fs: fs_struct use seqlock

Use a seqlock in the fs_struct to enable us to take an atomic copy of the
complete cwd and root paths. Use this in the RCU lookup path to avoid a
thread-shared spinlock in RCU lookup operations.

Multi-threaded apps may now perform path lookups with scalability matching
multi-process apps. Operations such as stat(2) become very scalable for
multi-threaded workload.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>

1 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/fs_struct.h b/include/linux/fs_struct.h
index a42b5bf02f8b..003dc0fd7347 100644
--- a/include/linux/fs_struct.h
+++ b/include/linux/fs_struct.h
@@ -2,10 +2,13 @@
 #define _LINUX_FS_STRUCT_H
 
 #include <linux/path.h>
+#include <linux/spinlock.h>
+#include <linux/seqlock.h>
 
 struct fs_struct {
         int users;
         spinlock_t lock;
+        seqcount_t seq;
         int umask;
         int in_exec;
         struct path root, pwd;