libfs: Re-arrange locking in offset_iterate_dir()

Liam and Matthew say that once the RCU read lock is released,
xa_state is not safe to re-use for the next xas_find() call. But the
RCU read lock must be released on each loop iteration so that
dput(), which might_sleep(), can be called safely.

Thus we are forced to walk the offset tree with fresh state for each
directory entry. xa_find() can do this for us, though it might be a
little less efficient than maintaining xa_state locally.

We believe that in the current code base, inode->i_rwsem provides
protection for the xa_state maintained in
offset_iterate_dir(). However, there is no guarantee that will
continue to be the case in the future.

Since offset_iterate_dir() doesn't build xa_state locally any more,
there's no longer a strong need for offset_find_next(). Clean up by
rolling these two helpers together.

Suggested-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Message-ID: <170785993027.11135.8830043889278631735.stgit@91.116.238.104.host.secureserver.net>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://lore.kernel.org/r/170820142021.6328.15047865406275957018.stgit@91.116.238.104.host.secureserver.net
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/libfs.c b/fs/libfs.c
index eec6031..752e24c 100644 (file)
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -402,12 +402,13 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
        return vfs_setpos(file, offset, U32_MAX);
 }
 
-static struct dentry *offset_find_next(struct xa_state *xas)
+static struct dentry *offset_find_next(struct offset_ctx *octx, loff_t offset)
 {
        struct dentry *child, *found = NULL;
+       XA_STATE(xas, &octx->xa, offset);
 
        rcu_read_lock();
-       child = xas_next_entry(xas, U32_MAX);
+       child = xas_next_entry(&xas, U32_MAX);
        if (!child)
                goto out;
        spin_lock(&child->d_lock);
@@ -430,12 +431,11 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
 
 static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
 {
-       struct offset_ctx *so_ctx = inode->i_op->get_offset_ctx(inode);
-       XA_STATE(xas, &so_ctx->xa, ctx->pos);
+       struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
        struct dentry *dentry;
 
        while (true) {
-               dentry = offset_find_next(&xas);
+               dentry = offset_find_next(octx, ctx->pos);
                if (!dentry)
                        return ERR_PTR(-ENOENT);
 
@@ -444,8 +444,8 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
                        break;
                }
 
+               ctx->pos = dentry2offset(dentry) + 1;
                dput(dentry);
-               ctx->pos = xas.xa_index + 1;
        }
        return NULL;
 }