| Message ID | 20200224125916.17321-1-jack@suse.cz |
|---|---|
| State | Not Applicable |
| Headers | show |
| Series | ext2: Silence lockdep warning about reclaim under xattr_sem | expand |
On Mon, Feb 24, 2020 at 01:59:16PM +0100, Jan Kara wrote: > + /* > + * 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(!down_write_trylock(&EXT2_I(inode)->xattr_sem))) > + return; Shouldn't this be a WARN_ON_ONCE? Just in case the impossible happens that avoids spamming dmesg over and over.
On Mon 24-02-20 11:46:55, Christoph Hellwig wrote: > On Mon, Feb 24, 2020 at 01:59:16PM +0100, Jan Kara wrote: > > + /* > > + * 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(!down_write_trylock(&EXT2_I(inode)->xattr_sem))) > > + return; > > Shouldn't this be a WARN_ON_ONCE? Just in case the impossible happens > that avoids spamming dmesg over and over. Fair enough, I'll switch to WARN_ON_ONCE here. Thanks for the review. Honza
diff --git a/fs/ext2/xattr.c b/fs/ext2/xattr.c index 0456bc990b5e..4a2eea3ca188 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(!down_write_trylock(&EXT2_I(inode)->xattr_sem))) + return; if (!EXT2_I(inode)->i_file_acl) goto cleanup;
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(-) I plan to queue this patch to my tree.