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Date:	Fri, 26 Aug 2011 11:56:43 +0800
From:	Tao Ma <tm@....ma>
To:	Theodore Ts'o <tytso@....edu>
CC:	Jiaying Zhang <jiayingz@...gle.com>, linux-ext4@...r.kernel.org
Subject: Re: [URGENT PATCH] ext4: fix potential deadlock in ext4_evict_inode()

Hi Ted,
On 08/26/2011 11:33 AM, Theodore Ts'o wrote:
> Note: this will probably need to be sent to Linus as an emergency
> bugfix ASAP, since it was introduced in 3.1-rc1, so it represents a
> regression.
> 
> Jiayingz, I'd appreciate if you could review this, since this is a
> partial undo of commit 2581fdc810, which you authored.  I don't think
> taking out the call to ext4_flush_complted_IO() should should cause any
> problems, since it should only delay how long it takes for an inode to
> be evicted, and in some cases we are already waiting for a truncate or
> journal commit to complete.  But I don't want to take any chances, so a
> second pair of eyes would be appreciated.  Thanks!!
I do agree that the revert can help to resolve that lockdep issue, but I
think jiaying's patch and the deadlock described in her commit log does
make sense. So I am working on another way to resolve it and hope to
send it out today. Please review the patch when it is ready.

Thanks
Tao
> 
> 	      	     	     	  	   - Ted
> 
> From 18271e31ece46955c0fd61e726fa7540fddf8924 Mon Sep 17 00:00:00 2001
> From: Theodore Ts'o <tytso@....edu>
> Date: Thu, 25 Aug 2011 23:26:01 -0400
> Subject: [PATCH] ext4: fix potential deadlock in ext4_evict_inode()
> 
> Commit 2581fdc810 moved ext4_ioend_wait() from ext4_destroy_inode() to
> ext4_evict_inode().  It also added code to explicitly call
> ext4_flush_completed_IO(inode):
> 
> 	mutex_lock(&inode->i_mutex);
> 	ext4_flush_completed_IO(inode);
> 	mutex_unlock(&inode->i_mutex);
> 
> Unfortunately, we can't take the i_mutex lock in ext4_evict_inode()
> without potentially causing a deadlock.
> 
> Fix this by removing the code sequence altogether.  This may result in
> ext4_evict_inode() taking longer to complete, but that's ok, we're not
> in a rush here.  That just means we have to wait until the workqueue
> is scheduled, which is OK; there's nothing that says we have to do
> this work on the current thread, which would require taking a lock
> that might lead to a deadlock condition.
> 
> See Kernel Bugzilla #41682 for one example of the circular locking
> problem that arise.  Another one can be seen here:
> 
> =======================================================
> [ INFO: possible circular locking dependency detected ]
> 3.1.0-rc3-00012-g2a22fc1 #1839
> -------------------------------------------------------
> dd/7677 is trying to acquire lock:
>  (&type->s_umount_key#18){++++..}, at: [<c021ea77>] writeback_inodes_sb_if_idle+0x26/0x3d
> 
> but task is already holding lock:
>  (&sb->s_type->i_mutex_key#3){+.+.+.}, at: [<c01d5956>] generic_file_aio_write+0x52/0xba
> 
> which lock already depends on the new lock.
> 
> the existing dependency chain (in reverse order) is:
> 
> -> #1 (&sb->s_type->i_mutex_key#3){+.+.+.}:
>        [<c018eb02>] lock_acquire+0x99/0xbd
>        [<c06a53b5>] __mutex_lock_common+0x33/0x2fb
>        [<c06a572b>] mutex_lock_nested+0x26/0x2f
>        [<c026c2db>] ext4_evict_inode+0x3e/0x2bd
>        [<c0214bb0>] evict+0x8e/0x131
>        [<c0214de6>] dispose_list+0x36/0x40
>        [<c0215239>] evict_inodes+0xcd/0xd5
>        [<c0204a23>] generic_shutdown_super+0x3d/0xaa
>        [<c0204ab2>] kill_block_super+0x22/0x5e
>        [<c0204cb8>] deactivate_locked_super+0x22/0x4e
>        [<c02055b2>] deactivate_super+0x3d/0x43
>        [<c0218427>] mntput_no_expire+0xda/0xdf
>        [<c0219486>] sys_umount+0x286/0x2ab
>        [<c02194bd>] sys_oldumount+0x12/0x14
>        [<c06a6ac5>] syscall_call+0x7/0xb
> 
> -> #0 (&type->s_umount_key#18){++++..}:
>        [<c018e262>] __lock_acquire+0x967/0xbd2
>        [<c018eb02>] lock_acquire+0x99/0xbd
>        [<c06a5991>] down_read+0x28/0x65
>        [<c021ea77>] writeback_inodes_sb_if_idle+0x26/0x3d
>        [<c0269630>] ext4_nonda_switch+0xd0/0xe1
>        [<c026e953>] ext4_da_write_begin+0x3c/0x1cf
>        [<c01d46ad>] generic_file_buffered_write+0xc0/0x1b4
>        [<c01d58d3>] __generic_file_aio_write+0x254/0x285
>        [<c01d596e>] generic_file_aio_write+0x6a/0xba
>        [<c026732f>] ext4_file_write+0x1d6/0x227
>        [<c0202789>] do_sync_write+0x8f/0xca
>        [<c02030d5>] vfs_write+0x85/0xe3
>        [<c02031d4>] sys_write+0x40/0x65
>        [<c06a6ac5>] syscall_call+0x7/0xb
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=41682
> 
> Cc: stable@...nel.org
> Cc: Jiaying Zhang <jiayingz@...gle.com>
> Signed-off-by: "Theodore Ts'o" <tytso@....edu>
> ---
>  fs/ext4/inode.c |    3 ---
>  1 files changed, 0 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 29b7148..cf0b515 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -121,9 +121,6 @@ void ext4_evict_inode(struct inode *inode)
>  
>  	trace_ext4_evict_inode(inode);
>  
> -	mutex_lock(&inode->i_mutex);
> -	ext4_flush_completed_IO(inode);
> -	mutex_unlock(&inode->i_mutex);
>  	ext4_ioend_wait(inode);
>  
>  	if (inode->i_nlink) {

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