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SubjectRe: [PATCH] remove BKL from driverfs
Patrick Mochel wrote:
>>>I see no reason to hold the BKL in your situation. I replaced it with
>>>i_sem in some places and just plain removed it in others. I believe
>>>that you get all of the protection that you need from dcache_lock in
>>>the dentry insert and activate. Can you prove me wrong?
>>
>>No, and I'm not about to try very hard. It appears that the place you
>>removed it should be fine. In driverfs_unlink, you replace it with i_sem.
>>ramfs, which driverfs mimmicks, doesn't hold any lock during unlink. It
>>seems it could be removed altogether.
>
>
> Actually, taking i_sem is completely wrong. Look at vfs_unlink() in
> fs/namei.c:
>
> down(&dentry->d_inode->i_sem);
> if (d_mountpoint(dentry))
> error = -EBUSY;
> else {
> error = dir->i_op->unlink(dir, dentry);
> if (!error)
> d_delete(dentry);
> }
> up(&dentry->d_inode->i_sem);
>
> Then, in driverfs_unlink:
>
> struct inode *inode = dentry->d_inode;
>
> down(&inode->i_sem);
>
>
> You didn't test this on file removal did you? A good way to verify that
> you have most of your bases covered is to plug/unplug a USB device a few
> times. I learned that one from Greg, and it's caught several bugs.

I need to get some USB devices that work in Linux!

> Anyway, I say that the lock can be removed altogether. Ditto for mknod as
> well.

I agree. It looks like vfs_unlink() provides all of the protection
that is needed. No BKL, no more i_sem uses. I'm asking Al Viro about
driverfs_get_inode(). It looks like it will be safe and correct to
use i_sem there, but stay tuned, it may not be necessary at all.

--
Dave Hansen
haveblue@us.ibm.com

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