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Commit 8b19e341 authored by Linus Torvalds's avatar Linus Torvalds
Browse files

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: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 5762482f

fs/dcache.c

+12 −11
Original line number Diff line number Diff line
@@ -3015,15 +3015,16 @@ Elong: 
	return ERR_PTR(-ENAMETOOLONG);

}



static inline void get_fs_root_and_pwd(struct fs_struct *fs, struct path *root,

static void get_fs_root_and_pwd_rcu(struct fs_struct *fs, struct path *root,

				    struct path *pwd)

{

	spin_lock(&fs->lock);

	unsigned seq;



	do {

		seq = read_seqcount_begin(&fs->seq);

		*root = fs->root;

	path_get(root);

		*pwd = fs->pwd;

	path_get(pwd);

	spin_unlock(&fs->lock);

	} 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;

}