[PATCH 6/6] fuse: fix deadlock between atomic O_TRUNC and page invalidation

[Yang Bo <yyyeer.bo@gmail.com>]

From: Miklos Szeredi <mszeredi@redhat.com>

commit 2fdbb8dd01556e1501132b5ad3826e8f71e24a8b upstream.

[backport for 5.10.y]

fuse_finish_open() will be called with FUSE_NOWRITE set in case of atomic
O_TRUNC open(), so commit 76224355db75 ("fuse: truncate pagecache on
atomic_o_trunc") replaced invalidate_inode_pages2() by truncate_pagecache()
in such a case to avoid the A-A deadlock. However, we found another A-B-B-A
deadlock related to the case above, which will cause the xfstests
generic/464 testcase hung in our virtio-fs test environment.

For example, consider two processes concurrently open one same file, one
with O_TRUNC and another without O_TRUNC. The deadlock case is described
below, if open(O_TRUNC) is already set_nowrite(acquired A), and is trying
to lock a page (acquiring B), open() could have held the page lock
(acquired B), and waiting on the page writeback (acquiring A). This would
lead to deadlocks.

open(O_TRUNC)
----------------------------------------------------------------
fuse_open_common
  inode_lock            [C acquire]
  fuse_set_nowrite      [A acquire]

  fuse_finish_open
    truncate_pagecache
      lock_page         [B acquire]
      truncate_inode_page
      unlock_page       [B release]

  fuse_release_nowrite  [A release]
  inode_unlock          [C release]
----------------------------------------------------------------

open()
----------------------------------------------------------------
fuse_open_common
  fuse_finish_open
    invalidate_inode_pages2
      lock_page         [B acquire]
        fuse_launder_page
          fuse_wait_on_page_writeback [A acquire & release]
      unlock_page       [B release]
----------------------------------------------------------------

Besides this case, all calls of invalidate_inode_pages2() and
invalidate_inode_pages2_range() in fuse code also can deadlock with
open(O_TRUNC).

Fix by moving the truncate_pagecache() call outside the nowrite protected
region.  The nowrite protection is only for delayed writeback
(writeback_cache) case, where inode lock does not protect against
truncation racing with writes on the server.  Write syscalls racing with
page cache truncation still get the inode lock protection.

This patch also changes the order of filemap_invalidate_lock()
vs. fuse_set_nowrite() in fuse_open_common().  This new order matches the
order found in fuse_file_fallocate() and fuse_do_setattr().

Reported-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
Tested-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
Fixes: e4648309b85a ("fuse: truncate pending writes on O_TRUNC")
Cc: <stable@vger.kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Yang Bo <yb203166@antfin.com>
---
 fs/fuse/dir.c  |  5 +++++
 fs/fuse/file.c | 29 +++++++++++++++++------------
 2 files changed, 22 insertions(+), 12 deletions(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index bdb04bea0da9..e3b9b7d188e6 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -537,6 +537,7 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry,
 	struct fuse_entry_out outentry;
 	struct fuse_inode *fi;
 	struct fuse_file *ff;
+	bool trunc = flags & O_TRUNC;
 
 	/* Userspace expects S_IFREG in create mode */
 	BUG_ON((mode & S_IFMT) != S_IFREG);
@@ -604,6 +605,10 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry,
 	} else {
 		file->private_data = ff;
 		fuse_finish_open(inode, file);
+		if (fm->fc->atomic_o_trunc && trunc)
+			truncate_pagecache(inode, 0);
+		else if (!(ff->open_flags & FOPEN_KEEP_CACHE))
+			invalidate_inode_pages2(inode->i_mapping);
 	}
 	return err;
 
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 94fe2c690676..13d97547eaf6 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -206,14 +206,10 @@ void fuse_finish_open(struct inode *inode, struct file *file)
 		fi->attr_version = atomic64_inc_return(&fc->attr_version);
 		i_size_write(inode, 0);
 		spin_unlock(&fi->lock);
-		truncate_pagecache(inode, 0);
 		fuse_invalidate_attr(inode);
 		if (fc->writeback_cache)
 			file_update_time(file);
-	} else if (!(ff->open_flags & FOPEN_KEEP_CACHE)) {
-		invalidate_inode_pages2(inode->i_mapping);
 	}
-
 	if ((file->f_mode & FMODE_WRITE) && fc->writeback_cache)
 		fuse_link_write_file(file);
 }
@@ -236,30 +232,39 @@ int fuse_open_common(struct inode *inode, struct file *file, bool isdir)
 	if (err)
 		return err;
 
-	if (is_wb_truncate || dax_truncate) {
+	if (is_wb_truncate || dax_truncate)
 		inode_lock(inode);
-		fuse_set_nowrite(inode);
-	}
 
 	if (dax_truncate) {
 		down_write(&get_fuse_inode(inode)->i_mmap_sem);
 		err = fuse_dax_break_layouts(inode, 0, 0);
 		if (err)
-			goto out;
+			goto out_inode_unlock;
 	}
 
+	if (is_wb_truncate || dax_truncate)
+		fuse_set_nowrite(inode);
+
 	err = fuse_do_open(fm, get_node_id(inode), file, isdir);
 	if (!err)
 		fuse_finish_open(inode, file);
 
-out:
+	if (is_wb_truncate || dax_truncate)
+		fuse_release_nowrite(inode);
+	if (!err) {
+		struct fuse_file *ff = file->private_data;
+
+		if (fc->atomic_o_trunc && (file->f_flags & O_TRUNC))
+			truncate_pagecache(inode, 0);
+		else if (!(ff->open_flags & FOPEN_KEEP_CACHE))
+			invalidate_inode_pages2(inode->i_mapping);
+	}
 	if (dax_truncate)
 		up_write(&get_fuse_inode(inode)->i_mmap_sem);
 
-	if (is_wb_truncate | dax_truncate) {
-		fuse_release_nowrite(inode);
+out_inode_unlock:
+	if (is_wb_truncate || dax_truncate)
 		inode_unlock(inode);
-	}
 
 	return err;
 }
-- 
2.40.0