Re: [PATCH v6 5/7] kernfs: use i_lock to protect concurrent inode updates

From: Miklos Szeredi <miklos@szeredi.hu>
To: Ian Kent <raven@themaw.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Tejun Heo <tj@kernel.org>, Eric Sandeen <sandeen@sandeen.net>,
	Fox Chen <foxhlchen@gmail.com>,
	Brice Goglin <brice.goglin@gmail.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Rick Lindsley <ricklind@linux.vnet.ibm.com>,
	David Howells <dhowells@redhat.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Carlos Maiolino <cmaiolino@redhat.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v6 5/7] kernfs: use i_lock to protect concurrent inode updates
Date: Fri, 11 Jun 2021 15:11:48 +0200	[thread overview]
Message-ID: <CAJfpegvYeNjWKc_9Aah+8NMicTus1CqqwQqLGUeYiQSKN9=gmQ@mail.gmail.com> (raw)
In-Reply-To: <162322868275.361452.17585267026652222121.stgit@web.messagingengine.com>

On Wed, 9 Jun 2021 at 10:52, Ian Kent <raven@themaw.net> wrote:
>
> The inode operations .permission() and .getattr() use the kernfs node
> write lock but all that's needed is to keep the rb tree stable while
> updating the inode attributes as well as protecting the update itself
> against concurrent changes.
>
> And .permission() is called frequently during path walks and can cause
> quite a bit of contention between kernfs node operations and path
> walks when the number of concurrent walks is high.
>
> To change kernfs_iop_getattr() and kernfs_iop_permission() to take
> the rw sem read lock instead of the write lock an additional lock is
> needed to protect against multiple processes concurrently updating
> the inode attributes and link count in kernfs_refresh_inode().
>
> The inode i_lock seems like the sensible thing to use to protect these
> inode attribute updates so use it in kernfs_refresh_inode().
>
> The last hunk in the patch, applied to kernfs_fill_super(), is possibly
> not needed but taking the lock was present originally and I prefer to
> continue to take it so the rb tree is held stable during the call to
> kernfs_refresh_inode() made by kernfs_get_inode().
>
> Signed-off-by: Ian Kent <raven@themaw.net>

Reviewed-by: Miklos Szeredi <mszeredi@redhat.com>