[v2] ext2: Silence lockdep warning about reclaim under xattr_sem
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Message ID 20200225120803.7901-1-jack@suse.cz
State Not Applicable
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  • [v2] ext2: Silence lockdep warning about reclaim under xattr_sem
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Commit Message

Jan Kara Feb. 25, 2020, 12:08 p.m. UTC
Lockdep complains about a chain:
  sb_internal#2 --> &ei->xattr_sem#2 --> fs_reclaim

and shrink_dentry_list -> ext2_evict_inode -> ext2_xattr_delete_inode ->
down_write(ei->xattr_sem) creating a locking cycle in the reclaim path.
This is however a false positive because when we are in
ext2_evict_inode() we are the only holder of the inode reference and
nobody else should touch xattr_sem of that inode. So we cannot ever
block on acquiring the xattr_sem in the reclaim path.

Silence the lockdep warning by using down_write_trylock() in
ext2_xattr_delete_inode() to not create false locking dependency.

Reported-by: "J. R. Okajima" <hooanon05g@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/ext2/xattr.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

Changes since v1:
- changed WARN_ON to WARN_ON_ONCE

Comments

Ritesh Harjani Feb. 26, 2020, 11:32 a.m. UTC | #1
On 2/25/20 5:38 PM, Jan Kara wrote:
> Lockdep complains about a chain:
>    sb_internal#2 --> &ei->xattr_sem#2 --> fs_reclaim
> 
> and shrink_dentry_list -> ext2_evict_inode -> ext2_xattr_delete_inode ->
> down_write(ei->xattr_sem) creating a locking cycle in the reclaim path.
> This is however a false positive because when we are in
> ext2_evict_inode() we are the only holder of the inode reference and
> nobody else should touch xattr_sem of that inode. So we cannot ever
> block on acquiring the xattr_sem in the reclaim path.
> 
> Silence the lockdep warning by using down_write_trylock() in
> ext2_xattr_delete_inode() to not create false locking dependency.
> 
> Reported-by: "J. R. Okajima" <hooanon05g@gmail.com>
> Signed-off-by: Jan Kara <jack@suse.cz>

Agreed with evict() will only be called when it's the last reference 
going down and so we won't be blocked on xattr_sem.
Thanks for clearly explaining the problem in the cover letter.

Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>


> ---
>   fs/ext2/xattr.c | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
> 
> Changes since v1:
> - changed WARN_ON to WARN_ON_ONCE
> 
> diff --git a/fs/ext2/xattr.c b/fs/ext2/xattr.c
> index 0456bc990b5e..9ad07c7ef0b3 100644
> --- a/fs/ext2/xattr.c
> +++ b/fs/ext2/xattr.c
> @@ -790,7 +790,15 @@ ext2_xattr_delete_inode(struct inode *inode)
>   	struct buffer_head *bh = NULL;
>   	struct ext2_sb_info *sbi = EXT2_SB(inode->i_sb);
> 
> -	down_write(&EXT2_I(inode)->xattr_sem);
> +	/*
> +	 * We are the only ones holding inode reference. The xattr_sem should
> +	 * better be unlocked! We could as well just not acquire xattr_sem at
> +	 * all but this makes the code more futureproof. OTOH we need trylock
> +	 * here to avoid false-positive warning from lockdep about reclaim
> +	 * circular dependency.
> +	 */
> +	if (WARN_ON_ONCE(!down_write_trylock(&EXT2_I(inode)->xattr_sem)))
> +		return;
>   	if (!EXT2_I(inode)->i_file_acl)
>   		goto cleanup;
>
Jan Kara Feb. 26, 2020, 12:16 p.m. UTC | #2
On Wed 26-02-20 17:02:18, Ritesh Harjani wrote:
> 
> 
> On 2/25/20 5:38 PM, Jan Kara wrote:
> > Lockdep complains about a chain:
> >    sb_internal#2 --> &ei->xattr_sem#2 --> fs_reclaim
> > 
> > and shrink_dentry_list -> ext2_evict_inode -> ext2_xattr_delete_inode ->
> > down_write(ei->xattr_sem) creating a locking cycle in the reclaim path.
> > This is however a false positive because when we are in
> > ext2_evict_inode() we are the only holder of the inode reference and
> > nobody else should touch xattr_sem of that inode. So we cannot ever
> > block on acquiring the xattr_sem in the reclaim path.
> > 
> > Silence the lockdep warning by using down_write_trylock() in
> > ext2_xattr_delete_inode() to not create false locking dependency.
> > 
> > Reported-by: "J. R. Okajima" <hooanon05g@gmail.com>
> > Signed-off-by: Jan Kara <jack@suse.cz>
> 
> Agreed with evict() will only be called when it's the last reference going
> down and so we won't be blocked on xattr_sem.
> Thanks for clearly explaining the problem in the cover letter.
> 
> Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>

Thanks for review! I've now pushed the patch to my tree.

								Honza
> 
> 
> > ---
> >   fs/ext2/xattr.c | 10 +++++++++-
> >   1 file changed, 9 insertions(+), 1 deletion(-)
> > 
> > Changes since v1:
> > - changed WARN_ON to WARN_ON_ONCE
> > 
> > diff --git a/fs/ext2/xattr.c b/fs/ext2/xattr.c
> > index 0456bc990b5e..9ad07c7ef0b3 100644
> > --- a/fs/ext2/xattr.c
> > +++ b/fs/ext2/xattr.c
> > @@ -790,7 +790,15 @@ ext2_xattr_delete_inode(struct inode *inode)
> >   	struct buffer_head *bh = NULL;
> >   	struct ext2_sb_info *sbi = EXT2_SB(inode->i_sb);
> > 
> > -	down_write(&EXT2_I(inode)->xattr_sem);
> > +	/*
> > +	 * We are the only ones holding inode reference. The xattr_sem should
> > +	 * better be unlocked! We could as well just not acquire xattr_sem at
> > +	 * all but this makes the code more futureproof. OTOH we need trylock
> > +	 * here to avoid false-positive warning from lockdep about reclaim
> > +	 * circular dependency.
> > +	 */
> > +	if (WARN_ON_ONCE(!down_write_trylock(&EXT2_I(inode)->xattr_sem)))
> > +		return;
> >   	if (!EXT2_I(inode)->i_file_acl)
> >   		goto cleanup;
> > 
>

Patch
diff mbox series

diff --git a/fs/ext2/xattr.c b/fs/ext2/xattr.c
index 0456bc990b5e..9ad07c7ef0b3 100644
--- a/fs/ext2/xattr.c
+++ b/fs/ext2/xattr.c
@@ -790,7 +790,15 @@  ext2_xattr_delete_inode(struct inode *inode)
 	struct buffer_head *bh = NULL;
 	struct ext2_sb_info *sbi = EXT2_SB(inode->i_sb);
 
-	down_write(&EXT2_I(inode)->xattr_sem);
+	/*
+	 * We are the only ones holding inode reference. The xattr_sem should
+	 * better be unlocked! We could as well just not acquire xattr_sem at
+	 * all but this makes the code more futureproof. OTOH we need trylock
+	 * here to avoid false-positive warning from lockdep about reclaim
+	 * circular dependency.
+	 */
+	if (WARN_ON_ONCE(!down_write_trylock(&EXT2_I(inode)->xattr_sem)))
+		return;
 	if (!EXT2_I(inode)->i_file_acl)
 		goto cleanup;