[PATCH] mm/cma.c: remove redundant cma_mutex lock

[Lecopzer Chen]

The cma_mutex which protects alloc_contig_range() was first appeared in
commit 7ee793a62fa8c ("cma: Remove potential deadlock situation"),
at that time, there is no guarantee the behavior of concurrency inside
alloc_contig_range().

After the commit 2c7452a075d4db2dc
("mm/page_isolation.c: make start_isolate_page_range() fail if already isolated")
  > However, two subsystems (CMA and gigantic
  > huge pages for example) could attempt operations on the same range.  If
  > this happens, one thread may 'undo' the work another thread is doing.
  > This can result in pageblocks being incorrectly left marked as
  > MIGRATE_ISOLATE and therefore not available for page allocation.
The concurrency inside alloc_contig_range() was clarified.

Now we can find that hugepage and virtio call alloc_contig_range() without
any lock, thus cma_mutex is "redundant" in cma_alloc() now.

Signed-off-by: Lecopzer Chen <lecopzer.chen@xxxxxxxxxxxx>
---
 mm/cma.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/mm/cma.c b/mm/cma.c
index 7f415d7cda9f..3692a34e2353 100644
--- a/mm/cma.c
+++ b/mm/cma.c
@@ -38,7 +38,6 @@
 
 struct cma cma_areas[MAX_CMA_AREAS];
 unsigned cma_area_count;
-static DEFINE_MUTEX(cma_mutex);
 
 phys_addr_t cma_get_base(const struct cma *cma)
 {
@@ -454,10 +453,9 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
 		mutex_unlock(&cma->lock);
 
 		pfn = cma->base_pfn + (bitmap_no << cma->order_per_bit);
-		mutex_lock(&cma_mutex);
 		ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA,
 				     GFP_KERNEL | (no_warn ? __GFP_NOWARN : 0));
-		mutex_unlock(&cma_mutex);
+
 		if (ret == 0) {
 			page = pfn_to_page(pfn);
 			break;
-- 
2.18.0