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Subject[RESEND PATCH v7 5/8] kernfs: Use a per-fs rwsem to protect per-fs list of kernfs_super_info.
Date
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>
---
fs/kernfs/dir.c | 1 +
fs/kernfs/file.c | 2 ++
fs/kernfs/kernfs-internal.h | 1 +
fs/kernfs/mount.c | 8 ++++----
include/linux/kernfs.h | 1 +
5 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 61a8edc4ba8b..17b438498c0b 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -917,6 +917,7 @@ struct kernfs_root *kernfs_create_root(struct kernfs_syscall_ops *scops,
idr_init(&root->ino_idr);
init_rwsem(&root->kernfs_rwsem);
INIT_LIST_HEAD(&root->supers);
+ init_rwsem(&root->supers_rwsem);

/*
* On 64bit ino setups, id is ino. On 32bit, low 32bits are ino.
diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 95426df9f030..07003d47343d 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -854,6 +854,7 @@ static void kernfs_notify_workfn(struct work_struct *work)
/* kick fsnotify */
down_write(&root->kernfs_rwsem);

+ down_write(&root->supers_rwsem);
list_for_each_entry(info, &kernfs_root(kn)->supers, node) {
struct kernfs_node *parent;
struct inode *p_inode = NULL;
@@ -889,6 +890,7 @@ static void kernfs_notify_workfn(struct work_struct *work)

iput(inode);
}
+ up_write(&root->supers_rwsem);

up_write(&root->kernfs_rwsem);
kernfs_put(kn);
diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index 25c3329bd60e..6e6398a72578 100644
--- a/fs/kernfs/kernfs-internal.h
+++ b/fs/kernfs/kernfs-internal.h
@@ -47,6 +47,7 @@ struct kernfs_root {

wait_queue_head_t deactivate_waitq;
struct rw_semaphore kernfs_rwsem;
+ struct rw_semaphore supers_rwsem;
};

/* +1 to avoid triggering overflow warning when negating it */
diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index 809b738739b1..d35142226c34 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -347,9 +347,9 @@ int kernfs_get_tree(struct fs_context *fc)
}
sb->s_flags |= SB_ACTIVE;

- down_write(&root->kernfs_rwsem);
+ down_write(&root->supers_rwsem);
list_add(&info->node, &info->root->supers);
- up_write(&root->kernfs_rwsem);
+ up_write(&root->supers_rwsem);
}

fc->root = dget(sb->s_root);
@@ -376,9 +376,9 @@ 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->supers_rwsem);
list_del(&info->node);
- up_write(&root->kernfs_rwsem);
+ up_write(&root->supers_rwsem);

/*
* Remove the superblock from fs_supers/s_instances
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index e82e57c007e9..9f0926f553fc 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -267,6 +267,7 @@ struct kernfs_root {

wait_queue_head_t deactivate_waitq;
struct rw_semaphore kernfs_rwsem;
+ struct rw_semaphore supers_rwsem;
};
#endif

--
2.30.2
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