kernfs: Use a per-fs rwsem to protect per-fs list of kernfs_super_info.

Right now per-fs kernfs_rwsem protects list of kernfs_super_info instances
for a kernfs_root. Since kernfs_rwsem is used to synchronize several other
operations across kernfs and since most of these operations don't impact
kernfs_super_info, we can use a separate per-fs rwsem to synchronize access
to list of kernfs_super_info.
This helps in reducing contention around kernfs_rwsem and also allows
operations that change/access list of kernfs_super_info to proceed without
contending for kernfs_rwsem.

Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
Link: https://lore.kernel.org/r/20230309110932.2889010-3-imran.f.khan@oracle.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 953b2717c60e632fbbf09b1ab144e780d56effd7..2cdb8516e5287e2f72f41adb500333d514a1a6a2 100644 (file)
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -944,6 +944,7 @@ struct kernfs_root *kernfs_create_root(struct kernfs_syscall_ops *scops,
        idr_init(&root->ino_idr);
        init_rwsem(&root->kernfs_rwsem);
        init_rwsem(&root->kernfs_iattr_rwsem);
+       init_rwsem(&root->kernfs_supers_rwsem);
        INIT_LIST_HEAD(&root->supers);
 
        /*

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index e4a50e4ff0d2340282ae6ab1902783e2cf4f20cf..40c4661f15b7c3f7cd8f57bdfca02e48df37ae6b 100644 (file)
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -922,8 +922,8 @@ repeat:
 
        root = kernfs_root(kn);
        /* kick fsnotify */
-       down_write(&root->kernfs_rwsem);
 
+       down_read(&root->kernfs_supers_rwsem);
        list_for_each_entry(info, &kernfs_root(kn)->supers, node) {
                struct kernfs_node *parent;
                struct inode *p_inode = NULL;
@@ -960,7 +960,7 @@ repeat:
                iput(inode);
        }
 
-       up_write(&root->kernfs_rwsem);
+       up_read(&root->kernfs_supers_rwsem);
        kernfs_put(kn);
        goto repeat;
 }

diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index 3297093c920dee4e0698260bb0eb2fea3d051e77..a9b854cdfdb5f73e45f25fbea2cdce50c6b00931 100644 (file)
--- a/fs/kernfs/kernfs-internal.h
+++ b/fs/kernfs/kernfs-internal.h
@@ -48,6 +48,7 @@ struct kernfs_root {
        wait_queue_head_t       deactivate_waitq;
        struct rw_semaphore     kernfs_rwsem;
        struct rw_semaphore     kernfs_iattr_rwsem;
+       struct rw_semaphore     kernfs_supers_rwsem;
 };
 
 /* +1 to avoid triggering overflow warning when negating it */

diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index e08e8d9998070d8cdad85ec7e79cd1d9b992d7a2..d49606accb07b1a702db70e17fdbd813a668a661 100644 (file)
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -351,9 +351,9 @@ int kernfs_get_tree(struct fs_context *fc)
                }
                sb->s_flags |= SB_ACTIVE;
 
-               down_write(&root->kernfs_rwsem);
+               down_write(&root->kernfs_supers_rwsem);
                list_add(&info->node, &info->root->supers);
-               up_write(&root->kernfs_rwsem);
+               up_write(&root->kernfs_supers_rwsem);
        }
 
        fc->root = dget(sb->s_root);
@@ -380,9 +380,9 @@ void kernfs_kill_sb(struct super_block *sb)
        struct kernfs_super_info *info = kernfs_info(sb);
        struct kernfs_root *root = info->root;
 
-       down_write(&root->kernfs_rwsem);
+       down_write(&root->kernfs_supers_rwsem);
        list_del(&info->node);
-       up_write(&root->kernfs_rwsem);
+       up_write(&root->kernfs_supers_rwsem);
 
        /*
         * Remove the superblock from fs_supers/s_instances