[PATCH 1/5] fs: make syncfs(2) killable on s_umount

[Josef Bacik]

A syncfs(2) which has to wait for s_umount sits in D state and can't be
killed, for as long as whoever has the lock takes.

That can be very long.  Another syncfs(2) holds s_umount shared while it
writes the filesystem out, which on a slow or dead device takes forever.
That alone doesn't keep other readers out, but as soon as somebody wants
the lock exclusively, a remount say, that writer queues up, and every
reader which comes after it queues up behind the writer.

Taking the lock is the first thing syncfs(2) does and it can return an
error, so use down_read_killable().

This only covers the wait for the lock.  Once syncfs(2) has it, it still
waits for the writeback uninterruptibly.

Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
---
 fs/sync.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/sync.c b/fs/sync.c
index 0681bfcb2da6..ea54af59cf4a 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -156,7 +156,9 @@ SYSCALL_DEFINE1(syncfs, int, fd)
 		return -EBADF;
 	sb = fd_file(f)->f_path.dentry->d_sb;
 
-	down_read(&sb->s_umount);
+	ret = down_read_killable(&sb->s_umount);
+	if (ret)
+		return ret;
 	ret = sync_filesystem(sb);
 	up_read(&sb->s_umount);
 

-- 
2.55.0