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@xxxxxxxxx> 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.
Thanks,
Fengguang
---
fs/drop_caches.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- 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;
--
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