9p: add refcount to p9_fid struct

Fix race issue in fid contention.

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 parallel accesses.
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 contention 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.

Link: http://lkml.kernel.org/r/20200923141146.90046-5-jianyong.wu@arm.com
Fixes: 478ba09edc1f ("fs/9p: search open fids first")
Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

1 files changed, 13 insertions, 3 deletions

diff --git a/fs/9p/xattr.c b/fs/9p/xattr.c
index ac8ff8ca4c11..87217dd0433e 100644
--- a/fs/9p/xattr.c
+++ b/fs/9p/xattr.c
@@ -71,14 +71,17 @@ ssize_t v9fs_xattr_get(struct dentry *dentry, const char *name,
 		       void *buffer, size_t buffer_size)
 {
 	struct p9_fid *fid;
+	int ret;
 
 	p9_debug(P9_DEBUG_VFS, "name = %s value_len = %zu\n",
 		 name, buffer_size);
 	fid = v9fs_fid_lookup(dentry);
 	if (IS_ERR(fid))
 		return PTR_ERR(fid);
+	ret = v9fs_fid_xattr_get(fid, name, buffer, buffer_size);
+	p9_client_clunk(fid);
 
-	return v9fs_fid_xattr_get(fid, name, buffer, buffer_size);
+	return ret;
 }
 
 /*
@@ -96,8 +99,15 @@ ssize_t v9fs_xattr_get(struct dentry *dentry, const char *name,
 int v9fs_xattr_set(struct dentry *dentry, const char *name,
 		   const void *value, size_t value_len, int flags)
 {
-	struct p9_fid *fid = v9fs_fid_lookup(dentry);
-	return v9fs_fid_xattr_set(fid, name, value, value_len, flags);
+	int ret;
+	struct p9_fid *fid;
+
+	fid  = v9fs_fid_lookup(dentry);
+	if (IS_ERR(fid))
+		return PTR_ERR(fid);
+	ret = v9fs_fid_xattr_set(fid, name, value, value_len, flags);
+	p9_client_clunk(fid);
+	return ret;
 }
 
 int v9fs_fid_xattr_set(struct p9_fid *fid, const char *name,