From 8b19e34188a32d63a7da94ea8a3f5e39b0c66050 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Thu, 12 Sep 2013 10:35:47 -0700 Subject: [PATCH] vfs: make getcwd() get the root and pwd path under rcu This allows us to skip all the crazy spinlocks and reference count updates, and instead use the fs sequence read-lock to get an atomic snapshot of the root and cwd information. We might want to make the rule that "prepend_path()" is always called with the RCU lock held, but the RCU lock nests fine and this is the minimal fix. Signed-off-by: Linus Torvalds --- fs/dcache.c | 23 ++++++++++++----------- 1 file changed, 12 insertions(+), 11 deletions(-) diff --git a/fs/dcache.c b/fs/dcache.c index 4df68e27cbc7..99d4d7226203 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -3015,15 +3015,16 @@ Elong: return ERR_PTR(-ENAMETOOLONG); } -static inline void get_fs_root_and_pwd(struct fs_struct *fs, struct path *root, - struct path *pwd) +static void get_fs_root_and_pwd_rcu(struct fs_struct *fs, struct path *root, + struct path *pwd) { - spin_lock(&fs->lock); - *root = fs->root; - path_get(root); - *pwd = fs->pwd; - path_get(pwd); - spin_unlock(&fs->lock); + unsigned seq; + + do { + seq = read_seqcount_begin(&fs->seq); + *root = fs->root; + *pwd = fs->pwd; + } while (read_seqcount_retry(&fs->seq, seq)); } /* @@ -3053,7 +3054,8 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size) if (!page) return -ENOMEM; - get_fs_root_and_pwd(current->fs, &root, &pwd); + rcu_read_lock(); + get_fs_root_and_pwd_rcu(current->fs, &root, &pwd); error = -ENOENT; br_read_lock(&vfsmount_lock); @@ -3088,8 +3090,7 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size) } out: - path_put(&pwd); - path_put(&root); + rcu_read_unlock(); free_page((unsigned long) page); return error; } -- 2.20.1