From: Trond Myklebust <trond.myklebust@primarydata.com>
Date: Wed, 19 Nov 2014 17:47:50 +0000 (-0500)
Subject: nfsd: Fix slot wake up race in the nfsv4.1 callback code
X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=65b3703021a99e5078b0092c88ef904a25179208;p=GitHub%2FLineageOS%2Fandroid_kernel_samsung_universal7580.git

nfsd: Fix slot wake up race in the nfsv4.1 callback code

commit c6c15e1ed303ffc47e696ea1c9a9df1761c1f603 upstream.

The currect code for nfsd41_cb_get_slot() and nfsd4_cb_done() has no
locking in order to guarantee atomicity, and so allows for races of
the form.

Task 1                                  Task 2
======                                  ======
if (test_and_set_bit(0) != 0) {
                                        clear_bit(0)
                                        rpc_wake_up_next(queue)
        rpc_sleep_on(queue)
        return false;
}

This patch breaks the race condition by adding a retest of the bit
after the call to rpc_sleep_on().

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---

diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
index cc8c5b32043..f42bbe5fbc0 100644
--- a/fs/nfsd/nfs4callback.c
+++ b/fs/nfsd/nfs4callback.c
@@ -784,8 +784,12 @@ static bool nfsd41_cb_get_slot(struct nfs4_client *clp, struct rpc_task *task)
 {
 	if (test_and_set_bit(0, &clp->cl_cb_slot_busy) != 0) {
 		rpc_sleep_on(&clp->cl_cb_waitq, task, NULL);
-		dprintk("%s slot is busy\n", __func__);
-		return false;
+		/* Race breaker */
+		if (test_and_set_bit(0, &clp->cl_cb_slot_busy) != 0) {
+			dprintk("%s slot is busy\n", __func__);
+			return false;
+		}
+		rpc_wake_up_queued_task(&clp->cl_cb_waitq, task);
 	}
 	return true;
 }