mm/memblock: fix a race between search and remove

no-map-fixup feature for dma-removed reserve region does a
late memblock remove. This late removal is a slight
deviation from conventional memblock flow. That leads to a
race between pfn_valid->memblock_search, and
memblock_remove.

To fix this, use a read seqlock in memblock_search, which
would ensure minimum overhead in search path. And export
two APIs to let code doing late memblock remove apply
write seqlock. write seqlock would ensure that search
retries if the list is updated concurrently.

The two exported APIs which should be called before
and after modifying memblock regions, late in boot, are
   - memblock_region_resize_late_begin
   - memblock_region_resize_late_end

The code to alter memblock regions should be guarded by
these APIs.

CRs-fixed: 967728
Change-Id: I6a10c3e980002048aafeaf829a16119848c6a099
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>

drivers/base/dma-removed.c

@@ -184,8 +184,10 @@ static void removed_region_fixup(struct removed_region *dma_mem, int index)
 		return;
 
 	/* carve-out */
+	memblock_region_resize_late_begin();
 	memblock_free(dma_mem->base, dma_mem->nr_pages * PAGE_SIZE);
 	memblock_remove(dma_mem->base, index * PAGE_SIZE);
+	memblock_region_resize_late_end();
 
 	/* clear page-mappings */
 	base_pfn = dma_mem->base >> PAGE_SHIFT;

include/linux/memblock.h

@@ -83,6 +83,8 @@ int memblock_mark_hotplug(phys_addr_t base, phys_addr_t size);
 int memblock_clear_hotplug(phys_addr_t base, phys_addr_t size);
 int memblock_mark_mirror(phys_addr_t base, phys_addr_t size);
 ulong choose_memblock_flags(void);
+void memblock_region_resize_late_begin(void);
+void memblock_region_resize_late_end(void);
 
 /* Low level functions */
 int memblock_add_range(struct memblock_type *type,

mm/memblock.c

@@ -19,6 +19,8 @@
 #include <linux/debugfs.h>
 #include <linux/seq_file.h>
 #include <linux/memblock.h>
+#include <linux/preempt.h>
+#include <linux/seqlock.h>
 
 #include <asm-generic/sections.h>
 #include <linux/io.h>
@@ -31,6 +33,7 @@ static struct memblock_region memblock_reserved_init_regions[INIT_MEMBLOCK_REGIO
 static struct memblock_region memblock_physmem_init_regions[INIT_PHYSMEM_REGIONS] __initdata_memblock;
 #endif
 
+static seqcount_t memblock_seq;
 struct memblock memblock __initdata_memblock = {
 	.memory.regions		= memblock_memory_init_regions,
 	.memory.cnt		= 1,	/* empty dummy entry */
@@ -1493,7 +1496,7 @@ void __init memblock_enforce_memory_limit(phys_addr_t limit)
 			      (phys_addr_t)ULLONG_MAX);
 }
 
-static int __init_memblock memblock_search(struct memblock_type *type, phys_addr_t addr)
+static int __init_memblock __memblock_search(struct memblock_type *type, phys_addr_t addr)
 {
 	unsigned int left = 0, right = type->cnt;
 
@@ -1511,6 +1514,19 @@ static int __init_memblock memblock_search(struct memblock_type *type, phys_addr
 	return -1;
 }
 
+static int __init_memblock memblock_search(struct memblock_type *type, phys_addr_t addr)
+{
+	int ret;
+	unsigned long seq;
+
+	do {
+		seq = raw_read_seqcount_begin(&memblock_seq);
+		ret = __memblock_search(type, addr);
+	} while (unlikely(read_seqcount_retry(&memblock_seq, seq)));
+
+	return ret;
+}
+
 int __init memblock_is_reserved(phys_addr_t addr)
 {
 	return memblock_search(&memblock.reserved, addr) != -1;
@@ -1659,6 +1675,32 @@ void __init memblock_allow_resize(void)
 	memblock_can_resize = 1;
 }
 
+static void __init_memblock memblock_resize_late(int begin)
+{
+	static int memblock_can_resize_old;
+
+	if (begin) {
+		preempt_disable();
+		memblock_can_resize_old = memblock_can_resize;
+		memblock_can_resize = 0;
+		raw_write_seqcount_begin(&memblock_seq);
+	} else {
+		raw_write_seqcount_end(&memblock_seq);
+		memblock_can_resize = memblock_can_resize_old;
+		preempt_enable();
+	}
+}
+
+void __init_memblock memblock_region_resize_late_begin(void)
+{
+	memblock_resize_late(1);
+}
+
+void __init_memblock memblock_region_resize_late_end(void)
+{
+	memblock_resize_late(0);
+}
+
 static int __init early_memblock(char *p)
 {
 	if (p && strstr(p, "debug"))