pNFS: Fix a deadlock between read resends and layoutreturn

We must not call nfs_pageio_init_read() on a new nfs_pageio_descriptor
while holding a reference to a layout segment, as that can deadlock
pnfs_update_layout().

Fixes: d67ae825a5 ("pnfs/flexfiles: Add the FlexFile Layout Driver")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: stable@vger.kernel.org # v4.0+

fs/nfs/pnfs.c

@@ -2286,6 +2286,10 @@ void pnfs_read_resend_pnfs(struct nfs_pgio_header *hdr)
 	struct nfs_pageio_descriptor pgio;
 
 	if (!test_and_set_bit(NFS_IOHDR_REDO, &hdr->flags)) {
+		/* Prevent deadlocks with layoutreturn! */
+		pnfs_put_lseg(hdr->lseg);
+		hdr->lseg = NULL;
+
 		nfs_pageio_init_read(&pgio, hdr->inode, false,
 					hdr->completion_ops);
 		hdr->task.tk_status = nfs_pageio_resend(&pgio, hdr);