diff --git a/mm/percpu.c b/mm/percpu.c
index 4c8a419..bfe6a3a 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -62,6 +62,7 @@
 #include <linux/pfn.h>
 #include <linux/rbtree.h>
 #include <linux/slab.h>
+#include <linux/spinlock.h>
 #include <linux/vmalloc.h>
 #include <linux/workqueue.h>
 
@@ -101,20 +102,28 @@
 static int pcpu_reserved_chunk_limit;
 
 /*
- * One mutex to rule them all.
+ * Synchronization rules.
  *
- * The following mutex is grabbed in the outermost public alloc/free
- * interface functions and released only when the operation is
- * complete.  As such, every function in this file other than the
- * outermost functions are called under pcpu_mutex.
+ * There are two locks - pcpu_alloc_mutex and pcpu_lock.  The former
+ * protects allocation/reclaim paths, chunks and chunk->page arrays.
+ * The latter is a spinlock and protects the index data structures -
+ * chunk slots, rbtree, chunks and area maps in chunks.
  *
- * It can easily be switched to use spinlock such that only the area
- * allocation and page population commit are protected with it doing
- * actual [de]allocation without holding any lock.  However, given
- * what this allocator does, I think it's better to let them run
- * sequentially.
+ * During allocation, pcpu_alloc_mutex is kept locked all the time and
+ * pcpu_lock is grabbed and released as necessary.  All actual memory
+ * allocations are done using GFP_KERNEL with pcpu_lock released.
+ *
+ * Free path accesses and alters only the index data structures, so it
+ * can be safely called from atomic context.  When memory needs to be
+ * returned to the system, free path schedules reclaim_work which
+ * grabs both pcpu_alloc_mutex and pcpu_lock, unlinks chunks to be
+ * reclaimed, release both locks and frees the chunks.  Note that it's
+ * necessary to grab both locks to remove a chunk from circulation as
+ * allocation path might be referencing the chunk with only
+ * pcpu_alloc_mutex locked.
  */
-static DEFINE_MUTEX(pcpu_mutex);
+static DEFINE_MUTEX(pcpu_alloc_mutex);	/* protects whole alloc and reclaim */
+static DEFINE_SPINLOCK(pcpu_lock);	/* protects index data structures */
 
 static struct list_head *pcpu_slot __read_mostly; /* chunk list slots */
 static struct rb_root pcpu_addr_root = RB_ROOT;	/* chunks by address */
@@ -176,6 +185,9 @@
  * kzalloc() is used; otherwise, vmalloc() is used.  The returned
  * memory is always zeroed.
  *
+ * CONTEXT:
+ * Does GFP_KERNEL allocation.
+ *
  * RETURNS:
  * Pointer to the allocated area on success, NULL on failure.
  */
@@ -215,6 +227,9 @@
  * New slot according to the changed state is determined and @chunk is
  * moved to the slot.  Note that the reserved chunk is never put on
  * chunk slots.
+ *
+ * CONTEXT:
+ * pcpu_lock.
  */
 static void pcpu_chunk_relocate(struct pcpu_chunk *chunk, int oslot)
 {
@@ -260,6 +275,9 @@
  * searchs for the chunk with the highest start address which isn't
  * beyond @addr.
  *
+ * CONTEXT:
+ * pcpu_lock.
+ *
  * RETURNS:
  * The address of the found chunk.
  */
@@ -300,6 +318,9 @@
  * @new: chunk to insert
  *
  * Insert @new into address rb tree.
+ *
+ * CONTEXT:
+ * pcpu_lock.
  */
 static void pcpu_chunk_addr_insert(struct pcpu_chunk *new)
 {
@@ -319,6 +340,10 @@
  * A single allocation can split an area into three areas, so this
  * function makes sure that @chunk->map has at least two extra slots.
  *
+ * CONTEXT:
+ * pcpu_alloc_mutex, pcpu_lock.  pcpu_lock is released and reacquired
+ * if area map is extended.
+ *
  * RETURNS:
  * 0 if noop, 1 if successfully extended, -errno on failure.
  */
@@ -332,13 +357,25 @@
 	if (chunk->map_alloc >= chunk->map_used + 2)
 		return 0;
 
+	spin_unlock_irq(&pcpu_lock);
+
 	new_alloc = PCPU_DFL_MAP_ALLOC;
 	while (new_alloc < chunk->map_used + 2)
 		new_alloc *= 2;
 
 	new = pcpu_mem_alloc(new_alloc * sizeof(new[0]));
-	if (!new)
+	if (!new) {
+		spin_lock_irq(&pcpu_lock);
 		return -ENOMEM;
+	}
+
+	/*
+	 * Acquire pcpu_lock and switch to new area map.  Only free
+	 * could have happened inbetween, so map_used couldn't have
+	 * grown.
+	 */
+	spin_lock_irq(&pcpu_lock);
+	BUG_ON(new_alloc < chunk->map_used + 2);
 
 	size = chunk->map_alloc * sizeof(chunk->map[0]);
 	memcpy(new, chunk->map, size);
@@ -371,6 +408,9 @@
  * is inserted after the target block.
  *
  * @chunk->map must have enough free slots to accomodate the split.
+ *
+ * CONTEXT:
+ * pcpu_lock.
  */
 static void pcpu_split_block(struct pcpu_chunk *chunk, int i,
 			     int head, int tail)
@@ -406,6 +446,9 @@
  *
  * @chunk->map must have at least two free slots.
  *
+ * CONTEXT:
+ * pcpu_lock.
+ *
  * RETURNS:
  * Allocated offset in @chunk on success, -1 if no matching area is
  * found.
@@ -495,6 +538,9 @@
  * Free area starting from @freeme to @chunk.  Note that this function
  * only modifies the allocation map.  It doesn't depopulate or unmap
  * the area.
+ *
+ * CONTEXT:
+ * pcpu_lock.
  */
 static void pcpu_free_area(struct pcpu_chunk *chunk, int freeme)
 {
@@ -580,6 +626,9 @@
  * For each cpu, depopulate and unmap pages [@page_start,@page_end)
  * from @chunk.  If @flush is true, vcache is flushed before unmapping
  * and tlb after.
+ *
+ * CONTEXT:
+ * pcpu_alloc_mutex.
  */
 static void pcpu_depopulate_chunk(struct pcpu_chunk *chunk, int off, int size,
 				  bool flush)
@@ -658,6 +707,9 @@
  *
  * For each cpu, populate and map pages [@page_start,@page_end) into
  * @chunk.  The area is cleared on return.
+ *
+ * CONTEXT:
+ * pcpu_alloc_mutex, does GFP_KERNEL allocation.
  */
 static int pcpu_populate_chunk(struct pcpu_chunk *chunk, int off, int size)
 {
@@ -748,15 +800,16 @@
  * @align: alignment of area (max PAGE_SIZE)
  * @reserved: allocate from the reserved chunk if available
  *
- * Allocate percpu area of @size bytes aligned at @align.  Might
- * sleep.  Might trigger writeouts.
+ * Allocate percpu area of @size bytes aligned at @align.
+ *
+ * CONTEXT:
+ * Does GFP_KERNEL allocation.
  *
  * RETURNS:
  * Percpu pointer to the allocated area on success, NULL on failure.
  */
 static void *pcpu_alloc(size_t size, size_t align, bool reserved)
 {
-	void *ptr = NULL;
 	struct pcpu_chunk *chunk;
 	int slot, off;
 
@@ -766,27 +819,37 @@
 		return NULL;
 	}
 
-	mutex_lock(&pcpu_mutex);
+	mutex_lock(&pcpu_alloc_mutex);
+	spin_lock_irq(&pcpu_lock);
 
 	/* serve reserved allocations from the reserved chunk if available */
 	if (reserved && pcpu_reserved_chunk) {
 		chunk = pcpu_reserved_chunk;
 		if (size > chunk->contig_hint ||
 		    pcpu_extend_area_map(chunk) < 0)
-			goto out_unlock;
+			goto fail_unlock;
 		off = pcpu_alloc_area(chunk, size, align);
 		if (off >= 0)
 			goto area_found;
-		goto out_unlock;
+		goto fail_unlock;
 	}
 
+restart:
 	/* search through normal chunks */
 	for (slot = pcpu_size_to_slot(size); slot < pcpu_nr_slots; slot++) {
 		list_for_each_entry(chunk, &pcpu_slot[slot], list) {
 			if (size > chunk->contig_hint)
 				continue;
-			if (pcpu_extend_area_map(chunk) < 0)
-				goto out_unlock;
+
+			switch (pcpu_extend_area_map(chunk)) {
+			case 0:
+				break;
+			case 1:
+				goto restart;	/* pcpu_lock dropped, restart */
+			default:
+				goto fail_unlock;
+			}
+
 			off = pcpu_alloc_area(chunk, size, align);
 			if (off >= 0)
 				goto area_found;
@@ -794,27 +857,36 @@
 	}
 
 	/* hmmm... no space left, create a new chunk */
+	spin_unlock_irq(&pcpu_lock);
+
 	chunk = alloc_pcpu_chunk();
 	if (!chunk)
-		goto out_unlock;
+		goto fail_unlock_mutex;
+
+	spin_lock_irq(&pcpu_lock);
 	pcpu_chunk_relocate(chunk, -1);
 	pcpu_chunk_addr_insert(chunk);
-
-	off = pcpu_alloc_area(chunk, size, align);
-	if (off < 0)
-		goto out_unlock;
+	goto restart;
 
 area_found:
+	spin_unlock_irq(&pcpu_lock);
+
 	/* populate, map and clear the area */
 	if (pcpu_populate_chunk(chunk, off, size)) {
+		spin_lock_irq(&pcpu_lock);
 		pcpu_free_area(chunk, off);
-		goto out_unlock;
+		goto fail_unlock;
 	}
 
-	ptr = __addr_to_pcpu_ptr(chunk->vm->addr + off);
-out_unlock:
-	mutex_unlock(&pcpu_mutex);
-	return ptr;
+	mutex_unlock(&pcpu_alloc_mutex);
+
+	return __addr_to_pcpu_ptr(chunk->vm->addr + off);
+
+fail_unlock:
+	spin_unlock_irq(&pcpu_lock);
+fail_unlock_mutex:
+	mutex_unlock(&pcpu_alloc_mutex);
+	return NULL;
 }
 
 /**
@@ -825,6 +897,9 @@
  * Allocate percpu area of @size bytes aligned at @align.  Might
  * sleep.  Might trigger writeouts.
  *
+ * CONTEXT:
+ * Does GFP_KERNEL allocation.
+ *
  * RETURNS:
  * Percpu pointer to the allocated area on success, NULL on failure.
  */
@@ -843,6 +918,9 @@
  * percpu area if arch has set it up; otherwise, allocation is served
  * from the same dynamic area.  Might sleep.  Might trigger writeouts.
  *
+ * CONTEXT:
+ * Does GFP_KERNEL allocation.
+ *
  * RETURNS:
  * Percpu pointer to the allocated area on success, NULL on failure.
  */
@@ -856,6 +934,9 @@
  * @work: unused
  *
  * Reclaim all fully free chunks except for the first one.
+ *
+ * CONTEXT:
+ * workqueue context.
  */
 static void pcpu_reclaim(struct work_struct *work)
 {
@@ -863,7 +944,8 @@
 	struct list_head *head = &pcpu_slot[pcpu_nr_slots - 1];
 	struct pcpu_chunk *chunk, *next;
 
-	mutex_lock(&pcpu_mutex);
+	mutex_lock(&pcpu_alloc_mutex);
+	spin_lock_irq(&pcpu_lock);
 
 	list_for_each_entry_safe(chunk, next, head, list) {
 		WARN_ON(chunk->immutable);
@@ -876,7 +958,8 @@
 		list_move(&chunk->list, &todo);
 	}
 
-	mutex_unlock(&pcpu_mutex);
+	spin_unlock_irq(&pcpu_lock);
+	mutex_unlock(&pcpu_alloc_mutex);
 
 	list_for_each_entry_safe(chunk, next, &todo, list) {
 		pcpu_depopulate_chunk(chunk, 0, pcpu_unit_size, false);
@@ -888,18 +971,22 @@
  * free_percpu - free percpu area
  * @ptr: pointer to area to free
  *
- * Free percpu area @ptr.  Might sleep.
+ * Free percpu area @ptr.
+ *
+ * CONTEXT:
+ * Can be called from atomic context.
  */
 void free_percpu(void *ptr)
 {
 	void *addr = __pcpu_ptr_to_addr(ptr);
 	struct pcpu_chunk *chunk;
+	unsigned long flags;
 	int off;
 
 	if (!ptr)
 		return;
 
-	mutex_lock(&pcpu_mutex);
+	spin_lock_irqsave(&pcpu_lock, flags);
 
 	chunk = pcpu_chunk_addr_search(addr);
 	off = addr - chunk->vm->addr;
@@ -917,7 +1004,7 @@
 			}
 	}
 
-	mutex_unlock(&pcpu_mutex);
+	spin_unlock_irqrestore(&pcpu_lock, flags);
 }
 EXPORT_SYMBOL_GPL(free_percpu);