From c17a113eea230c21a1a8f4c4c086721ff59130da Mon Sep 17 00:00:00 2001
From: Yedidya Feldblum <yfeldblum@fb.com>
Date: Tue, 19 Jul 2016 14:38:12 -0700
Subject: [PATCH] Make the mprotect variant of asymmetricHeavyBarrier work when
 mlock fails

Summary: [Folly] Make the `mprotect` variant of `asymmetricHeavyBarrier` work when `mlock` fails.

Reviewed By: djwatson

Differential Revision: D3585948

fbshipit-source-id: c3a46884434b7f9da9caa9cf203573f9e3ce7444
---
 folly/experimental/AsymmetricMemoryBarrier.cpp | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/folly/experimental/AsymmetricMemoryBarrier.cpp b/folly/experimental/AsymmetricMemoryBarrier.cpp
index 31c34cb7..c5a5d2b8 100644
--- a/folly/experimental/AsymmetricMemoryBarrier.cpp
+++ b/folly/experimental/AsymmetricMemoryBarrier.cpp
@@ -42,16 +42,18 @@ struct DummyPageCreator {
     auto ptr = mmap(nullptr, 1, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
     checkUnixError(reinterpret_cast<uintptr_t>(ptr), "mmap");
 
-    // Lock the memory so it can't get paged out. If it gets paged out, changing
-    // its protection won't accomplish anything.
+    // Optimistically try to lock the page so it stays resident. Could make
+    // the heavy barrier faster.
     auto r = mlock(ptr, 1);
-    checkUnixError(r, "mlock");
+    if (r != 0) {
+      // Do nothing.
+    }
 
     return ptr;
   }
 };
 
-// Make sure dummy page is always initialized before shutdown
+// Make sure dummy page is always initialized before shutdown.
 DummyPageCreator dummyPageCreator;
 
 void mprotectMembarrier() {
@@ -63,9 +65,17 @@ void mprotectMembarrier() {
   std::lock_guard<std::mutex> lg(*mprotectMutex);
 
   int r = 0;
+
+  // We want to downgrade the page while it is resident. To do that, it must
+  // first be upgraded and forced to be resident.
   r = mprotect(dummyPage, 1, PROT_READ | PROT_WRITE);
   checkUnixError(r, "mprotect");
 
+  // Force the page to be resident. If it is already resident, almost no-op.
+  *static_cast<char*>(dummyPage) = 0;
+
+  // Downgrade the page. Forces a memory barrier in every core running any
+  // of the process's threads. On a sane platform.
   r = mprotect(dummyPage, 1, PROT_READ);
   checkUnixError(r, "mprotect");
 }
-- 
2.34.1