Date:   Wed, 04 Nov 2020 12:32:02 +0100
From:   Christian Schoenebeck <qemu_oss@...debyte.com>
To:     Dominique Martinet <asmadeus@...ewreck.org>
Cc:     Jianyong Wu <jianyong.wu@....com>, ericvh@...il.com,
        lucho@...kov.net, groug@...d.org,
        v9fs-developer@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        justin.he@....com
Subject: Re: [PATCH RFC v2 4/4] 9p: fix race issue in fid contention.

On Dienstag, 3. November 2020 11:41:16 CET Dominique Martinet wrote:
> Jianyong,
> 
> Jianyong Wu wrote on Wed, Sep 23, 2020:
> > Eric's and Greg's patch offer a mechanism to fix open-unlink-f*syscall
> > bug in 9p. But there is race issue in fid comtention.
> > As Greg's patch stores all of fids from opened files into according inode,
> > so all the lookup fid ops can retrieve fid from inode preferentially. But
> > there is no mechanism to handle the fid comtention issue. For example,
> > there are two threads get the same fid in the same time and one of them
> > clunk the fid before the other thread ready to discard the fid. In this
> > scenario, it will lead to some fatal problems, even kernel core dump.
> > 
> > I introduce a mechanism to fix this race issue. A counter field introduced
> > into p9_fid struct to store the reference counter to the fid. When a fid
> > is allocated from the inode or dentry, the counter will increase, and
> > will decrease at the end of its occupation. It is guaranteed that the
> > fid won't be clunked before the reference counter go down to 0, then
> > we can avoid the clunked fid to be used.
> > 
> > tests:
> > race issue test from the old test case:
> > for file in {01..50}; do touch f.${file}; done
> > seq 1 1000 | xargs -n 1 -P 50 -I{} cat f.* > /dev/null
> > 
> > open-unlink-f*syscall test:
> > I have tested for f*syscall include: ftruncate fstat fchown fchmod
> > faccessat.
> > 
> > Signed-off-by: Jianyong Wu <jianyong.wu@....com>
> 
> Ok so I've finally taken some time to test -- sorry for the delay.
> I didn't bother with qemu but the use-after-close f* calls work with
> nfs-ganesha and it doesn't introduce any obvious regression with the
> current qemu either, so this is good for me.
> 
> Greg, Christian - from what I understood (in private, hopefully I'm
> allowed to repeat!), he won't be able to contribute to qemu because of
> company policies and I'm unlikely to take the time either right now.
> I don't think it's a problem to continue as is though, we can land linux
> kernel support (it's still useful for non-qemu servers) and if someone
> is interested later on they'll just need to finish that bit.
> 

Hmm, no idea what kind of policy that is; there is no GPL3 in qemu at least 
that some companies are concerned about, but OK not my business.

I actually thought this would still take a while on kernel side, so in the 
meantime we layed the ground in qemu for resolving this issue independent of 
clients and independent of any guest OS installation by introducing test cases 
using the 9pfs 'local' filesystem driver:

https://github.com/qemu/qemu/blob/master/tests/qtest/virtio-9p-test.c

So the idea was to resolve that chicken egg problem of this issue that way and 
also handle it a bit more systematically. If you now run qemu's 9p tests with 
latest git version (or at least with yesterday's QEMU 5.2 rc1 tarball):

cd qemu/build
make
export QTEST_QEMU_BINARY=x86_64-softmmu/qemu-system-x86_64
tests/qtest/qos-test

these tests will now create a test directory qtest-9p-local-XXXXXX under the 
current directory (i.e. the build directory) where they are creating real 
directories and files like on a production system would do, just without a 
guest OS.

As you can see, there are already 9p tests for creating and deleting 
directories, files, symlinks and hard links, etc.

Maybe somebody interested to see this issue resolved in qemu might help by 
rebasing Greg's old patches and testing it with some test cases this way. 
Personally I need to work on some other things in the next couple weeks, but 
if somebody needs help, questions, review, etc., I'll be there.

> 
> I'm not seeing any obvious problem now so I'll take these patches in
> -next within the next few days, with this extra patch on top that
> basically applies the requests I had:
>  - removing the extra atomic_set in fs/9p/fid.c
>  - convert from atomic to refcount API (overflow checks)
>  - rename a no-composant walk to clone_fid()
> 
> I'll just run some more instrumented tests first to check we're not
> leaking anything but so far I haven't found any problem.
> 
> If you noticed anything else please speak up.
> Thanks for taking the time to finish this! :)
> ---
>  fs/9p/fid.c             | 10 ++++------
>  fs/9p/fid.h             |  2 +-
>  include/net/9p/client.h |  2 +-
>  net/9p/client.c         |  4 ++--
>  4 files changed, 8 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/9p/fid.c b/fs/9p/fid.c
> index 89643dabcdae..50118ec72a92 100644
> --- a/fs/9p/fid.c
> +++ b/fs/9p/fid.c
> @@ -28,7 +28,6 @@
> 
>  static inline void __add_fid(struct dentry *dentry, struct p9_fid *fid)
>  {
> -	atomic_set(&fid->count, 1);
>  	hlist_add_head(&fid->dlist, (struct hlist_head *)&dentry->d_fsdata);
>  }
> 
> @@ -62,7 +61,7 @@ static struct p9_fid *v9fs_fid_find_inode(struct inode
> *inode, kuid_t uid) }
>  	}
>  	if (ret && !IS_ERR(ret))
> -		atomic_inc(&ret->count);
> +		refcount_inc(&ret->count);
>  	spin_unlock(&inode->i_lock);
>  	return ret;
>  }
> @@ -77,7 +76,6 @@ 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);
>  	hlist_add_head(&fid->ilist, (struct hlist_head *)&inode->i_private);
>  	spin_unlock(&inode->i_lock);
>  }
> @@ -110,7 +108,7 @@ static struct p9_fid *v9fs_fid_find(struct dentry
> *dentry, kuid_t uid, int any) hlist_for_each_entry(fid, h, dlist) {
>  			if (any || uid_eq(fid->uid, uid)) {
>  				ret = fid;
> -				atomic_inc(&ret->count);
> +				refcount_inc(&ret->count);
>  				break;
>  			}
>  		}
> @@ -201,7 +199,7 @@ static struct p9_fid *v9fs_fid_lookup_with_uid(struct
> dentry *dentry, }
>  	/* If we are root ourself just return that */
>  	if (dentry->d_sb->s_root == dentry) {
> -		atomic_inc(&fid->count);
> +		refcount_inc(&fid->count);
>  		return fid;
>  	}
>  	/*
> @@ -250,7 +248,7 @@ static struct p9_fid *v9fs_fid_lookup_with_uid(struct
> dentry *dentry, fid = ERR_PTR(-ENOENT);
>  		} else {
>  			__add_fid(dentry, fid);
> -			atomic_inc(&fid->count);
> +			refcount_inc(&fid->count);
>  			spin_unlock(&dentry->d_lock);
>  		}
>  	}
> diff --git a/fs/9p/fid.h b/fs/9p/fid.h
> index 1fed96546728..f7f33509e169 100644
> --- a/fs/9p/fid.h
> +++ b/fs/9p/fid.h
> @@ -28,7 +28,7 @@ static inline struct p9_fid *v9fs_fid_clone(struct dentry
> *dentry) if (!fid || IS_ERR(fid))
>  		return fid;
> 
> -	nfid = p9_client_walk(fid, 0, NULL, 1);
> +	nfid = clone_fid(fid);
>  	p9_client_clunk(fid);
>  	return nfid;
>  }
> diff --git a/include/net/9p/client.h b/include/net/9p/client.h
> index 58ed9bd306bd..e1c308d8d288 100644
> --- a/include/net/9p/client.h
> +++ b/include/net/9p/client.h
> @@ -149,7 +149,7 @@ enum fid_source {
>  struct p9_fid {
>  	struct p9_client *clnt;
>  	u32 fid;
> -	atomic_t count;
> +	refcount_t count;
>  	int mode;
>  	struct p9_qid qid;
>  	u32 iounit;
> diff --git a/net/9p/client.c b/net/9p/client.c
> index a6c8a915e0d8..ba4910138c5b 100644
> --- a/net/9p/client.c
> +++ b/net/9p/client.c
> @@ -901,7 +901,7 @@ static struct p9_fid *p9_fid_create(struct p9_client
> *clnt) fid->clnt = clnt;
>  	fid->rdir = NULL;
>  	fid->fid = 0;
> -	atomic_set(&fid->count, 1);
> +	refcount_set(&fid->count, 1);
> 
>  	idr_preload(GFP_KERNEL);
>  	spin_lock_irq(&clnt->lock);
> @@ -1466,7 +1466,7 @@ int p9_client_clunk(struct p9_fid *fid)
>  		dump_stack();
>  		return 0;
>  	}
> -	if (!atomic_dec_and_test(&fid->count))
> +	if (!refcount_dec_and_test(&fid->count))
>  		return 0;
> 
>  again:

Best regards,
Christian Schoenebeck

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