Re: [PATCH][BUG] Lack of mutex_lock in drop_pagecache_sb()

[Jan Kara <jack@suse.cz>]

On Tue 24-03-09 15:44:57, Wu Fengguang wrote:
> Hi Masayoshi,
> 
> On Tue, Mar 24, 2009 at 03:06:45PM +0800, Masayoshi MIZUMA wrote:
> > Hi, Fengguang
> > 
> > On Mon, 23 Mar 2009 18:38:46 +0800
> > Wu Fengguang <fengguang.wu@intel.com> wrote:
> > 
> > > Masasyoshi, 
> > > 
> > > On Wed, Mar 18, 2009 at 05:13:35PM +0900, Masasyoshi MIZUMA wrote:
> > > > I create the patch which fixes lack of mutex_lock in drop_pagecache_sb().
> > > > Please check the bug and the patch (below).
> > > 
> > Thank you for your comment, and I apologize to you for my lack
> > of explanation.
> > 
> > > Is this a real producible bug or a theory one?
> > This is a real bug.
> > 
> > > IMHO the I_FREEING flag should avoid the race.
> > I supplement the explanation for this problem.
> > 
> > clear_inode() is called by dispose_list(), and sets the inode's 
> > i_state to I_CLEAR. Therefore, the following conditional expression 
> > doesn't match for the inode:
> > "if (inode->i_state & (I_FREEING|I_WILL_FREE)) continue;"
> > As the result, this problem can happen.
> > 
> > > 
> > > > ----------------------------------------------------------------------
> > > > 
> > > > When drop_pagecache_sb() frees inodes, it doesn't get mutex_lock of 
> > > > iprune_mutex. Therefore, if it races the process which frees inodes 
> > > > (ex. prune_icache()), OS panic may happen.
> > > > 
> > > > An example of the panic flow is the following:
> > > > ----------------------------------------------------------------------
> > > >             [process A]               |         [process B]
> > > >  |                                    |
> > > >  |  shrink_icache_memory()            |
> > > >  |      |                             |
> > > >  |      V                             |
> > > >  |    prune_icache()                  |  drop_pagecache()
> > > >  |      mutex_lock(&iprune_mutex)     |      |
> > > >  |      spin_lock(&inode_lock)        |      |
> > > >  |          |                         |      V
> > > >  |          |                         |    drop_pagecache_sb()
> > > >  |          |                         |        |
> > > 
> > >           inode->i_state |= I_FREEING;
> > > 
> > > >  |          V                         |        V
> > > >  |      spin_unlock(&inode_lock)      |      spin_lock(&inode_lock)
> > > >  |          |                         |          |
> > > 
> > >                                                 if (inode->i_state & (I_FREEING|I_WILL_FREE))
> > >                                                         continue;
> > > 
> > > >  |          |                         |          |
> > > >  |          V                         |          V
> > > >  |      dispose_list()                |        __iget()
> > > >  |        list_del()                  |            |
> > > >  |            |                       |            |
> > > >  |            V                       |            V
> > > >  |        spin_lock(&inode_lock)      |          list_move() <----- PANIC !!
> > > >  |                                    |
> > > >  V                                    |
> > > > (time)
> > > > ----------------------------------------------------------------------
> > > > If the inode which Process B do list_move() with is the same as the one which
> > > > Process A did list_del() with, OS may panic.
> > 
> > I applied your comment and then modified the panic flow figure.
> > Please check below:
> > ----------------------------------------------------------------------
> >             [process A]               |        [process B]
> >  |                                    |
> >  |  shrink_icache_memory()            |
> >  |      |                             |
> >  |      V                             |
> >  |    prune_icache()                  | drop_pagecache()
> >  |      mutex_lock(&iprune_mutex)     |     |
> >  |      spin_lock(&inode_lock)        |     |
> >  |          |                         |     V
> >  |          |                         |   drop_pagecache_sb()
> >  |          |                         |       |
> >  |          V                         |       |
> >  |      inode->i_state |= I_FREEING;  |       |
> >  |          |                         |       |
> >  |          V                         |       V
> >  |      spin_unlock(&inode_lock)      |     spin_lock(&inode_lock)
> >  |          |                         |         |
> >  |          |                         |         |
> >  |          V                         |         |
> >  |      dispose_list()                |         |
> >  |        list_del()                  |         |
> >  |            |                       |         |
> >  |            V                       |         |
> >  |        clear_inode()               |         |
> >  |          inode->i_state = I_CLEAR  |         |
> >  |            |                       |         |
> >  |            |                       |         V
> >  |            |                       |      if (inode->i_state & (I_FREEING|I_WILL_FREE))
> >  |            |                       |              continue;           <---- NOT MATCH
> >  |            |                       |           |
> >  |            |                       |           V
> >  |            |                       |      __iget()   
> >  |            |                       |            |
> >  |            V                       |            V
> >  |        spin_lock(&inode_lock)      |        list_move() <----- PANIC !!
> >  |                                    |
> >  V                                    |
> > (time)
> > ----------------------------------------------------------------------
> 
> Ah thanks for the explanation!
> 
> How about this lightweight fix? Since s_umount is already taken in
> drop_pagecache(), it's not necessary to take iprune_mutex again.
  Yes, the patch looks fine like this. But I believe there are more places
which might miss similar check. Probably dquot.c: add_dquot_ref() needs
the same check and fs-writeback.c: generic_sync_sb_inodes() also needs
adding I_CLEAR to the test, doesn't it?

								Honza

> --- mm.orig/fs/drop_caches.c
> +++ mm/fs/drop_caches.c
> @@ -18,7 +18,7 @@ static void drop_pagecache_sb(struct sup
>  
>  	spin_lock(&inode_lock);
>  	list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
> -		if (inode->i_state & (I_FREEING|I_WILL_FREE))
> +		if (inode->i_state & (I_FREEING|I_CLEAR|I_WILL_FREE))
>  			continue;
>  		if (inode->i_mapping->nrpages == 0)
>  			continue;
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR