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Subject[PATCH 12/17] drbd: fix potential deadlock when trying to detach during handshake
Date
From: Lars Ellenberg <lars.ellenberg@linbit.com>

When requesting a detach, we first suspend IO, and also inhibit meta-data IO
by means of drbd_md_get_buffer(), because we don't want to "fail" the disk
while there is IO in-flight: the transition into D_FAILED for detach purposes
may get misinterpreted as actual IO error in a confused endio function.

We wrap it all into wait_event(), to retry in case the drbd_req_state()
returns SS_IN_TRANSIENT_STATE, as it does for example during an ongoing
connection handshake.

In that example, the receiver thread may need to grab drbd_md_get_buffer()
during the handshake to make progress. To avoid potential deadlock with
detach, detach needs to grab and release the meta data buffer inside of
that wait_event retry loop. To avoid lock inversion between
mutex_lock(&device->state_mutex) and drbd_md_get_buffer(device),
introduce a new enum chg_state_flag CS_INHIBIT_MD_IO, and move the
call to drbd_md_get_buffer() inside the state_mutex grabbed in
drbd_req_state().

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

diff --git a/drivers/block/drbd/drbd_nl.c b/drivers/block/drbd/drbd_nl.c
index c383b6c..6bb58a6 100644
--- a/drivers/block/drbd/drbd_nl.c
+++ b/drivers/block/drbd/drbd_nl.c
@@ -2149,34 +2149,13 @@ int drbd_adm_attach(struct sk_buff *skb, struct genl_info *info)

static int adm_detach(struct drbd_device *device, int force)
{
- enum drbd_state_rv retcode;
- void *buffer;
- int ret;
-
if (force) {
set_bit(FORCE_DETACH, &device->flags);
drbd_force_state(device, NS(disk, D_FAILED));
- retcode = SS_SUCCESS;
- goto out;
+ return SS_SUCCESS;
}

- drbd_suspend_io(device); /* so no-one is stuck in drbd_al_begin_io */
- buffer = drbd_md_get_buffer(device, __func__); /* make sure there is no in-flight meta-data IO */
- if (buffer) {
- retcode = drbd_request_state(device, NS(disk, D_FAILED));
- drbd_md_put_buffer(device);
- } else /* already <= D_FAILED */
- retcode = SS_NOTHING_TO_DO;
- /* D_FAILED will transition to DISKLESS. */
- drbd_resume_io(device);
- ret = wait_event_interruptible(device->misc_wait,
- device->state.disk != D_FAILED);
- if ((int)retcode == (int)SS_IS_DISKLESS)
- retcode = SS_NOTHING_TO_DO;
- if (ret)
- retcode = ERR_INTR;
-out:
- return retcode;
+ return drbd_request_detach_interruptible(device);
}

/* Detaching the disk is a process in multiple stages. First we need to lock
diff --git a/drivers/block/drbd/drbd_state.c b/drivers/block/drbd/drbd_state.c
index 306f116..0813c65 100644
--- a/drivers/block/drbd/drbd_state.c
+++ b/drivers/block/drbd/drbd_state.c
@@ -579,11 +579,14 @@ drbd_req_state(struct drbd_device *device, union drbd_state mask,
unsigned long flags;
union drbd_state os, ns;
enum drbd_state_rv rv;
+ void *buffer = NULL;

init_completion(&done);

if (f & CS_SERIALIZE)
mutex_lock(device->state_mutex);
+ if (f & CS_INHIBIT_MD_IO)
+ buffer = drbd_md_get_buffer(device, __func__);

spin_lock_irqsave(&device->resource->req_lock, flags);
os = drbd_read_state(device);
@@ -636,6 +639,8 @@ drbd_req_state(struct drbd_device *device, union drbd_state mask,
}

abort:
+ if (buffer)
+ drbd_md_put_buffer(device);
if (f & CS_SERIALIZE)
mutex_unlock(device->state_mutex);

@@ -664,6 +669,47 @@ _drbd_request_state(struct drbd_device *device, union drbd_state mask,
return rv;
}

+/*
+ * We grab drbd_md_get_buffer(), because we don't want to "fail" the disk while
+ * there is IO in-flight: the transition into D_FAILED for detach purposes
+ * may get misinterpreted as actual IO error in a confused endio function.
+ *
+ * We wrap it all into wait_event(), to retry in case the drbd_req_state()
+ * returns SS_IN_TRANSIENT_STATE.
+ *
+ * To avoid potential deadlock with e.g. the receiver thread trying to grab
+ * drbd_md_get_buffer() while trying to get out of the "transient state", we
+ * need to grab and release the meta data buffer inside of that wait_event loop.
+ */
+static enum drbd_state_rv
+request_detach(struct drbd_device *device)
+{
+ return drbd_req_state(device, NS(disk, D_FAILED),
+ CS_VERBOSE | CS_ORDERED | CS_INHIBIT_MD_IO);
+}
+
+enum drbd_state_rv
+drbd_request_detach_interruptible(struct drbd_device *device)
+{
+ enum drbd_state_rv rv;
+ int ret;
+
+ drbd_suspend_io(device); /* so no-one is stuck in drbd_al_begin_io */
+ wait_event_interruptible(device->state_wait,
+ (rv = request_detach(device)) != SS_IN_TRANSIENT_STATE);
+ drbd_resume_io(device);
+
+ ret = wait_event_interruptible(device->misc_wait,
+ device->state.disk != D_FAILED);
+
+ if (rv == SS_IS_DISKLESS)
+ rv = SS_NOTHING_TO_DO;
+ if (ret)
+ rv = ERR_INTR;
+
+ return rv;
+}
+
enum drbd_state_rv
_drbd_request_state_holding_state_mutex(struct drbd_device *device, union drbd_state mask,
union drbd_state val, enum chg_state_flags f)
diff --git a/drivers/block/drbd/drbd_state.h b/drivers/block/drbd/drbd_state.h
index 6c9d5d4..0276c98 100644
--- a/drivers/block/drbd/drbd_state.h
+++ b/drivers/block/drbd/drbd_state.h
@@ -71,6 +71,10 @@ enum chg_state_flags {
CS_DC_SUSP = 1 << 10,
CS_DC_MASK = CS_DC_ROLE + CS_DC_PEER + CS_DC_CONN + CS_DC_DISK + CS_DC_PDSK,
CS_IGN_OUTD_FAIL = 1 << 11,
+
+ /* Make sure no meta data IO is in flight, by calling
+ * drbd_md_get_buffer(). Used for graceful detach. */
+ CS_INHIBIT_MD_IO = 1 << 12,
};

/* drbd_dev_state and drbd_state are different types. This is to stress the
@@ -156,6 +160,10 @@ static inline int drbd_request_state(struct drbd_device *device,
return _drbd_request_state(device, mask, val, CS_VERBOSE + CS_ORDERED);
}

+/* for use in adm_detach() (drbd_adm_detach(), drbd_adm_down()) */
+enum drbd_state_rv
+drbd_request_detach_interruptible(struct drbd_device *device);
+
enum drbd_role conn_highest_role(struct drbd_connection *connection);
enum drbd_role conn_highest_peer(struct drbd_connection *connection);
enum drbd_disk_state conn_highest_disk(struct drbd_connection *connection);
--
2.7.4
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