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 46a4763f8e7c..4a8ec485c91d 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 0279bf9dc722..14901cb82c18 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 */