Re: [PATCH RESEND] vfs: Fix missed wakeup in I_NEW handling

[Al Viro <viro@ZenIV.linux.org.uk>]

On Wed, Mar 07, 2012 at 06:05:40PM +0100, Jan Kara wrote:
> Commit 250df6ed removed wake_up_inode() (in particular a memory barrier before
> wake_up_bit()) on the basis that i_state transitions are protected by i_lock.
> That would be fine if all the readers of i_state were using i_lock as well. But
> wait_on_inode() doesn't use i_lock and thus the following can happen due to
> reordering:
> 
>    CPU 1				CPU 2
> unlock_new_inode()
>   spin_lock(&inode->i_lock);
>   wake_up_bit(&inode->i_state, __I_NEW);
> 					wait_on_inode()
> 					  wait_on_bit(&inode->i_state, __I_NEW);
>   inode->i_state &= ~I_NEW;
>   ^^^ this store was reordered
>   spin_unlock(&inode->i_lock);
> 
> And waiter on CPU2 sleeps forever (or for a really long time).
> 
> We fix the issue by using i_lock in wait_on_inode() in the spirit of commit
> 250df6ed.

Applied (along with a bunch of other stuff; will push to Linus tomorrow)