UBI: simplify PEB protection code

UBI has 2 RB-trees to implement PEB protection, which is too
much for simply prevent PEB from being moved for some time.
This patch implements this using lists. The benefits:

1. No need to allocate protection entry on each PEB get.
2. No need to maintain balanced trees and walk them.

Signed-off-by: Xiaochuan-Xu <xiaochuan-xu@cqu.edu.cn>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

2 files changed, 162 insertions, 241 deletions

diff --git a/drivers/mtd/ubi/ubi.h b/drivers/mtd/ubi/ubi.h
index 46a4763..4a8ec48 100644
--- a/drivers/mtd/ubi/ubi.h
+++ b/drivers/mtd/ubi/ubi.h
@@ -74,6 +74,13 @@
 #define UBI_IO_RETRIES 3
 
 /*
+ * Length of the protection queue. The length is effectively equivalent to the
+ * number of (global) erase cycles PEBs are protected from the wear-leveling
+ * worker.
+ */
+#define UBI_PROT_QUEUE_LEN 10
+
+/*
  * Error codes returned by the I/O sub-system.
  *
  * UBI_IO_PEB_EMPTY: the physical eraseblock is empty, i.e. it contains only
@@ -96,6 +103,7 @@ enum {
 /**
  * struct ubi_wl_entry - wear-leveling entry.
  * @u.rb: link in the corresponding (free/used) RB-tree
+ * @u.list: link in the protection queue
  * @ec: erase counter
  * @pnum: physical eraseblock number
  *
@@ -106,6 +114,7 @@ enum {
 struct ubi_wl_entry {
 	union {
 		struct rb_node rb;
+		struct list_head list;
 	} u;
 	int ec;
 	int pnum;
@@ -290,7 +299,7 @@ struct ubi_wl_entry;
  * @beb_rsvd_level: normal level of PEBs reserved for bad PEB handling
  *
  * @autoresize_vol_id: ID of the volume which has to be auto-resized at the end
- *                     of UBI ititializetion
+ *                     of UBI initialization
  * @vtbl_slots: how many slots are available in the volume table
  * @vtbl_size: size of the volume table in bytes
  * @vtbl: in-RAM volume table copy
@@ -308,18 +317,17 @@ struct ubi_wl_entry;
  * @used: RB-tree of used physical eraseblocks
  * @free: RB-tree of free physical eraseblocks
  * @scrub: RB-tree of physical eraseblocks which need scrubbing
- * @prot: protection trees
- * @prot.pnum: protection tree indexed by physical eraseblock numbers
- * @prot.aec: protection tree indexed by absolute erase counter value
- * @wl_lock: protects the @used, @free, @prot, @lookuptbl, @abs_ec, @move_from,
- *           @move_to, @move_to_put @erase_pending, @wl_scheduled, and @works
- *           fields
+ * @pq: protection queue (contain physical eraseblocks which are temporarily
+ *      protected from the wear-leveling worker)
+ * @pq_head: protection queue head
+ * @wl_lock: protects the @used, @free, @pq, @pq_head, @lookuptbl, @move_from,
+ * 	     @move_to, @move_to_put @erase_pending, @wl_scheduled and @works
+ * 	     fields
  * @move_mutex: serializes eraseblock moves
- * @work_sem: sycnhronizes the WL worker with use tasks
+ * @work_sem: synchronizes the WL worker with use tasks
  * @wl_scheduled: non-zero if the wear-leveling was scheduled
  * @lookuptbl: a table to quickly find a &struct ubi_wl_entry object for any
  *             physical eraseblock
- * @abs_ec: absolute erase counter
  * @move_from: physical eraseblock from where the data is being moved
  * @move_to: physical eraseblock where the data is being moved to
  * @move_to_put: if the "to" PEB was put
@@ -353,11 +361,11 @@ struct ubi_wl_entry;
  *
  * @peb_buf1: a buffer of PEB size used for different purposes
  * @peb_buf2: another buffer of PEB size used for different purposes
- * @buf_mutex: proptects @peb_buf1 and @peb_buf2
+ * @buf_mutex: protects @peb_buf1 and @peb_buf2
  * @ckvol_mutex: serializes static volume checking when opening
- * @mult_mutex: serializes operations on multiple volumes, like re-nameing
+ * @mult_mutex: serializes operations on multiple volumes, like re-naming
  * @dbg_peb_buf: buffer of PEB size used for debugging
- * @dbg_buf_mutex: proptects @dbg_peb_buf
+ * @dbg_buf_mutex: protects @dbg_peb_buf
  */
 struct ubi_device {
 	struct cdev cdev;
@@ -394,16 +402,13 @@ struct ubi_device {
 	struct rb_root used;
 	struct rb_root free;
 	struct rb_root scrub;
-	struct {
-		struct rb_root pnum;
-		struct rb_root aec;
-	} prot;
+	struct list_head pq[UBI_PROT_QUEUE_LEN];
+	int pq_head;
 	spinlock_t wl_lock;
 	struct mutex move_mutex;
 	struct rw_semaphore work_sem;
 	int wl_scheduled;
 	struct ubi_wl_entry **lookuptbl;
-	unsigned long long abs_ec;
 	struct ubi_wl_entry *move_from;
 	struct ubi_wl_entry *move_to;
 	int move_to_put;
diff --git a/drivers/mtd/ubi/wl.c b/drivers/mtd/ubi/wl.c
index 0279bf9..14901cb 100644
--- a/drivers/mtd/ubi/wl.c
+++ b/drivers/mtd/ubi/wl.c
@@ -22,7 +22,7 @@
  * UBI wear-leveling sub-system.
  *
  * This sub-system is responsible for wear-leveling. It works in terms of
- * physical* eraseblocks and erase counters and knows nothing about logical
+ * physical eraseblocks and erase counters and knows nothing about logical
  * eraseblocks, volumes, etc. From this sub-system's perspective all physical
  * eraseblocks are of two types - used and free. Used physical eraseblocks are
  * those that were "get" by the 'ubi_wl_get_peb()' function, and free physical
@@ -55,8 +55,39 @@
  *
  * As it was said, for the UBI sub-system all physical eraseblocks are either
  * "free" or "used". Free eraseblock are kept in the @wl->free RB-tree, while
- * used eraseblocks are kept in a set of different RB-trees: @wl->used,
- * @wl->prot.pnum, @wl->prot.aec, and @wl->scrub.
+ * used eraseblocks are kept in @wl->used or @wl->scrub RB-trees, or
+ * (temporarily) in the @wl->pq queue.
+ *
+ * When the WL sub-system returns a physical eraseblock, the physical
+ * eraseblock is protected from being moved for some "time". For this reason,
+ * the physical eraseblock is not directly moved from the @wl->free tree to the
+ * @wl->used tree. There is a protection queue in between where this
+ * physical eraseblock is temporarily stored (@wl->pq).
+ *
+ * All this protection stuff is needed because:
+ *  o we don't want to move physical eraseblocks just after we have given them
+ *    to the user; instead, we first want to let users fill them up with data;
+ *
+ *  o there is a chance that the user will put the physical eraseblock very
+ *    soon, so it makes sense not to move it for some time, but wait; this is
+ *    especially important in case of "short term" physical eraseblocks.
+ *
+ * Physical eraseblocks stay protected only for limited time. But the "time" is
+ * measured in erase cycles in this case. This is implemented with help of the
+ * protection queue. Eraseblocks are put to the tail of this queue when they
+ * are returned by the 'ubi_wl_get_peb()', and eraseblocks are removed from the
+ * head of the queue on each erase operation (for any eraseblock). So the
+ * length of the queue defines how may (global) erase cycles PEBs are protected.
+ *
+ * To put it differently, each physical eraseblock has 2 main states: free and
+ * used. The former state corresponds to the @wl->free tree. The latter state
+ * is split up on several sub-states:
+ * o the WL movement is allowed (@wl->used tree);
+ * o the WL movement is temporarily prohibited (@wl->pq queue);
+ * o scrubbing is needed (@wl->scrub tree).
+ *
+ * Depending on the sub-state, wear-leveling entries of the used physical
+ * eraseblocks may be kept in one of those structures.
  *
  * Note, in this implementation, we keep a small in-RAM object for each physical
  * eraseblock. This is surely not a scalable solution. But it appears to be good
@@ -70,9 +101,6 @@
  * target PEB, we pick a PEB with the highest EC if our PEB is "old" and we
  * pick target PEB with an average EC if our PEB is not very "old". This is a
  * room for future re-works of the WL sub-system.
- *
- * Note: the stuff with protection trees looks too complex and is difficult to
- * understand. Should be fixed.
  */
 
 #include <linux/slab.h>
@@ -85,14 +113,6 @@
 #define WL_RESERVED_PEBS 1
 
 /*
- * How many erase cycles are short term, unknown, and long term physical
- * eraseblocks protected.
- */
-#define ST_PROTECTION 16
-#define U_PROTECTION  10
-#define LT_PROTECTION 4
-
-/*
  * Maximum difference between two erase counters. If this threshold is
  * exceeded, the WL sub-system starts moving data from used physical
  * eraseblocks with low erase counter to free physical eraseblocks with high
@@ -120,64 +140,9 @@
 #define WL_MAX_FAILURES 32
 
 /**
- * struct ubi_wl_prot_entry - PEB protection entry.
- * @rb_pnum: link in the @wl->prot.pnum RB-tree
- * @rb_aec: link in the @wl->prot.aec RB-tree
- * @abs_ec: the absolute erase counter value when the protection ends
- * @e: the wear-leveling entry of the physical eraseblock under protection
- *
- * When the WL sub-system returns a physical eraseblock, the physical
- * eraseblock is protected from being moved for some "time". For this reason,
- * the physical eraseblock is not directly moved from the @wl->free tree to the
- * @wl->used tree. There is one more tree in between where this physical
- * eraseblock is temporarily stored (@wl->prot).
- *
- * All this protection stuff is needed because:
- *  o we don't want to move physical eraseblocks just after we have given them
- *    to the user; instead, we first want to let users fill them up with data;
- *
- *  o there is a chance that the user will put the physical eraseblock very
- *    soon, so it makes sense not to move it for some time, but wait; this is
- *    especially important in case of "short term" physical eraseblocks.
- *
- * Physical eraseblocks stay protected only for limited time. But the "time" is
- * measured in erase cycles in this case. This is implemented with help of the
- * absolute erase counter (@wl->abs_ec). When it reaches certain value, the
- * physical eraseblocks are moved from the protection trees (@wl->prot.*) to
- * the @wl->used tree.
- *
- * Protected physical eraseblocks are searched by physical eraseblock number
- * (when they are put) and by the absolute erase counter (to check if it is
- * time to move them to the @wl->used tree). So there are actually 2 RB-trees
- * storing the protected physical eraseblocks: @wl->prot.pnum and
- * @wl->prot.aec. They are referred to as the "protection" trees. The
- * first one is indexed by the physical eraseblock number. The second one is
- * indexed by the absolute erase counter. Both trees store
- * &struct ubi_wl_prot_entry objects.
- *
- * Each physical eraseblock has 2 main states: free and used. The former state
- * corresponds to the @wl->free tree. The latter state is split up on several
- * sub-states:
- * o the WL movement is allowed (@wl->used tree);
- * o the WL movement is temporarily prohibited (@wl->prot.pnum and
- * @wl->prot.aec trees);
- * o scrubbing is needed (@wl->scrub tree).
- *
- * Depending on the sub-state, wear-leveling entries of the used physical
- * eraseblocks may be kept in one of those trees.
- */
-struct ubi_wl_prot_entry {
-	struct rb_node rb_pnum;
-	struct rb_node rb_aec;
-	unsigned long long abs_ec;
-	struct ubi_wl_entry *e;
-};
-
-/**
  * struct ubi_work - UBI work description data structure.
  * @list: a link in the list of pending works
  * @func: worker function
- * @priv: private data of the worker function
  * @e: physical eraseblock to erase
  * @torture: if the physical eraseblock has to be tortured
  *
@@ -198,9 +163,11 @@ struct ubi_work {
 static int paranoid_check_ec(struct ubi_device *ubi, int pnum, int ec);
 static int paranoid_check_in_wl_tree(struct ubi_wl_entry *e,
 				     struct rb_root *root);
+static int paranoid_check_in_pq(struct ubi_device *ubi, struct ubi_wl_entry *e);
 #else
 #define paranoid_check_ec(ubi, pnum, ec) 0
 #define paranoid_check_in_wl_tree(e, root)
+#define paranoid_check_in_pq(ubi, e) 0
 #endif
 
 /**
@@ -355,49 +322,24 @@ static int in_wl_tree(struct ubi_wl_entry *e, struct rb_root *root)
 }
 
 /**
- * prot_tree_add - add physical eraseblock to protection trees.
+ * prot_queue_add - add physical eraseblock to the protection queue.
  * @ubi: UBI device description object
  * @e: the physical eraseblock to add
- * @pe: protection entry object to use
- * @ec: for how many erase operations this PEB should be protected
  *
- * @wl->lock has to be locked.
+ * This function adds @e to the tail of the protection queue @ubi->pq, where
+ * @e will stay for %UBI_PROT_QUEUE_LEN erase operations and will be
+ * temporarily protected from the wear-leveling worker. Note, @wl->lock has to
+ * be locked.
  */
-static void prot_tree_add(struct ubi_device *ubi, struct ubi_wl_entry *e,
-			  struct ubi_wl_prot_entry *pe, int ec)
+static void prot_queue_add(struct ubi_device *ubi, struct ubi_wl_entry *e)
 {
-	struct rb_node **p, *parent = NULL;
-	struct ubi_wl_prot_entry *pe1;
+	int pq_tail = ubi->pq_head - 1;
 
-	pe->e = e;
-	pe->abs_ec = ubi->abs_ec + ec;
-
-	p = &ubi->prot.pnum.rb_node;
-	while (*p) {
-		parent = *p;
-		pe1 = rb_entry(parent, struct ubi_wl_prot_entry, rb_pnum);
-
-		if (e->pnum < pe1->e->pnum)
-			p = &(*p)->rb_left;
-		else
-			p = &(*p)->rb_right;
-	}
-	rb_link_node(&pe->rb_pnum, parent, p);
-	rb_insert_color(&pe->rb_pnum, &ubi->prot.pnum);
-
-	p = &ubi->prot.aec.rb_node;
-	parent = NULL;
-	while (*p) {
-		parent = *p;
-		pe1 = rb_entry(parent, struct ubi_wl_prot_entry, rb_aec);
-
-		if (pe->abs_ec < pe1->abs_ec)
-			p = &(*p)->rb_left;
-		else
-			p = &(*p)->rb_right;
-	}
-	rb_link_node(&pe->rb_aec, parent, p);
-	rb_insert_color(&pe->rb_aec, &ubi->prot.aec);
+	if (pq_tail < 0)
+		pq_tail = UBI_PROT_QUEUE_LEN - 1;
+	ubi_assert(pq_tail >= 0 && pq_tail < UBI_PROT_QUEUE_LEN);
+	list_add_tail(&e->u.list, &ubi->pq[pq_tail]);
+	dbg_wl("added PEB %d EC %d to the protection queue", e->pnum, e->ec);
 }
 
 /**
@@ -442,17 +384,12 @@ static struct ubi_wl_entry *find_wl_entry(struct rb_root *root, int max)
  */
 int ubi_wl_get_peb(struct ubi_device *ubi, int dtype)
 {
-	int err, protect, medium_ec;
+	int err, medium_ec;
 	struct ubi_wl_entry *e, *first, *last;
-	struct ubi_wl_prot_entry *pe;
 
 	ubi_assert(dtype == UBI_LONGTERM || dtype == UBI_SHORTTERM ||
 		   dtype == UBI_UNKNOWN);
 
-	pe = kmalloc(sizeof(struct ubi_wl_prot_entry), GFP_NOFS);
-	if (!pe)
-		return -ENOMEM;
-
 retry:
 	spin_lock(&ubi->wl_lock);
 	if (!ubi->free.rb_node) {
@@ -460,16 +397,13 @@ retry:
 			ubi_assert(list_empty(&ubi->works));
 			ubi_err("no free eraseblocks");
 			spin_unlock(&ubi->wl_lock);
-			kfree(pe);
 			return -ENOSPC;
 		}
 		spin_unlock(&ubi->wl_lock);
 
 		err = produce_free_peb(ubi);
-		if (err < 0) {
-			kfree(pe);
+		if (err < 0)
 			return err;
-		}
 		goto retry;
 	}
 
@@ -482,7 +416,6 @@ retry:
 		 * %WL_FREE_MAX_DIFF.
 		 */
 		e = find_wl_entry(&ubi->free, WL_FREE_MAX_DIFF);
-		protect = LT_PROTECTION;
 		break;
 	case UBI_UNKNOWN:
 		/*
@@ -502,7 +435,6 @@ retry:
 			medium_ec = (first->ec + WL_FREE_MAX_DIFF)/2;
 			e = find_wl_entry(&ubi->free, medium_ec);
 		}
-		protect = U_PROTECTION;
 		break;
 	case UBI_SHORTTERM:
 		/*
@@ -510,63 +442,45 @@ retry:
 		 * lowest erase counter as we expect it will be erased soon.
 		 */
 		e = rb_entry(rb_first(&ubi->free), struct ubi_wl_entry, u.rb);
-		protect = ST_PROTECTION;
 		break;
 	default:
-		protect = 0;
-		e = NULL;
 		BUG();
 	}
 
+	paranoid_check_in_wl_tree(e, &ubi->free);
+
 	/*
-	 * Move the physical eraseblock to the protection trees where it will
+	 * Move the physical eraseblock to the protection queue where it will
 	 * be protected from being moved for some time.
 	 */
-	paranoid_check_in_wl_tree(e, &ubi->free);
 	rb_erase(&e->u.rb, &ubi->free);
-	prot_tree_add(ubi, e, pe, protect);
-
-	dbg_wl("PEB %d EC %d, protection %d", e->pnum, e->ec, protect);
+	dbg_wl("PEB %d EC %d", e->pnum, e->ec);
+	prot_queue_add(ubi, e);
 	spin_unlock(&ubi->wl_lock);
-
 	return e->pnum;
 }
 
 /**
- * prot_tree_del - remove a physical eraseblock from the protection trees
+ * prot_queue_del - remove a physical eraseblock from the protection queue.
  * @ubi: UBI device description object
  * @pnum: the physical eraseblock to remove
  *
- * This function returns PEB @pnum from the protection trees and returns zero
- * in case of success and %-ENODEV if the PEB was not found in the protection
- * trees.
+ * This function deletes PEB @pnum from the protection queue and returns zero
+ * in case of success and %-ENODEV if the PEB was not found.
  */
-static int prot_tree_del(struct ubi_device *ubi, int pnum)
+static int prot_queue_del(struct ubi_device *ubi, int pnum)
 {
-	struct rb_node *p;
-	struct ubi_wl_prot_entry *pe = NULL;
-
-	p = ubi->prot.pnum.rb_node;
-	while (p) {
-
-		pe = rb_entry(p, struct ubi_wl_prot_entry, rb_pnum);
-
-		if (pnum == pe->e->pnum)
-			goto found;
+	struct ubi_wl_entry *e;
 
-		if (pnum < pe->e->pnum)
-			p = p->rb_left;
-		else
-			p = p->rb_right;
-	}
+	e = ubi->lookuptbl[pnum];
+	if (!e)
+		return -ENODEV;
 
-	return -ENODEV;
+	if (paranoid_check_in_pq(ubi, e))
+		return -ENODEV;
 
-found:
-	ubi_assert(pe->e->pnum == pnum);
-	rb_erase(&pe->rb_aec, &ubi->prot.aec);
-	rb_erase(&pe->rb_pnum, &ubi->prot.pnum);
-	kfree(pe);
+	list_del(&e->u.list);
+	dbg_wl("deleted PEB %d from the protection queue", e->pnum);
 	return 0;
 }
 
@@ -632,47 +546,47 @@ out_free:
 }
 
 /**
- * check_protection_over - check if it is time to stop protecting some PEBs.
+ * serve_prot_queue - check if it is time to stop protecting PEBs.
  * @ubi: UBI device description object
  *
- * This function is called after each erase operation, when the absolute erase
- * counter is incremented, to check if some physical eraseblock  have not to be
- * protected any longer. These physical eraseblocks are moved from the
- * protection trees to the used tree.
+ * This function is called after each erase operation and removes PEBs from the
+ * tail of the protection queue. These PEBs have been protected for long enough
+ * and should be moved to the used tree.
  */
-static void check_protection_over(struct ubi_device *ubi)
+static void serve_prot_queue(struct ubi_device *ubi)
 {
-	struct ubi_wl_prot_entry *pe;
+	struct ubi_wl_entry *e, *tmp;
+	int count;
 
 	/*
 	 * There may be several protected physical eraseblock to remove,
 	 * process them all.
 	 */
-	while (1) {
-		spin_lock(&ubi->wl_lock);
-		if (!ubi->prot.aec.rb_node) {
-			spin_unlock(&ubi->wl_lock);
-			break;
-		}
-
-		pe = rb_entry(rb_first(&ubi->prot.aec),
-			      struct ubi_wl_prot_entry, rb_aec);
+repeat:
+	count = 0;
+	spin_lock(&ubi->wl_lock);
+	list_for_each_entry_safe(e, tmp, &ubi->pq[ubi->pq_head], u.list) {
+		dbg_wl("PEB %d EC %d protection over, move to used tree",
+			e->pnum, e->ec);
 
-		if (pe->abs_ec > ubi->abs_ec) {
+		list_del(&e->u.list);
+		wl_tree_add(e, &ubi->used);
+		if (count++ > 32) {
+			/*
+			 * Let's be nice and avoid holding the spinlock for
+			 * too long.
+			 */
 			spin_unlock(&ubi->wl_lock);
-			break;
+			cond_resched();
+			goto repeat;
 		}
-
-		dbg_wl("PEB %d protection over, abs_ec %llu, PEB abs_ec %llu",
-		       pe->e->pnum, ubi->abs_ec, pe->abs_ec);
-		rb_erase(&pe->rb_aec, &ubi->prot.aec);
-		rb_erase(&pe->rb_pnum, &ubi->prot.pnum);
-		wl_tree_add(pe->e, &ubi->used);
-		spin_unlock(&ubi->wl_lock);
-
-		kfree(pe);
-		cond_resched();
 	}
+
+	ubi->pq_head += 1;
+	if (ubi->pq_head == UBI_PROT_QUEUE_LEN)
+		ubi->pq_head = 0;
+	ubi_assert(ubi->pq_head >= 0 && ubi->pq_head < UBI_PROT_QUEUE_LEN);
+	spin_unlock(&ubi->wl_lock);
 }
 
 /**
@@ -680,8 +594,8 @@ static void check_protection_over(struct ubi_device *ubi)
  * @ubi: UBI device description object
  * @wrk: the work to schedule
  *
- * This function enqueues a work defined by @wrk to the tail of the pending
- * works list.
+ * This function adds a work defined by @wrk to the tail of the pending works
+ * list.
  */
 static void schedule_ubi_work(struct ubi_device *ubi, struct ubi_work *wrk)
 {
@@ -740,7 +654,6 @@ static int wear_leveling_worker(struct ubi_device *ubi, struct ubi_work *wrk,
 				int cancel)
 {
 	int err, scrubbing = 0, torture = 0;
-	struct ubi_wl_prot_entry *uninitialized_var(pe);
 	struct ubi_wl_entry *e1, *e2;
 	struct ubi_vid_hdr *vid_hdr;
 
@@ -857,23 +770,17 @@ static int wear_leveling_worker(struct ubi_device *ubi, struct ubi_work *wrk,
 		 * The LEB has not been moved because the volume is being
 		 * deleted or the PEB has been put meanwhile. We should prevent
 		 * this PEB from being selected for wear-leveling movement
-		 * again, so put it to the protection tree.
+		 * again, so put it to the protection queue.
 		 */
 
 		dbg_wl("canceled moving PEB %d", e1->pnum);
 		ubi_assert(err == 1);
 
-		pe = kmalloc(sizeof(struct ubi_wl_prot_entry), GFP_NOFS);
-		if (!pe) {
-			err = -ENOMEM;
-			goto out_error;
-		}
-
 		ubi_free_vid_hdr(ubi, vid_hdr);
 		vid_hdr = NULL;
 
 		spin_lock(&ubi->wl_lock);
-		prot_tree_add(ubi, e1, pe, U_PROTECTION);
+		prot_queue_add(ubi, e1);
 		ubi_assert(!ubi->move_to_put);
 		ubi->move_from = ubi->move_to = NULL;
 		ubi->wl_scheduled = 0;
@@ -1075,7 +982,6 @@ static int erase_worker(struct ubi_device *ubi, struct ubi_work *wl_wrk,
 		kfree(wl_wrk);
 
 		spin_lock(&ubi->wl_lock);
-		ubi->abs_ec += 1;
 		wl_tree_add(e, &ubi->free);
 		spin_unlock(&ubi->wl_lock);
 
@@ -1083,7 +989,7 @@ static int erase_worker(struct ubi_device *ubi, struct ubi_work *wl_wrk,
 		 * One more erase operation has happened, take care about
 		 * protected physical eraseblocks.
 		 */
-		check_protection_over(ubi);
+		serve_prot_queue(ubi);
 
 		/* And take care about wear-leveling */
 		err = ensure_wear_leveling(ubi);
@@ -1220,7 +1126,7 @@ retry:
 			paranoid_check_in_wl_tree(e, &ubi->scrub);
 			rb_erase(&e->u.rb, &ubi->scrub);
 		} else {
-			err = prot_tree_del(ubi, e->pnum);
+			err = prot_queue_del(ubi, e->pnum);
 			if (err) {
 				ubi_err("PEB %d not found", pnum);
 				ubi_ro_mode(ubi);
@@ -1284,7 +1190,7 @@ retry:
 	} else {
 		int err;
 
-		err = prot_tree_del(ubi, e->pnum);
+		err = prot_queue_del(ubi, e->pnum);
 		if (err) {
 			ubi_err("PEB %d not found", pnum);
 			ubi_ro_mode(ubi);
@@ -1315,7 +1221,7 @@ int ubi_wl_flush(struct ubi_device *ubi)
 	int err;
 
 	/*
-	 * Erase while the pending works queue is not empty, but not more then
+	 * Erase while the pending works queue is not empty, but not more than
 	 * the number of currently pending works.
 	 */
 	dbg_wl("flush (%d pending works)", ubi->works_count);
@@ -1461,15 +1367,13 @@ static void cancel_pending(struct ubi_device *ubi)
  */
 int ubi_wl_init_scan(struct ubi_device *ubi, struct ubi_scan_info *si)
 {
-	int err;
+	int err, i;
 	struct rb_node *rb1, *rb2;
 	struct ubi_scan_volume *sv;
 	struct ubi_scan_leb *seb, *tmp;
 	struct ubi_wl_entry *e;
 
-
 	ubi->used = ubi->free = ubi->scrub = RB_ROOT;
-	ubi->prot.pnum = ubi->prot.aec = RB_ROOT;
 	spin_lock_init(&ubi->wl_lock);
 	mutex_init(&ubi->move_mutex);
 	init_rwsem(&ubi->work_sem);
@@ -1483,6 +1387,10 @@ int ubi_wl_init_scan(struct ubi_device *ubi, struct ubi_scan_info *si)
 	if (!ubi->lookuptbl)
 		return err;
 
+	for (i = 0; i < UBI_PROT_QUEUE_LEN; i++)
+		INIT_LIST_HEAD(&ubi->pq[i]);
+	ubi->pq_head = 0;
+
 	list_for_each_entry_safe(seb, tmp, &si->erase, u.list) {
 		cond_resched();
 
@@ -1577,33 +1485,18 @@ out_free:
 }
 
 /**
- * protection_trees_destroy - destroy the protection RB-trees.
+ * protection_queue_destroy - destroy the protection queue.
  * @ubi: UBI device description object
  */
-static void protection_trees_destroy(struct ubi_device *ubi)
+static void protection_queue_destroy(struct ubi_device *ubi)
 {
-	struct rb_node *rb;
-	struct ubi_wl_prot_entry *pe;
+	int i;
+	struct ubi_wl_entry *e, *tmp;
 
-	rb = ubi->prot.aec.rb_node;
-	while (rb) {
-		if (rb->rb_left)
-			rb = rb->rb_left;
-		else if (rb->rb_right)
-			rb = rb->rb_right;
-		else {
-			pe = rb_entry(rb, struct ubi_wl_prot_entry, rb_aec);
-
-			rb = rb_parent(rb);
-			if (rb) {
-				if (rb->rb_left == &pe->rb_aec)
-					rb->rb_left = NULL;
-				else
-					rb->rb_right = NULL;
-			}
-
-			kmem_cache_free(ubi_wl_entry_slab, pe->e);
-			kfree(pe);
+	for (i = 0; i < UBI_PROT_QUEUE_LEN; ++i) {
+		list_for_each_entry_safe(e, tmp, &ubi->pq[i], u.list) {
+			list_del(&e->u.list);
+			kmem_cache_free(ubi_wl_entry_slab, e);
 		}
 	}
 }
@@ -1616,7 +1509,7 @@ void ubi_wl_close(struct ubi_device *ubi)
 {
 	dbg_wl("close the WL sub-system");
 	cancel_pending(ubi);
-	protection_trees_destroy(ubi);
+	protection_queue_destroy(ubi);
 	tree_destroy(&ubi->used);
 	tree_destroy(&ubi->free);
 	tree_destroy(&ubi->scrub);
@@ -1686,4 +1579,27 @@ static int paranoid_check_in_wl_tree(struct ubi_wl_entry *e,
 	return 1;
 }
 
+/**
+ * paranoid_check_in_pq - check if wear-leveling entry is in the protection
+ *                        queue.
+ * @ubi: UBI device description object
+ * @e: the wear-leveling entry to check
+ *
+ * This function returns zero if @e is in @ubi->pq and %1 if it is not.
+ */
+static int paranoid_check_in_pq(struct ubi_device *ubi, struct ubi_wl_entry *e)
+{
+	struct ubi_wl_entry *p;
+	int i;
+
+	for (i = 0; i < UBI_PROT_QUEUE_LEN; ++i)
+		list_for_each_entry(p, &ubi->pq[i], u.list)
+			if (p == e)
+				return 0;
+
+	ubi_err("paranoid check failed for PEB %d, EC %d, Protect queue",
+		e->pnum, e->ec);
+	ubi_dbg_dump_stack();
+	return 1;
+}
 #endif /* CONFIG_MTD_UBI_DEBUG_PARANOID */