Commit 3f6d8106 authored by Chuck Lever's avatar Chuck Lever Committed by Christian Brauner
Browse files

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: default avatarLiam R. Howlett <Liam.Howlett@Oracle.com>
Message-ID: <170785993027.11135.8830043889278631735.stgit@91.116.238.104.host.secureserver.net>
Signed-off-by: default avatarChuck Lever <chuck.lever@oracle.com>
Link: https://lore.kernel.org/r/170820142021.6328.15047865406275957018.stgit@91.116.238.104.host.secureserver.net


Reviewed-by: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent 6613476e

fs/libfs.c

+6 −6
Original line number Diff line number Diff line
@@ -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;

}