ceph: fix potential race condition on operations with CEPH_I_ODIRECT flag

The Coverity Scan service has detected potential
race conditions in ceph_block_o_direct(), ceph_start_io_read(),
ceph_block_buffered(), and ceph_start_io_direct() [1 - 4].

The CID 1590942, 1590665, 1589664, 1590377 contain explanation:
"The value of the shared data will be determined by
the interleaving of thread execution. Thread shared data is accessed
without holding an appropriate lock, possibly causing
a race condition (CWE-366)".

This patch reworks the pattern of accessing/modification of
CEPH_I_ODIRECT flag by means of adding smp_mb__before_atomic()
before reading the status of CEPH_I_ODIRECT flag and
smp_mb__after_atomic() after clearing set/clear this flag.
Also, it was reworked the pattern of using of ci->i_ceph_lock
in ceph_block_o_direct(), ceph_start_io_read(),
ceph_block_buffered(), and ceph_start_io_direct() methods.

[1] https://scan5.scan.coverity.com/#/project-view/64304/10063?selectedIssue=1590942
[2] https://scan5.scan.coverity.com/#/project-view/64304/10063?selectedIssue=1590665
[3] https://scan5.scan.coverity.com/#/project-view/64304/10063?selectedIssue=1589664
[4] https://scan5.scan.coverity.com/#/project-view/64304/10063?selectedIssue=1590377

Signed-off-by: Viacheslav Dubeyko <[e-mail]>
Reviewed-by: Alex Markuze <[e-mail]>
Signed-off-by: Ilya Dryomov <[e-mail]>

fs/ceph/io.c

···
 /* Call with exclusively locked inode->i_rwsem */
 static void ceph_block_o_direct(struct ceph_inode_info *ci, struct inode *inode)
 {
+	bool is_odirect;
+
 	lockdep_assert_held_write(&inode->i_rwsem);
 
-	if (READ_ONCE(ci->i_ceph_flags) & CEPH_I_ODIRECT) {
-		spin_lock(&ci->i_ceph_lock);
-		ci->i_ceph_flags &= ~CEPH_I_ODIRECT;
-		spin_unlock(&ci->i_ceph_lock);
-		inode_dio_wait(inode);
+	spin_lock(&ci->i_ceph_lock);
+	/* ensure that bit state is consistent */
+	smp_mb__before_atomic();
+	is_odirect = READ_ONCE(ci->i_ceph_flags) & CEPH_I_ODIRECT;
+	if (is_odirect) {
+		clear_bit(CEPH_I_ODIRECT_BIT, &ci->i_ceph_flags);
+		/* ensure modified bit is visible */
+		smp_mb__after_atomic();
 	}
+	spin_unlock(&ci->i_ceph_lock);
+
+	if (is_odirect)
+		inode_dio_wait(inode);
 }
 
 /**
···
 int ceph_start_io_read(struct inode *inode)
 {
 	struct ceph_inode_info *ci = ceph_inode(inode);
+	bool is_odirect;
 	int err;
 
 	/* Be an optimist! */
···
 	if (err)
 		return err;
 
-	if (!(READ_ONCE(ci->i_ceph_flags) & CEPH_I_ODIRECT))
+	spin_lock(&ci->i_ceph_lock);
+	/* ensure that bit state is consistent */
+	smp_mb__before_atomic();
+	is_odirect = READ_ONCE(ci->i_ceph_flags) & CEPH_I_ODIRECT;
+	spin_unlock(&ci->i_ceph_lock);
+	if (!is_odirect)
 		return 0;
 	up_read(&inode->i_rwsem);
 
···
 /* Call with exclusively locked inode->i_rwsem */
 static void ceph_block_buffered(struct ceph_inode_info *ci, struct inode *inode)
 {
+	bool is_odirect;
+
 	lockdep_assert_held_write(&inode->i_rwsem);
 
-	if (!(READ_ONCE(ci->i_ceph_flags) & CEPH_I_ODIRECT)) {
-		spin_lock(&ci->i_ceph_lock);
-		ci->i_ceph_flags |= CEPH_I_ODIRECT;
-		spin_unlock(&ci->i_ceph_lock);
+	spin_lock(&ci->i_ceph_lock);
+	/* ensure that bit state is consistent */
+	smp_mb__before_atomic();
+	is_odirect = READ_ONCE(ci->i_ceph_flags) & CEPH_I_ODIRECT;
+	if (!is_odirect) {
+		set_bit(CEPH_I_ODIRECT_BIT, &ci->i_ceph_flags);
+		/* ensure modified bit is visible */
+		smp_mb__after_atomic();
+	}
+	spin_unlock(&ci->i_ceph_lock);
+
+	if (!is_odirect) {
 		/* FIXME: unmap_mapping_range? */
 		filemap_write_and_wait(inode->i_mapping);
 	}
···
 int ceph_start_io_direct(struct inode *inode)
 {
 	struct ceph_inode_info *ci = ceph_inode(inode);
+	bool is_odirect;
 	int err;
 
 	/* Be an optimist! */
···
 	if (err)
 		return err;
 
-	if (READ_ONCE(ci->i_ceph_flags) & CEPH_I_ODIRECT)
+	spin_lock(&ci->i_ceph_lock);
+	/* ensure that bit state is consistent */
+	smp_mb__before_atomic();
+	is_odirect = READ_ONCE(ci->i_ceph_flags) & CEPH_I_ODIRECT;
+	spin_unlock(&ci->i_ceph_lock);
+	if (is_odirect)
 		return 0;
 	up_read(&inode->i_rwsem);
 

fs/ceph/super.h

···
 #define CEPH_I_FLUSH_SNAPS	(1 << 8)  /* need flush snapss */
 #define CEPH_I_ERROR_WRITE	(1 << 9) /* have seen write errors */
 #define CEPH_I_ERROR_FILELOCK	(1 << 10) /* have seen file lock errors */
-#define CEPH_I_ODIRECT		(1 << 11) /* inode in direct I/O mode */
+#define CEPH_I_ODIRECT_BIT	(11) /* inode in direct I/O mode */
+#define CEPH_I_ODIRECT		(1 << CEPH_I_ODIRECT_BIT)
 #define CEPH_ASYNC_CREATE_BIT	(12)	  /* async create in flight for this */
 #define CEPH_I_ASYNC_CREATE	(1 << CEPH_ASYNC_CREATE_BIT)
 #define CEPH_I_SHUTDOWN		(1 << 13) /* inode is no longer usable */