linux-kernel - RE: [PATCH RFC v2 4/4] 9p: fix race issue in fid contention.

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Message-ID: <HE1PR0802MB25556D1AAF1336F4EE3CA8DBF4390@HE1PR0802MB2555.eurprd08.prod.outlook.com>
Date:   Thu, 24 Sep 2020 08:38:01 +0000
From:   Jianyong Wu <Jianyong.Wu@....com>
To:     Dominique Martinet <asmadeus@...ewreck.org>
CC:     "ericvh@...il.com" <ericvh@...il.com>,
        "lucho@...kov.net" <lucho@...kov.net>,
        "qemu_oss@...debyte.com" <qemu_oss@...debyte.com>,
        "groug@...d.org" <groug@...d.org>,
        "v9fs-developer@...ts.sourceforge.net" 
        <v9fs-developer@...ts.sourceforge.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Justin He <Justin.He@....com>
Subject: RE: [PATCH RFC v2 4/4] 9p: fix race issue in fid contention.

Hi Dominique,

> -----Original Message-----
> From: Dominique Martinet <asmadeus@...ewreck.org>
> Sent: Wednesday, September 23, 2020 10:50 PM
> To: Jianyong Wu <Jianyong.Wu@....com>
> Cc: ericvh@...il.com; lucho@...kov.net; qemu_oss@...debyte.com;
> groug@...d.org; v9fs-developer@...ts.sourceforge.net; linux-
> kernel@...r.kernel.org; Justin He <Justin.He@....com>
> Subject: Re: [PATCH RFC v2 4/4] 9p: fix race issue in fid contention.
>
>
> Overall looks good; a few comments.

Thanks
>
> Jianyong Wu wrote on Wed, Sep 23, 2020:
> > open-unlink-f*syscall test:
> > I have tested for f*syscall include: ftruncate fstat fchown fchmod faccessat.
>
> Given the other thread, what did you test this with?
Er, I just use Greg's qemu of https://github.com/gkurz/qemu.git: 9p-attr-fixes. I should have referenced it in commit message.

> Since qemu doesn't work apparently do you have a in-house server at arm I
> could test?
> (I'll try with ganesha otherwise, it keeps files open so it should work I think...)
>
Yeah, I test this on my arm server. But as these codes are arch-free, we can test it in whatever a machine.
(also the server in arm can't be accessed by outer space, so sorry)
But I think that this test are far from serious and complete.

> > +atomic_set(&fid->count, 1);
>
> I kind of like the refcount API becauese it has some extra overflow checks;
> but it requires a bit more work around clunk (instead of bailing out early if
> counter hits 0, you need to have it call a separate function in case it does)
>
> That's mostly esthetics though I'm not going to fuss over that.
>
Sorry, I'm not sure what's the context of this line, does this line lie in "__add_fid”. I'm not sure about
why it do harm to clunk?

> > @@ -74,6 +77,7 @@ static struct p9_fid *v9fs_fid_find_inode(struct
> > inode *inode, kuid_t uid)  void v9fs_open_fid_add(struct inode *inode,
> > struct p9_fid *fid)  {
> >  spin_lock(&inode->i_lock);
> > +atomic_set(&fid->count, 1);
>
> Hm, that should be done at fid creation time in net/9p/client.c p9_fid_create ;
> no ?
> (you do it there already, I don't see what reseting count here brings except
> confusion)
>
I put this counter set op before the fids are added to hlist. So I can make sure the counter value is 1 before
fids are used. It's redundant code. I can remove it in both "__add_fid" and "v9fs_open_fid_add", but we must take care of it that
no clunk is called between fids are created and added to hlist. Both are good for me.

> > diff --git a/fs/9p/fid.h b/fs/9p/fid.h index
> > dfa11df02818..1fed96546728 100644
> > --- a/fs/9p/fid.h
> > +++ b/fs/9p/fid.h
> > @@ -22,6 +22,14 @@ static inline struct p9_fid *clone_fid(struct
> > p9_fid *fid)  }  static inline struct p9_fid *v9fs_fid_clone(struct
> > dentry *dentry)  {
> > -return clone_fid(v9fs_fid_lookup(dentry));
> > +struct p9_fid *fid, *nfid;
> > +
> > +fid = v9fs_fid_lookup(dentry);
> > +if (!fid || IS_ERR(fid))
> > +return fid;
> > +
> > +nfid = p9_client_walk(fid, 0, NULL, 1);
>
> I think you clone_fid() here is slightly easier to understand; everyone doesn't
> know that a walk with no component is a clone.
> The compiler will optimize that IS_ERR(fid) is checked twice, it's fine.
>
Er, I rewrite it because I must acquire the intermedia fid from v9fs_fid_lookup and clunk it.

> > diff --git a/include/net/9p/client.h b/include/net/9p/client.h index
> > ce7882da8e86..58ed9bd306bd 100644
> > --- a/include/net/9p/client.h
> > +++ b/include/net/9p/client.h
> > @@ -140,10 +140,16 @@ struct p9_client {
> >   *
> >   * TODO: This needs lots of explanation.
> >   */
> > +enum fid_source {
> > +FID_FROM_OTHER,
> > +FID_FROM_INODE,
> > +FID_FROM_DENTRY,
> > +};
>
> leftovers from previous iteration.
>
Sorry, need remove it.

>
> Overall looks good to me.
> I'd need to spend some time checking the actual counting part & hammering
> the fs a bit then confirming no fid got forgotten (there's a pr_info at umount
> time) but I'm happy with this ; thanks!
>
Yeah, it's a tedious job to do that. Also we need to find a reliable test suit. Then we can check
this patch both from code and test.

Thanks
Jianyong
> --
> Dominique
>
>
>

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