new file mode 100644
--- /dev/null
+++ b/src/event/ngx_event_busy_lock.c
@@ -0,0 +1,345 @@
+
+#include <ngx_config.h>
+#include <ngx_core.h>
+#include <ngx_event.h>
+
+/*
+ * TODO: eliminate mutex and use atomic_xchg():
+ *       ev->next = ev; ngx_atomic_xchg(ngx_posted_events, ev->next);
+ *       in ngx_event_busy_unlock() and ngx_event_busy_lock_handler()
+ */
+
+
+static int ngx_event_busy_lock_look_cachable(ngx_event_busy_lock_t *bl,
+                                             ngx_event_busy_lock_ctx_t *ctx);
+static void ngx_event_busy_lock_handler(ngx_event_t *ev);
+static void ngx_event_busy_lock_posted_handler(ngx_event_t *ev);
+
+
+/*
+ * NGX_OK:     the busy lock is held
+ * NGX_BUSY:   there are many the busy locks or many the waiting locks
+ * NGX_AGAIN:  the all busy locks are held but we will wait the specified time
+ * NGX_ERROR:  there was error while the mutex locking
+ */
+
+ngx_int_t ngx_event_busy_lock(ngx_event_busy_lock_t *bl,
+                              ngx_event_busy_lock_ctx_t *ctx)
+{
+    ngx_int_t  rc;
+
+#if (NGX_THREADS)
+    if (ngx_mutex_lock(bl->mutex) == NGX_ERROR) {
+        return NGX_ERROR;
+    }
+#endif
+
+    ngx_log_debug2(NGX_LOG_DEBUG_EVENT, ctx->event->log, 0,
+                   "event busy lock: b:%d mb:%d",
+                   bl->busy, bl->max_busy);
+
+    if (bl->busy < bl->max_busy) {
+        bl->busy++;
+        rc = NGX_OK;
+
+    } else if (ctx->timer && bl->waiting < bl->max_waiting) {
+        bl->waiting++;
+        ngx_add_timer(ctx->event, ctx->timer);
+        ctx->event->event_handler = ngx_event_busy_lock_handler;
+
+        if (bl->events == NULL) {
+            bl->events = ctx;
+        } else {
+            bl->last->next = ctx;
+        }
+        bl->last = ctx;
+
+        rc = NGX_AGAIN;
+
+    } else {
+        rc = NGX_BUSY;
+    }
+
+#if (NGX_THREADS)
+    ngx_mutex_unlock(bl->mutex);
+#endif
+
+    return rc;
+}
+
+
+ngx_int_t ngx_event_busy_lock_cachable(ngx_event_busy_lock_t *bl,
+                                       ngx_event_busy_lock_ctx_t *ctx)
+{
+    ngx_int_t  rc;
+
+#if (NGX_THREADS)
+    if (ngx_mutex_lock(bl->mutex) == NGX_ERROR) {
+        return NGX_ERROR;
+    }
+#endif
+
+    rc = ngx_event_busy_lock_look_cachable(bl, ctx);
+
+    ngx_log_debug3(NGX_LOG_DEBUG_EVENT, ctx->event->log, 0,
+                   "event busy lock: %d w:%d mw:%d",
+                   rc, bl->waiting, bl->max_waiting);
+
+    /*
+     * NGX_OK:     no the same request, there is free slot and we locked it
+     * NGX_BUSY:   no the same request and there is no free slot
+     * NGX_AGAIN:  the same request is processing
+     */
+
+    if (rc == NGX_AGAIN) {
+
+        if (ctx->timer && bl->waiting < bl->max_waiting) {
+            bl->waiting++;
+            ngx_add_timer(ctx->event, ctx->timer);
+            ctx->event->event_handler = ngx_event_busy_lock_handler;
+
+            if (bl->events == NULL) {
+                bl->events = ctx;
+            } else {
+                bl->last->next = ctx;
+            }
+            bl->last = ctx;
+
+        } else {
+            rc = NGX_BUSY;
+        }
+    }
+
+#if (NGX_THREADS)
+    ngx_mutex_unlock(bl->mutex);
+#endif
+
+    return rc;
+}
+
+
+ngx_int_t ngx_event_busy_unlock(ngx_event_busy_lock_t *bl,
+                                ngx_event_busy_lock_ctx_t *ctx)
+{
+    ngx_event_t                *ev;
+    ngx_event_busy_lock_ctx_t  *wakeup;
+
+#if (NGX_THREADS)
+    if (ngx_mutex_lock(bl->mutex) == NGX_ERROR) {
+        return NGX_ERROR;
+    }
+#endif
+
+    if (bl->events) {
+        wakeup = bl->events;
+        bl->events = bl->events->next;
+
+    } else {
+        wakeup = NULL;
+        bl->busy--;
+    }
+
+    /*
+     * MP:
+     * nocachable (bl->md5 == NULL): ngx_shared_mutex_unlock(mutex, !wakeup)
+     * cachable (bl->md5): ???
+     */
+
+    if (wakeup == NULL) {
+#if (NGX_THREADS)
+        ngx_mutex_unlock(bl->mutex);
+#endif
+        return NGX_OK;
+    }
+
+    if (ctx->md5) {
+        for (wakeup = bl->events; wakeup; wakeup = wakeup->next) {
+            if (wakeup->md5 == NULL) {
+                continue;
+            }
+
+            if (ngx_memcmp(ctx->md5, wakeup->md5, 16) != 0) {
+                continue;
+            }
+            
+            wakeup->handler = ngx_event_busy_lock_posted_handler;
+            wakeup->cache_updated = 1;
+
+            ev = wakeup->event;
+
+#if (NGX_THREADS)
+            if (ngx_mutex_lock(ngx_posted_events_mutex) == NGX_ERROR) {
+                return NGX_ERROR;
+            }
+#endif
+
+            ev->next = (ngx_event_t *) ngx_posted_events;
+            ngx_posted_events = ev;
+
+#if (NGX_THREADS)
+            ngx_mutex_unlock(ngx_posted_events_mutex);
+#endif
+        }
+
+#if (NGX_THREADS)
+        ngx_mutex_unlock(bl->mutex);
+#endif
+
+    } else {
+        bl->waiting--;
+
+#if (NGX_THREADS)
+        ngx_mutex_unlock(bl->mutex);
+#endif
+
+        wakeup->handler = ngx_event_busy_lock_posted_handler;
+        wakeup->locked = 1;
+
+        ev = wakeup->event;
+
+        if (ev->timer_set) {
+            ngx_del_timer(ev);
+        }
+
+#if (NGX_THREADS)
+        if (ngx_mutex_lock(ngx_posted_events_mutex) == NGX_ERROR) {
+            return NGX_ERROR;
+        }
+#endif
+
+        ev->next = (ngx_event_t *) ngx_posted_events;
+        ngx_posted_events = ev;
+
+#if (NGX_THREADS)
+        ngx_mutex_unlock(ngx_posted_events_mutex);
+#endif
+    }
+
+    return NGX_OK;
+}
+
+
+ngx_int_t ngx_event_busy_lock_cancel(ngx_event_busy_lock_t *bl,
+                                     ngx_event_busy_lock_ctx_t *ctx)
+{
+    ngx_event_busy_lock_ctx_t  *c, *p;
+
+#if (NGX_THREADS)
+    if (ngx_mutex_lock(bl->mutex) == NGX_ERROR) {
+        return NGX_ERROR;
+    }
+#endif
+
+    bl->waiting--;
+
+    if (ctx == bl->events) {
+        bl->events = ctx->next;
+
+    } else {
+        p = bl->events;
+        for (c = bl->events->next; c; c = c->next) {
+            if (c == ctx) {
+                p->next = ctx->next;
+                break;
+            }
+            p = c;
+        }
+    }
+
+#if (NGX_THREADS)
+    ngx_mutex_unlock(bl->mutex);
+#endif
+
+    return NGX_OK;
+}
+
+
+static int ngx_event_busy_lock_look_cachable(ngx_event_busy_lock_t *bl,
+                                             ngx_event_busy_lock_ctx_t *ctx)
+{
+    ngx_int_t    free;
+    ngx_uint_t   i, bit, cachable, mask;
+
+    bit = 0;
+    cachable = 0;
+    free = -1;
+
+#if (NGX_SUPPRESS_WARN)
+    mask = 0;
+#endif
+
+    for (i = 0; i < bl->max_busy; i++) {
+
+        if ((bit & 7) == 0) {
+            mask = bl->md5_mask[i / 8];
+        }
+
+        if (mask & 1) {
+            if (ngx_memcmp(&bl->md5[i * 16], ctx->md5, 16) == 0) {
+                return NGX_AGAIN;
+            }
+            cachable++;
+
+        } else if (free == -1) {
+            free = i;
+        }
+
+        if (cachable == bl->cachable) {
+            if (free == -1 && cachable < bl->max_busy) {
+                free = i + 1;
+            }
+
+            break;
+        }
+
+        mask >>= 1;
+        bit++;
+    }
+
+    if (free == -1) {
+        return NGX_BUSY;
+    }
+
+#if 0
+    if (bl->busy == bl->max_busy) {
+        return NGX_BUSY;
+    }
+#endif
+
+    ngx_memcpy(&bl->md5[free * 16], ctx->md5, 16);
+    bl->md5_mask[free / 8] |= 1 << (free & 7);
+    ctx->slot = free;
+
+    bl->cachable++;
+    bl->busy++;
+
+    return NGX_OK;
+}
+
+
+static void ngx_event_busy_lock_handler(ngx_event_t *ev)
+{
+    ev->event_handler = ngx_event_busy_lock_posted_handler;
+
+#if (NGX_THREADS)
+    if (ngx_mutex_lock(ngx_posted_events_mutex) == NGX_ERROR) {
+        return;
+    }
+#endif
+
+    ev->next = (ngx_event_t *) ngx_posted_events;
+    ngx_posted_events = ev;
+
+#if (NGX_THREADS)
+    ngx_mutex_unlock(ngx_posted_events_mutex);
+#endif
+}
+
+
+static void ngx_event_busy_lock_posted_handler(ngx_event_t *ev)
+{
+    ngx_event_busy_lock_ctx_t  *ctx;
+
+    ctx = ev->data;
+    ctx->handler(ev);
+}