[PATCH] fs: don't overwrite the nsec field if I_CTIME_QUERIED is already set

[Jeff Layton <jlayton@kernel.org>]

When fetching the ctime's nsec value for a stat-like operation, do a
simple fetch first and avoid the atomic_fetch_or if the flag is already
set.

Suggested-by: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
I'm running tests on this now, but I don't expect any problems.

This is based on top of Christian's vfs.mgtime branch. It may be best to
squash this into 6feb43ecdd8e ("fs: add infrastructure for multigrain
timestamps").
---
To: Alexander Viro <viro@zeniv.linux.org.uk>
To: Christian Brauner <brauner@kernel.org>
To: Jan Kara <jack@suse.cz>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 fs/stat.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/stat.c b/fs/stat.c
index f2bf7cca64b2..9eb6d9b2d010 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -35,8 +35,8 @@
  * @inode: inode from which to grab the c/mtime
  *
  * Given @inode, grab the ctime and mtime out if it and store the result
- * in @stat. When fetching the value, flag it as queried so the next write
- * will ensure a distinct timestamp.
+ * in @stat. When fetching the value, flag it as QUERIED (if not already)
+ * so the next write will record a distinct timestamp.
  */
 void fill_mg_cmtime(struct kstat *stat, u32 request_mask, struct inode *inode)
 {
@@ -50,7 +50,10 @@ void fill_mg_cmtime(struct kstat *stat, u32 request_mask, struct inode *inode)
 
 	stat->mtime = inode_get_mtime(inode);
 	stat->ctime.tv_sec = inode->i_ctime_sec;
-	stat->ctime.tv_nsec = ((u32)atomic_fetch_or(I_CTIME_QUERIED, pcn)) & ~I_CTIME_QUERIED;
+	stat->ctime.tv_nsec = (u32)atomic_read(pcn);
+	if (!(stat->ctime.tv_nsec & I_CTIME_QUERIED))
+		stat->ctime.tv_nsec = ((u32)atomic_fetch_or(I_CTIME_QUERIED, pcn));
+	stat->ctime.tv_nsec &= ~I_CTIME_QUERIED;
 	trace_fill_mg_cmtime(inode, &stat->ctime, &stat->mtime);
 }
 EXPORT_SYMBOL(fill_mg_cmtime);

---
base-commit: 9000eec2bdc08a0b9027427f9d3d9a3545694258
change-id: 20240809-mgtime-cbc2aa6dc0fe

Best regards,
-- 
Jeff Layton <jlayton@kernel.org>