fs: add inode_set_flags() for ext4 3.18 backport

Excerpted from commit 5f16f3225b062

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 31 insertions, 0 deletions

diff --git a/fs/inode.c b/fs/inode.c
index 63537c654..e3b0d8488 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1938,3 +1938,34 @@ void inode_dio_done(struct inode *inode)
 		wake_up_bit(&inode->i_state, __I_DIO_WAKEUP);
 }
 EXPORT_SYMBOL(inode_dio_done);
+
+/*
+ * inode_set_flags - atomically set some inode flags
+ *
+ * Note: the caller should be holding i_mutex, or else be sure that
+ * they have exclusive access to the inode structure (i.e., while the
+ * inode is being instantiated).  The reason for the cmpxchg() loop
+ * --- which wouldn't be necessary if all code paths which modify
+ * i_flags actually followed this rule, is that there is at least one
+ * code path which doesn't today --- for example,
+ * __generic_file_aio_write() calls file_remove_suid() without holding
+ * i_mutex --- so we use cmpxchg() out of an abundance of caution.
+ *
+ * In the long run, i_mutex is overkill, and we should probably look
+ * at using the i_lock spinlock to protect i_flags, and then make sure
+ * it is so documented in include/linux/fs.h and that all code follows
+ * the locking convention!!
+ */
+void inode_set_flags(struct inode *inode, unsigned int flags,
+		     unsigned int mask)
+{
+	unsigned int old_flags, new_flags;
+
+	WARN_ON_ONCE(flags & ~mask);
+	do {
+		old_flags = ACCESS_ONCE(inode->i_flags);
+		new_flags = (old_flags & ~mask) | flags;
+	} while (unlikely(cmpxchg(&inode->i_flags, old_flags,
+				  new_flags) != old_flags));
+}
+EXPORT_SYMBOL(inode_set_flags);