From: "J. Bruce Fields" Subject: Re: [PATCH v5 02/19] fs: don't take the i_lock in inode_inc_iversion Date: Thu, 18 Jan 2018 16:45:34 -0500 Message-ID: <20180118214534.GB5299@fieldses.org> References: <20180109141059.25929-1-jlayton@kernel.org> <20180109141059.25929-3-jlayton@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, viro@zeniv.linux.org.uk, linux-nfs@vger.kernel.org, neilb@suse.de, jack@suse.de, linux-ext4@vger.kernel.org, tytso@mit.edu, adilger.kernel@dilger.ca, linux-xfs@vger.kernel.org, darrick.wong@oracle.com, david@fromorbit.com, linux-btrfs@vger.kernel.org, clm@fb.com, jbacik@fb.com, dsterba@suse.com, linux-integrity@vger.kernel.org, zohar@linux.vnet.ibm.com, dmitry.kasatkin@gmail.com, linux-afs@lists.infradead.org, dhowells@redhat.com, jaltman@auristor.com, krzk@kernel.org To: Jeff Layton Return-path: Content-Disposition: inline In-Reply-To: <20180109141059.25929-3-jlayton@kernel.org> Sender: linux-btrfs-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Tue, Jan 09, 2018 at 09:10:42AM -0500, Jeff Layton wrote: > From: Jeff Layton > > The rationale for taking the i_lock when incrementing this value is > lost in antiquity. The readers of the field don't take it (at least > not universally), so my assumption is that it was only done here to > serialize incrementors. > > If that is indeed the case, then we can drop the i_lock from this > codepath and treat it as a atomic64_t for the purposes of > incrementing it. This allows us to use inode_inc_iversion without > any danger of lock inversion. > > Note that the read side is not fetched atomically with this change. > The assumption here is that that is not a critical issue since the > i_version is not fully synchronized with anything else anyway. So I guess it's theoretically possible that e.g. if you read while it's incrementing from 2^32-1 to 2^32 you could read 0, 1, or 2^32+1? If so then you could see an i_version value reused and incorrectly decide that a file hadn't changed. But it's such a tiny case, and I think you convert this to atomic64_t later anyway, so, whatever. --b. > > Signed-off-by: Jeff Layton > --- > include/linux/iversion.h | 7 ++++--- > 1 file changed, 4 insertions(+), 3 deletions(-) > > diff --git a/include/linux/iversion.h b/include/linux/iversion.h > index d09cc3a08740..5ad9eaa3a9b0 100644 > --- a/include/linux/iversion.h > +++ b/include/linux/iversion.h > @@ -104,12 +104,13 @@ inode_set_iversion_queried(struct inode *inode, u64 new) > static inline bool > inode_maybe_inc_iversion(struct inode *inode, bool force) > { > - spin_lock(&inode->i_lock); > - inode->i_version++; > - spin_unlock(&inode->i_lock); > + atomic64_t *ivp = (atomic64_t *)&inode->i_version; > + > + atomic64_inc(ivp); > return true; > } > > + > /** > * inode_inc_iversion - forcibly increment i_version > * @inode: inode that needs to be updated > -- > 2.14.3