remove unneeded spinlock

Summary: This spin lock is not required, and guards quite a complex region that could easily fallback to the slow path

Reviewed By: andriigrynenko

Differential Revision: D22839268

fbshipit-source-id: 045a3562a2d4358b4c95e155a4b9958b2b7f67d4

folly/fibers/GuardPageAllocator.cpp

@@ -276,25 +276,25 @@ class CacheManager {
   std::unique_ptr<StackCacheEntry> getStackCache(
       size_t stackSize,
       size_t guardPagesPerStack) {
-    std::lock_guard<folly::SpinLock> lg(lock_);
-    if (inUse_ < kMaxInUse) {
-      ++inUse_;
-      return std::make_unique<StackCacheEntry>(stackSize, guardPagesPerStack);
-    }
-
+    auto used = inUse_.load(std::memory_order_relaxed);
+    do {
+      if (used >= kMaxInUse) {
         return nullptr;
       }
+    } while (!inUse_.compare_exchange_weak(
+        used, used + 1, std::memory_order_acquire, std::memory_order_relaxed));
+    return std::make_unique<StackCacheEntry>(stackSize, guardPagesPerStack);
+  }
 
  private:
-  folly::SpinLock lock_;
-  size_t inUse_{0};
+  std::atomic<size_t> inUse_{0};
 
   friend class StackCacheEntry;
 
   void giveBack(std::unique_ptr<StackCache> /* stackCache_ */) {
-    std::lock_guard<folly::SpinLock> lg(lock_);
-    assert(inUse_ > 0);
-    --inUse_;
+    FOLLY_MAYBE_UNUSED auto wasUsed =
+        inUse_.fetch_sub(1, std::memory_order_release);
+    assert(wasUsed > 0);
     /* Note: we can add a free list for each size bucket
        if stack re-use is important.
        In this case this needs to be a folly::Singleton