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From: Amir Goldstein <amir73il@gmail.com>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>,
	Al Viro <viro@zeniv.linux.org.uk>,
	linux-unionfs@vger.kernel.org,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH v2 01/11] ovl: store path type in dentry
Date: Mon, 24 Apr 2017 16:41:39 +0300	[thread overview]
Message-ID: <CAOQ4uxiR1FsubAJs=0dZpnfp8dXJtRqWpFHKOA_PBfpaR_j=hw@mail.gmail.com> (raw)
In-Reply-To: <20170424133617.GA17437@redhat.com>

On Mon, Apr 24, 2017 at 4:36 PM, Vivek Goyal <vgoyal@redhat.com> wrote:
> On Mon, Apr 24, 2017 at 04:10:30PM +0300, Amir Goldstein wrote:
>> On Mon, Apr 24, 2017 at 3:59 PM, Vivek Goyal <vgoyal@redhat.com> wrote:
>> >
>> > On Mon, Apr 24, 2017 at 12:14:06PM +0300, Amir Goldstein wrote:
>> >
>> > [..]
>> > > diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
>> > > index c072a0c..671bac0 100644
>> > > --- a/fs/overlayfs/super.c
>> > > +++ b/fs/overlayfs/super.c
>> > > @@ -961,6 +961,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
>> > >       kfree(stack);
>> > >
>> > >       root_dentry->d_fsdata = oe;
>> > > +     ovl_update_type(root_dentry, true);
>> > >
>> > >       realinode = d_inode(ovl_dentry_real(root_dentry));
>> > >       ovl_inode_init(d_inode(root_dentry), realinode, !!upperpath.dentry);
>> > > diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
>> > > index 1953986..6a857fb 100644
>> > > --- a/fs/overlayfs/util.c
>> > > +++ b/fs/overlayfs/util.c
>> > > @@ -70,21 +70,38 @@ bool ovl_dentry_weird(struct dentry *dentry)
>> > >  enum ovl_path_type ovl_path_type(struct dentry *dentry)
>> > >  {
>> > >       struct ovl_entry *oe = dentry->d_fsdata;
>> > > -     enum ovl_path_type type = 0;
>> > > +     enum ovl_path_type type = oe->__type;
>> > >
>> > > -     if (oe->__upperdentry) {
>> > > -             type = __OVL_PATH_UPPER;
>> > > +     /* Matches smp_wmb() in ovl_update_type() */
>> > > +     smp_rmb();
>> > > +     return type;
>> >
>> > Hi Amir,
>> >
>> > I never manage to understand barriers so I will ask. Why this barrier is
>> > required and what can go wrong if we don't use this barrier.
>> >
>>
>> Hi Vivek,
>>
>> Miklos was kind enough to answer that question for me when he made
>> the comment about missing memmory barrier on v1 of the patch:
>> http://www.spinics.net/lists/linux-unionfs/msg01687.html
>>
>> Whether or not I got it right, we shall see shortly ;-)
>
> Hi Amir,
>
> Thanks. Ok, so we are making sure if other cpu sees updated ->type, then
> it is guaranteed that it isses updated ->upperdentry as well.
>
> I feel it is worth to put a shortened version of explanation from miklos
> in the comments. It will help to recall why did we put it. But it is just
> me. May be it is obvious to others.
>

I recon memory barriers are obvious to few.
However, I think the documentation I left is quite standard practice:
- smp_rmb() has a comment to point to the matching smp_wmb()
- smp_wmb() has a comment to explain what the barrier protects:
 * Make sure type is consistent with __upperdentry before making it
 * visible to ovl_path_type()
(i.e. to lockless readers of oe->__type)

Amir.

  reply	other threads:[~2017-04-24 13:41 UTC|newest]

Thread overview: 69+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-04-24  9:14 [PATCH v2 00/11] overlayfs constant inode numbers Amir Goldstein
2017-04-24  9:14 ` [PATCH v2 01/11] ovl: store path type in dentry Amir Goldstein
2017-04-24 12:59   ` Vivek Goyal
2017-04-24 13:10     ` Amir Goldstein
2017-04-24 13:36       ` Vivek Goyal
2017-04-24 13:41         ` Amir Goldstein [this message]
2017-04-24  9:14 ` [PATCH v2 02/11] ovl: cram opaque boolean into type flags Amir Goldstein
2017-04-24  9:14 ` [PATCH v2 03/11] ovl: check if all layers are on the same fs Amir Goldstein
2017-04-24  9:14 ` [PATCH v2 04/11] ovl: store file handle of lower inode on copy up Amir Goldstein
2017-04-24 13:32   ` kbuild test robot
2017-04-24 13:57     ` Amir Goldstein
2017-04-25 14:53   ` Miklos Szeredi
2017-04-26  5:47     ` Amir Goldstein
2017-04-26  9:21       ` Miklos Szeredi
2017-04-26  9:27         ` Amir Goldstein
2017-04-26  9:35           ` Miklos Szeredi
2017-04-26  9:39   ` Miklos Szeredi
2017-04-26  9:53     ` Amir Goldstein
2017-04-26  9:57       ` Miklos Szeredi
2017-04-24  9:14 ` [PATCH v2 05/11] ovl: lookup redirect by file handle Amir Goldstein
2017-04-25  8:10   ` Amir Goldstein
2017-04-25 15:13   ` Miklos Szeredi
2017-04-25 17:41     ` Amir Goldstein
2017-04-25 19:11       ` Amir Goldstein
2017-04-26  9:06         ` Miklos Szeredi
2017-04-26  9:40           ` Amir Goldstein
2017-04-26  9:55             ` Miklos Szeredi
2017-04-26 10:17               ` Amir Goldstein
2017-04-26 12:15                 ` Miklos Szeredi
2017-04-26 14:51                   ` Amir Goldstein
2017-04-27  6:27                     ` Amir Goldstein
2017-04-27  7:48                       ` Miklos Szeredi
2017-04-27  9:22                         ` Amir Goldstein
2017-04-27  9:26                         ` Miklos Szeredi
     [not found]                           ` <CAOQ4uxiweaqzR3eT-StgtDFAHBuYhGRvAJE6v=XpH33MevpmoA@mail.gmail.com>
     [not found]                             ` <CAJfpegtTJmcLVrLOeQbhu4Q6sM0Mi_FRgr+vStF0k95QsWm5uQ@mail.gmail.com>
2017-04-27 13:53                               ` Amir Goldstein
2017-04-27 14:46                                 ` Miklos Szeredi
2017-04-27 16:08                                   ` Amir Goldstein
2017-04-28  7:25                                     ` Amir Goldstein
2017-04-28  7:55                                       ` Miklos Szeredi
2017-04-28  8:15                                         ` Amir Goldstein
2017-04-28  9:37                                           ` Miklos Szeredi
2017-04-28  9:57                                             ` Amir Goldstein
2017-04-28 10:05                                               ` Miklos Szeredi
2017-04-28 10:45                                                 ` Amir Goldstein
2017-04-27  7:40                     ` Miklos Szeredi
2017-04-24  9:14 ` [PATCH v2 06/11] ovl: lookup non-dir inode copy up origin Amir Goldstein
2017-04-24  9:14 ` [PATCH v2 07/11] ovl: set the COPYUP type flag for non-dirs Amir Goldstein
2017-04-26 14:40   ` Miklos Szeredi
2017-04-26 14:53     ` Miklos Szeredi
2017-04-26 15:02       ` Amir Goldstein
2017-04-26 18:51         ` Amir Goldstein
2017-04-27  9:32         ` Miklos Szeredi
2017-04-26 14:57     ` Amir Goldstein
2017-04-24  9:14 ` [PATCH v2 08/11] ovl: redirect non-dir by path on rename Amir Goldstein
2017-04-24  9:14 ` [PATCH v2 09/11] ovl: constant st_ino/st_dev across copy up Amir Goldstein
2017-04-24  9:14 ` [PATCH v2 10/11] ovl: persistent and constant inode number for directories Amir Goldstein
2017-04-24  9:14 ` [PATCH v2 11/11] ovl: fix du --one-file-system on overlay mount Amir Goldstein
2017-04-24 18:40 ` [PATCH v2 12/12] ovl: persistent inode numbers for hardlinks Amir Goldstein
2017-04-24 18:51 ` [PATCH v2 00/11] overlayfs constant inode numbers Amir Goldstein
2017-04-25 11:52 ` Vivek Goyal
2017-04-25 12:05   ` Amir Goldstein
2017-04-25 12:16 ` Vivek Goyal
2017-04-25 12:41   ` Amir Goldstein
2017-04-25 12:52     ` Vivek Goyal
2017-04-25 13:23       ` Amir Goldstein
2017-04-25 13:29         ` Vivek Goyal
2017-04-25 13:49           ` Amir Goldstein
2017-04-25 13:53             ` Vivek Goyal
2017-04-25 14:20               ` Amir Goldstein

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