[PATCH v1 08/13] ceph: make ceph_start_io_write() killable

[Ionut Nechita (Wind River)]

From: Ionut Nechita <ionut.nechita@xxxxxxxxxxxxx>

When multiple processes write to the same file and one of them is
blocked waiting for MDS/OSD response (e.g., during MDS failover),
other processes block indefinitely on down_write(&inode->i_rwsem)
in ceph_start_io_write().

This causes hung task warnings:

  INFO: task dd:12345 blocked for more than 122 seconds.
  Call Trace:
    ceph_start_io_write+0x...
    ceph_write_iter+0x...

The i_rwsem is held by a process doing fsync/writeback that is
waiting for MDS or OSD response. Other writers queue up on the
rwsem and block indefinitely.

Fix this by using down_write_killable() instead of down_write().
This allows blocked processes to be killed with SIGKILL, preventing
indefinite hangs. The function now returns an error code that
callers must check.

Update ceph_write_iter() to handle the new error return from
ceph_start_io_write().

Signed-off-by: Ionut Nechita <ionut.nechita@xxxxxxxxxxxxx>
---
 fs/ceph/file.c | 9 +++++++--
 fs/ceph/io.c   | 9 +++++++--
 fs/ceph/io.h   | 2 +-
 3 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/fs/ceph/file.c b/fs/ceph/file.c
index 6587c2d5af1e0..01e4f31b1f2f3 100644
--- a/fs/ceph/file.c
+++ b/fs/ceph/file.c
@@ -2359,8 +2359,13 @@ static ssize_t ceph_write_iter(struct kiocb *iocb, struct iov_iter *from)
 retry_snap:
 	if (direct_lock)
 		ceph_start_io_direct(inode);
-	else
-		ceph_start_io_write(inode);
+	else {
+		err = ceph_start_io_write(inode);
+		if (err) {
+			ceph_free_cap_flush(prealloc_cf);
+			return err;
+		}
+	}
 
 	if (iocb->ki_flags & IOCB_APPEND) {
 		err = ceph_do_getattr(inode, CEPH_STAT_CAP_SIZE, false);
diff --git a/fs/ceph/io.c b/fs/ceph/io.c
index c456509b31c3f..f9ac89ec1d6a1 100644
--- a/fs/ceph/io.c
+++ b/fs/ceph/io.c
@@ -83,11 +83,16 @@ ceph_end_io_read(struct inode *inode)
  * Declare that a buffered write operation is about to start, and ensure
  * that we block all direct I/O.
  */
-void
+int
 ceph_start_io_write(struct inode *inode)
 {
-	down_write(&inode->i_rwsem);
+	int ret;
+
+	ret = down_write_killable(&inode->i_rwsem);
+	if (ret)
+		return ret;
 	ceph_block_o_direct(ceph_inode(inode), inode);
+	return 0;
 }
 
 /**
diff --git a/fs/ceph/io.h b/fs/ceph/io.h
index fa594cd77348a..94ce176df9997 100644
--- a/fs/ceph/io.h
+++ b/fs/ceph/io.h
@@ -4,7 +4,7 @@
 
 void ceph_start_io_read(struct inode *inode);
 void ceph_end_io_read(struct inode *inode);
-void ceph_start_io_write(struct inode *inode);
+int ceph_start_io_write(struct inode *inode);
 void ceph_end_io_write(struct inode *inode);
 void ceph_start_io_direct(struct inode *inode);
 void ceph_end_io_direct(struct inode *inode);
-- 
2.53.0