drbd: fix potential deadlock on detach

If we have contention in drbd_al_begin_iod (heavy randon IO),
an administrative request to detach the disk may deadlock
for similar reasons as the recently fixed deadlock if detaching
because of IO-error.

The approach taken here is to either go through the intermediate
cleanup state D_FAILED, or first lock out application io,
don't just go directly to D_DISKLESS.

We need an additional state bit (WAS_IO_ERROR) to distinguish
the -> D_FAILED because of IO-error from other failures.

Sanitize D_ATTACHING -> D_FAILED to D_ATTACHING -> D_DISKLESS.
If only attaching, ldev may be missing still, but would be referenced
from within the after_state_ch for -> D_FAILED, potentially
dereferencing a NULL pointer.

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

4 files changed, 113 insertions, 63 deletions

diff --git a/drivers/block/drbd/drbd_int.h b/drivers/block/drbd/drbd_int.h
index e0e0bf6f16a1..03c15e317c37 100644
--- a/drivers/block/drbd/drbd_int.h
+++ b/drivers/block/drbd/drbd_int.h
@@ -852,7 +852,8 @@ enum {
         BITMAP_IO,              /* suspend application io;
                                    once no more io in flight, start bitmap io */
         BITMAP_IO_QUEUED,       /* Started bitmap IO */
-        GO_DISKLESS,            /* Disk failed, local_cnt reached zero, we are going diskless */
+        GO_DISKLESS,            /* Disk is being detached, on io-error or admin request. */
+        WAS_IO_ERROR,           /* Local disk failed returned IO error */
         RESYNC_AFTER_NEG,       /* Resync after online grow after the attach&negotiate finished. */
         NET_CONGESTED,          /* The data socket is congested */
 
@@ -1281,6 +1282,7 @@ extern int drbd_bmio_set_n_write(struct drbd_conf *mdev);
 extern int drbd_bmio_clear_n_write(struct drbd_conf *mdev);
 extern int drbd_bitmap_io(struct drbd_conf *mdev, int (*io_fn)(struct drbd_conf *), char *why);
 extern void drbd_go_diskless(struct drbd_conf *mdev);
+extern void drbd_ldev_destroy(struct drbd_conf *mdev);
 
 
 /* Meta data layout
@@ -1798,17 +1800,17 @@ static inline void __drbd_chk_io_error_(struct drbd_conf *mdev, int forcedetach,
         case EP_PASS_ON:
                 if (!forcedetach) {
                         if (__ratelimit(&drbd_ratelimit_state))
-                                dev_err(DEV, "Local IO failed in %s."
-                                             "Passing error on...\n", where);
+                                dev_err(DEV, "Local IO failed in %s.\n", where);
                         break;
                 }
                 /* NOTE fall through to detach case if forcedetach set */
         case EP_DETACH:
         case EP_CALL_HELPER:
+                set_bit(WAS_IO_ERROR, &mdev->flags);
                 if (mdev->state.disk > D_FAILED) {
                         _drbd_set_state(_NS(mdev, disk, D_FAILED), CS_HARD, NULL);
-                        dev_err(DEV, "Local IO failed in %s."
-                                     "Detaching...\n", where);
+                        dev_err(DEV,
+                                "Local IO failed in %s. Detaching...\n", where);
                 }
                 break;
         }
@@ -2127,7 +2129,11 @@ static inline void put_ldev(struct drbd_conf *mdev)
         __release(local);
         D_ASSERT(i >= 0);
         if (i == 0) {
+                if (mdev->state.disk == D_DISKLESS)
+                        /* even internal references gone, safe to destroy */
+                        drbd_ldev_destroy(mdev);
                 if (mdev->state.disk == D_FAILED)
+                        /* all application IO references gone. */
                         drbd_go_diskless(mdev);
                 wake_up(&mdev->misc_wait);
         }
@@ -2138,6 +2144,10 @@ static inline int _get_ldev_if_state(struct drbd_conf *mdev, enum drbd_disk_stat
 {
         int io_allowed;
 
+        /* never get a reference while D_DISKLESS */
+        if (mdev->state.disk == D_DISKLESS)
+                return 0;
+
         atomic_inc(&mdev->local_cnt);
         io_allowed = (mdev->state.disk >= mins);
         if (!io_allowed)
diff --git a/drivers/block/drbd/drbd_main.c b/drivers/block/drbd/drbd_main.c
index 8d029b14e7cc..992c3aecdf7e 100644
--- a/drivers/block/drbd/drbd_main.c
+++ b/drivers/block/drbd/drbd_main.c
@@ -834,6 +834,15 @@ static union drbd_state sanitize_state(struct drbd_conf *mdev, union drbd_state
             ns.conn != C_UNCONNECTED && ns.conn != C_DISCONNECTING && ns.conn <= C_TEAR_DOWN)
                 ns.conn = os.conn;
 
+        /* we cannot fail (again) if we already detached */
+        if (ns.disk == D_FAILED && os.disk == D_DISKLESS)
+                ns.disk = D_DISKLESS;
+
+        /* if we are only D_ATTACHING yet,
+         * we can (and should) go directly to D_DISKLESS. */
+        if (ns.disk == D_FAILED && os.disk == D_ATTACHING)
+                ns.disk = D_DISKLESS;
+
         /* After C_DISCONNECTING only C_STANDALONE may follow */
         if (os.conn == C_DISCONNECTING && ns.conn != C_STANDALONE)
                 ns.conn = os.conn;
@@ -1055,7 +1064,15 @@ int __drbd_set_state(struct drbd_conf *mdev,
             !test_and_set_bit(CONFIG_PENDING, &mdev->flags))
                 set_bit(DEVICE_DYING, &mdev->flags);
 
-        mdev->state.i = ns.i;
+        /* if we are going -> D_FAILED or D_DISKLESS, grab one extra reference
+         * on the ldev here, to be sure the transition -> D_DISKLESS resp.
+         * drbd_ldev_destroy() won't happen before our corresponding
+         * after_state_ch works run, where we put_ldev again. */
+        if ((os.disk != D_FAILED && ns.disk == D_FAILED) ||
+            (os.disk != D_DISKLESS && ns.disk == D_DISKLESS))
+                atomic_inc(&mdev->local_cnt);
+
+        mdev->state = ns;
         wake_up(&mdev->misc_wait);
         wake_up(&mdev->state_wait);
 
@@ -1363,63 +1380,64 @@ static void after_state_ch(struct drbd_conf *mdev, union drbd_state os,
             os.disk > D_INCONSISTENT && ns.disk == D_INCONSISTENT)
                 drbd_queue_bitmap_io(mdev, &drbd_bmio_set_n_write, NULL, "set_n_write from invalidate");
 
-        /* first half of local IO error */
-        if (os.disk > D_FAILED && ns.disk == D_FAILED) {
-                enum drbd_io_error_p eh = EP_PASS_ON;
+        /* first half of local IO error, failure to attach,
+         * or administrative detach */
+        if (os.disk != D_FAILED && ns.disk == D_FAILED) {
+                enum drbd_io_error_p eh;
+                int was_io_error;
+                /* corresponding get_ldev was in __drbd_set_state, to serialize
+                 * our cleanup here with the transition to D_DISKLESS,
+                 * so it is safe to dreference ldev here. */
+                eh = mdev->ldev->dc.on_io_error;
+                was_io_error = test_and_clear_bit(WAS_IO_ERROR, &mdev->flags);
+
+                /* current state still has to be D_FAILED,
+                 * there is only one way out: to D_DISKLESS,
+                 * and that may only happen after our put_ldev below. */
+                if (mdev->state.disk != D_FAILED)
+                        dev_err(DEV,
+                                "ASSERT FAILED: disk is %s during detach\n",
+                                drbd_disk_str(mdev->state.disk));
 
                 if (drbd_send_state(mdev))
-                        dev_warn(DEV, "Notified peer that my disk is broken.\n");
+                        dev_warn(DEV, "Notified peer that I am detaching my disk\n");
                 else
-                        dev_err(DEV, "Sending state for drbd_io_error() failed\n");
+                        dev_err(DEV, "Sending state for detaching disk failed\n");
 
                 drbd_rs_cancel_all(mdev);
 
-                if (get_ldev_if_state(mdev, D_FAILED)) {
-                        eh = mdev->ldev->dc.on_io_error;
-                        put_ldev(mdev);
-                }
-                if (eh == EP_CALL_HELPER)
+                /* In case we want to get something to stable storage still,
+                 * this may be the last chance.
+                 * Following put_ldev may transition to D_DISKLESS. */
+                drbd_md_sync(mdev);
+                put_ldev(mdev);
+
+                if (was_io_error && eh == EP_CALL_HELPER)
                         drbd_khelper(mdev, "local-io-error");
         }
 
+        /* second half of local IO error, failure to attach,
+         * or administrative detach,
+         * after local_cnt references have reached zero again */
+        if (os.disk != D_DISKLESS && ns.disk == D_DISKLESS) {
+                /* We must still be diskless,
+                 * re-attach has to be serialized with this! */
+                if (mdev->state.disk != D_DISKLESS)
+                        dev_err(DEV,
+                                "ASSERT FAILED: disk is %s while going diskless\n",
+                                drbd_disk_str(mdev->state.disk));
 
-        /* second half of local IO error handling,
-         * after local_cnt references have reached zero: */
-        if (os.disk == D_FAILED && ns.disk == D_DISKLESS) {
-                mdev->rs_total = 0;
-                mdev->rs_failed = 0;
-                atomic_set(&mdev->rs_pending_cnt, 0);
-        }
-
-        if (os.disk > D_DISKLESS && ns.disk == D_DISKLESS) {
-                /* We must still be diskless,
-                 * re-attach has to be serialized with this! */
-                if (mdev->state.disk != D_DISKLESS)
-                        dev_err(DEV,
-                                "ASSERT FAILED: disk is %s while going diskless\n",
-                                drbd_disk_str(mdev->state.disk));
+                mdev->rs_total = 0;
+                mdev->rs_failed = 0;
+                atomic_set(&mdev->rs_pending_cnt, 0);
 
-                /* we cannot assert local_cnt == 0 here, as get_ldev_if_state
-                 * will inc/dec it frequently. Since we became D_DISKLESS, no
-                 * one has touched the protected members anymore, though, so we
-                 * are safe to free them here. */
                 if (drbd_send_state(mdev))
-                        dev_warn(DEV, "Notified peer that I detached my disk.\n");
+                        dev_warn(DEV, "Notified peer that I'm now diskless.\n");
                 else
-                        dev_err(DEV, "Sending state for detach failed\n");
-
-                lc_destroy(mdev->resync);
-                mdev->resync = NULL;
-                lc_destroy(mdev->act_log);
-                mdev->act_log = NULL;
-                __no_warn(local,
-                        drbd_free_bc(mdev->ldev);
-                        mdev->ldev = NULL;);
-
-                if (mdev->md_io_tmpp) {
-                        __free_page(mdev->md_io_tmpp);
-                        mdev->md_io_tmpp = NULL;
-                }
+                        dev_err(DEV, "Sending state for being diskless failed\n");
+                /* corresponding get_ldev in __drbd_set_state
+                 * this may finaly trigger drbd_ldev_destroy. */
+                put_ldev(mdev);
         }
 
         /* Disks got bigger while they were detached */
@@ -2897,7 +2915,6 @@ void drbd_mdev_cleanup(struct drbd_conf *mdev)
         D_ASSERT(list_empty(&mdev->resync_work.list));
         D_ASSERT(list_empty(&mdev->unplug_work.list));
         D_ASSERT(list_empty(&mdev->go_diskless.list));
-
 }
 
 
@@ -3756,19 +3773,31 @@ static int w_bitmap_io(struct drbd_conf *mdev, struct drbd_work *w, int unused)
         return 1;
 }
 
+void drbd_ldev_destroy(struct drbd_conf *mdev)
+{
+        lc_destroy(mdev->resync);
+        mdev->resync = NULL;
+        lc_destroy(mdev->act_log);
+        mdev->act_log = NULL;
+        __no_warn(local,
+                drbd_free_bc(mdev->ldev);
+                mdev->ldev = NULL;);
+
+        if (mdev->md_io_tmpp) {
+                __free_page(mdev->md_io_tmpp);
+                mdev->md_io_tmpp = NULL;
+        }
+        clear_bit(GO_DISKLESS, &mdev->flags);
+}
+
 static int w_go_diskless(struct drbd_conf *mdev, struct drbd_work *w, int unused)
 {
         D_ASSERT(mdev->state.disk == D_FAILED);
         /* we cannot assert local_cnt == 0 here, as get_ldev_if_state will
          * inc/dec it frequently. Once we are D_DISKLESS, no one will touch
-         * the protected members anymore, though, so in the after_state_ch work
-         * it will be safe to free them. */
+         * the protected members anymore, though, so once put_ldev reaches zero
+         * again, it will be safe to free them. */
         drbd_force_state(mdev, NS(disk, D_DISKLESS));
-        /* We need to wait for return of references checked out while we still
-         * have been D_FAILED, though (drbd_md_sync, bitmap io). */
-        wait_event(mdev->misc_wait, !atomic_read(&mdev->local_cnt));
-
-        clear_bit(GO_DISKLESS, &mdev->flags);
         return 1;
 }
 
@@ -3777,9 +3806,6 @@ void drbd_go_diskless(struct drbd_conf *mdev)
         D_ASSERT(mdev->state.disk == D_FAILED);
         if (!test_and_set_bit(GO_DISKLESS, &mdev->flags))
                 drbd_queue_work(&mdev->data.work, &mdev->go_diskless);
-                /* don't drbd_queue_work_front,
-                 * we need to serialize with the after_state_ch work
-                 * of the -> D_FAILED transition. */
 }
 
 /**
diff --git a/drivers/block/drbd/drbd_nl.c b/drivers/block/drbd/drbd_nl.c
index c498c4827de4..0cba7d3d2b5d 100644
--- a/drivers/block/drbd/drbd_nl.c
+++ b/drivers/block/drbd/drbd_nl.c
@@ -870,6 +870,11 @@ static int drbd_nl_disk_conf(struct drbd_conf *mdev, struct drbd_nl_cfg_req *nlp
                 retcode = ERR_DISK_CONFIGURED;
                 goto fail;
         }
+        /* It may just now have detached because of IO error.  Make sure
+         * drbd_ldev_destroy is done already, we may end up here very fast,
+         * e.g. if someone calls attach from the on-io-error handler,
+         * to realize a "hot spare" feature (not that I'd recommend that) */
+        wait_event(mdev->misc_wait, !atomic_read(&mdev->local_cnt));
 
         /* allocation not in the IO path, cqueue thread context */
         nbc = kzalloc(sizeof(struct drbd_backing_dev), GFP_KERNEL);
@@ -1262,7 +1267,7 @@ static int drbd_nl_disk_conf(struct drbd_conf *mdev, struct drbd_nl_cfg_req *nlp
  force_diskless_dec:
         put_ldev(mdev);
  force_diskless:
-        drbd_force_state(mdev, NS(disk, D_DISKLESS));
+        drbd_force_state(mdev, NS(disk, D_FAILED));
         drbd_md_sync(mdev);
  release_bdev2_fail:
         if (nbc)
@@ -1285,10 +1290,19 @@ static int drbd_nl_disk_conf(struct drbd_conf *mdev, struct drbd_nl_cfg_req *nlp
         return 0;
 }
 
+/* Detaching the disk is a process in multiple stages.  First we need to lock
+ * out application IO, in-flight IO, IO stuck in drbd_al_begin_io.
+ * Then we transition to D_DISKLESS, and wait for put_ldev() to return all
+ * internal references as well.
+ * Only then we have finally detached. */
 static int drbd_nl_detach(struct drbd_conf *mdev, struct drbd_nl_cfg_req *nlp,
                           struct drbd_nl_cfg_reply *reply)
 {
+        drbd_suspend_io(mdev); /* so no-one is stuck in drbd_al_begin_io */
         reply->ret_code = drbd_request_state(mdev, NS(disk, D_DISKLESS));
+        if (mdev->state.disk == D_DISKLESS)
+                wait_event(mdev->misc_wait, !atomic_read(&mdev->local_cnt));
+        drbd_resume_io(mdev);
         return 0;
 }
 
diff --git a/drivers/block/drbd/drbd_receiver.c b/drivers/block/drbd/drbd_receiver.c
index 6ec922c623a1..04a823b01da5 100644
--- a/drivers/block/drbd/drbd_receiver.c
+++ b/drivers/block/drbd/drbd_receiver.c
@@ -3363,7 +3363,7 @@ static int receive_state(struct drbd_conf *mdev, enum drbd_packets cmd, unsigned
                 if (ns.conn == C_MASK) {
                         ns.conn = C_CONNECTED;
                         if (mdev->state.disk == D_NEGOTIATING) {
-                                drbd_force_state(mdev, NS(disk, D_DISKLESS));
+                                drbd_force_state(mdev, NS(disk, D_FAILED));
                         } else if (peer_state.disk == D_NEGOTIATING) {
                                 dev_err(DEV, "Disk attach process on the peer node was aborted.\n");
                                 peer_state.disk = D_DISKLESS;