 at 12:03 -0400, Ming Zhang wrote:
> On Wed, 2007-04-04 at 12:52 -0400, John Anthony Kazos Jr. wrote:
> > > According to Documentation/filesystems/Locking, ->get_sb() is called
> > > with the BKL held, but looking through the code, I'm not able to find
> > > where it is being taken.
> > 
> > I noticed that too. Unless I'm just dumb and can't see it, I'm not able to 
> > find any BKL references during filesystem mounting until you get into 
> > FS-specific code. I looked through everything from sys_mount through to 
> > vfs_kern_mount. Documentation/filesystems/porting talks about several 
> > situations where the VFS code was modified to not take the BKL, and BLK 
> > calls were added by FS non-maintainers for safety until each FS could be 
> > audited independently, but that wouldn't be the case, would it?
> 
> 
> sys_mount->do_mount->do_new_mount->do_kern_mount path
> 
> part of sys_mount()
> 
> 1570                 goto out3;
> 1571 
> 1572         lock_kernel();
> 1573         retval = do_mount((char *)dev_page, dir_page, (char *)type_page,
> 1574                           flags, (void *)data_page);
> 1575         unlock_kernel();
> 1576         free_page(data_page);
> 1577 
> 1578 out3:
> 1579         free_page(dev_page)

Thanks.  I missed that somehow.  It seems the documentation is correct.
-- 
David Kleikamp
IBM Linux Technology Center

Re: (un)lock_kernel() ?

[Ming Zhang]

On Tue, 2007-04-10 at 11:10 -0500, Dave Kleikamp wrote:
> On Tue, 2007-04-10 at 12:03 -0400, Ming Zhang wrote:
> > On Wed, 2007-04-04 at 12:52 -0400, John Anthony Kazos Jr. wrote:
> > > > According to Documentation/filesystems/Locking, ->get_sb() is called
> > > > with the BKL held, but looking through the code, I'm not able to find
> > > > where it is being taken.
> > > 
> > > I noticed that too. Unless I'm just dumb and can't see it, I'm not able to 
> > > find any BKL references during filesystem mounting until you get into 
> > > FS-specific code. I looked through everything from sys_mount through to 
> > > vfs_kern_mount. Documentation/filesystems/porting talks about several 
> > > situations where the VFS code was modified to not take the BKL, and BLK 
> > > calls were added by FS non-maintainers for safety until each FS could be 
> > > audited independently, but that wouldn't be the case, would it?
> > 
> > 
> > sys_mount->do_mount->do_new_mount->do_kern_mount path
> > 
> > part of sys_mount()
> > 
> > 1570                 goto out3;
> > 1571 
> > 1572         lock_kernel();
> > 1573         retval = do_mount((char *)dev_page, dir_page, (char *)type_page,
> > 1574                           flags, (void *)data_page);
> > 1575         unlock_kernel();
> > 1576         free_page(data_page);
> > 1577 
> > 1578 out3:
> > 1579         free_page(dev_page)
> 
> Thanks.  I missed that somehow.  It seems the documentation is correct.

welcome. i last time spend 30 minutes on finding it, so i pinned this
piece of info in my brain memory...