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From:   Lars Ellenberg <lars.ellenberg@linbit.com>
To:     Jens Axboe <axboe@kernel.dk>, linux-kernel@vger.kernel.org,
        linux-block@vger.kernel.org
Cc:     drbd-dev@lists.linbit.com
Subject: [PATCH 01/17] drbd: narrow rcu_read_lock in drbd_sync_handshake
Date:   Thu, 20 Dec 2018 17:23:28 +0100
Message-Id: <20181220162344.8430-2-lars.ellenberg@linbit.com>
In-Reply-To: <20181220162344.8430-1-lars.ellenberg@linbit.com>
References: <20181220162344.8430-1-lars.ellenberg@linbit.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From: Roland Kammerer <roland.kammerer@linbit.com>

So far there was the possibility that we called
genlmsg_new(GFP_NOIO)/mutex_lock() while holding an rcu_read_lock().

This included cases like:

drbd_sync_handshake (acquire the RCU lock)
  drbd_asb_recover_1p
    drbd_khelper
      drbd_bcast_event
        genlmsg_new(GFP_NOIO) --> may sleep

drbd_sync_handshake (acquire the RCU lock)
  drbd_asb_recover_1p
    drbd_khelper
      notify_helper
        genlmsg_new(GFP_NOIO) --> may sleep

drbd_sync_handshake (acquire the RCU lock)
  drbd_asb_recover_1p
    drbd_khelper
      notify_helper
        mutex_lock --> may sleep

While using GFP_ATOMIC whould have been possible in the first two cases,
the real fix is to narrow the rcu_read_lock.

Reported-by: Jia-Ju Bai <baijiaju1990@163.com>
Reviewed-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Roland Kammerer <roland.kammerer@linbit.com>
---
 drivers/block/drbd/drbd_receiver.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/drivers/block/drbd/drbd_receiver.c b/drivers/block/drbd/drbd_receiver.c
index 61c392752fe4..1b9822f264d2 100644
--- a/drivers/block/drbd/drbd_receiver.c
+++ b/drivers/block/drbd/drbd_receiver.c
@@ -3364,7 +3364,7 @@ static enum drbd_conns drbd_sync_handshake(struct drbd_peer_device *peer_device,
 	enum drbd_conns rv = C_MASK;
 	enum drbd_disk_state mydisk;
 	struct net_conf *nc;
-	int hg, rule_nr, rr_conflict, tentative;
+	int hg, rule_nr, rr_conflict, tentative, always_asbp;
 
 	mydisk = device->state.disk;
 	if (mydisk == D_NEGOTIATING)
@@ -3415,8 +3415,12 @@ static enum drbd_conns drbd_sync_handshake(struct drbd_peer_device *peer_device,
 
 	rcu_read_lock();
 	nc = rcu_dereference(peer_device->connection->net_conf);
+	always_asbp = nc->always_asbp;
+	rr_conflict = nc->rr_conflict;
+	tentative = nc->tentative;
+	rcu_read_unlock();
 
-	if (hg == 100 || (hg == -100 && nc->always_asbp)) {
+	if (hg == 100 || (hg == -100 && always_asbp)) {
 		int pcount = (device->state.role == R_PRIMARY)
 			   + (peer_role == R_PRIMARY);
 		int forced = (hg == -100);
@@ -3455,9 +3459,6 @@ static enum drbd_conns drbd_sync_handshake(struct drbd_peer_device *peer_device,
 			     "Sync from %s node\n",
 			     (hg < 0) ? "peer" : "this");
 	}
-	rr_conflict = nc->rr_conflict;
-	tentative = nc->tentative;
-	rcu_read_unlock();
 
 	if (hg == -100) {
 		/* FIXME this log message is not correct if we end up here
-- 
2.17.1