Introduce 'markedForEviction' state for the Item. (#183)

Summary:
It is similar to 'moving' but requires ref count to be 0.

An item which is marked for eviction causes all incRef() calls to that item to fail.

This will be used to ensure that once item is selected for eviction, no one can interfere and prevent the eviction from suceeding.

'markedForEviction' relies on the same 'exlusive' bit as the 'moving' state. To distinguish between those two states, 'moving' add 1 to the refCount. This is hidden from the user, so getRefCount() will not return that extra ref.

Pull Request resolved: #183

Test Plan:
flatmemcache shadow result:

It seems there is a slight latency regression in allocation (~1 micro second)
https://fburl.com/ods/u3isn8b4
It's probably small enough to ignore. But we can come back if this starts showing up in large scale in production.

Imported from GitHub, without a `Test Plan:` line.

Reviewed By: therealgymmy

Differential Revision: D42089974

Pulled By: haowu14

fbshipit-source-id: 90477faa9443080a7ddefa37a524965a04b6f084

Author: igchor

cachelib/allocator/CacheAllocator-inl.h

@@ -832,28 +832,29 @@ CacheAllocator<CacheTrait>::releaseBackToAllocator(Item& it,
 
       removeFromMMContainer(*head);
 
-      // If this chained item is marked as exclusive, we will not free it.
-      // We must capture the exclusive state before we do the decRef when
+      // If this chained item is marked as moving, we will not free it.
+      // We must capture the moving state before we do the decRef when
       // we know the item must still be valid
-      const bool wasExclusive = head->isExclusive();
+      const bool wasMoving = head->isMoving();
+      XDCHECK(!head->isMarkedForEviction());
 
       // Decref and check if we were the last reference. Now if the item
-      // was marked exclusive, after decRef, it will be free to be released
+      // was marked moving, after decRef, it will be free to be released
       // by slab release thread
       const auto childRef = head->decRef();
 
-      // If the item is already exclusive and we already decremented the
+      // If the item is already moving and we already decremented the
       // refcount, we don't need to free this item. We'll let the slab
       // release thread take care of that
-      if (!wasExclusive) {
+      if (!wasMoving) {
         if (childRef != 0) {
           throw std::runtime_error(folly::sformat(
               "chained item refcount is not zero. We cannot proceed! "
               "Ref: {}, Chained Item: {}",
               childRef, head->toString()));
         }
 
-        // Item is not exclusive and refcount is 0, we can proceed to
+        // Item is not moving and refcount is 0, we can proceed to
         // free it or recylce the memory
         if (head == toRecycle) {
           XDCHECK(ReleaseRes::kReleased != res);
@@ -881,9 +882,12 @@ CacheAllocator<CacheTrait>::releaseBackToAllocator(Item& it,
 }
 
 template <typename CacheTrait>
-void CacheAllocator<CacheTrait>::incRef(Item& it) {
-  it.incRef();
-  ++handleCount_.tlStats();
+bool CacheAllocator<CacheTrait>::incRef(Item& it) {
+  if (it.incRef()) {
+    ++handleCount_.tlStats();
+    return true;
+  }
+  return false;
 }
 
 template <typename CacheTrait>
@@ -903,8 +907,12 @@ CacheAllocator<CacheTrait>::acquire(Item* it) {
 
   SCOPE_FAIL { stats_.numRefcountOverflow.inc(); };
 
-  incRef(*it);
-  return WriteHandle{it, *this};
+  if (LIKELY(incRef(*it))) {
+    return WriteHandle{it, *this};
+  } else {
+    // item is being evicted
+    return WriteHandle{};
+  }
 }
 
 template <typename CacheTrait>
@@ -1179,7 +1187,7 @@ bool CacheAllocator<CacheTrait>::moveChainedItem(ChainedItem& oldItem,
 
   // This item has been unlinked from its parent and we're the only
   // owner of it, so we're done here
-  if (!oldItem.isInMMContainer() || oldItem.isOnlyExclusive()) {
+  if (!oldItem.isInMMContainer() || oldItem.isOnlyMoving()) {
     return false;
   }
 
@@ -1210,7 +1218,7 @@ bool CacheAllocator<CacheTrait>::moveChainedItem(ChainedItem& oldItem,
 
   // In case someone else had removed this chained item from its parent by now
   // So we check again to see if the it has been unlinked from its parent
-  if (!oldItem.isInMMContainer() || oldItem.isOnlyExclusive()) {
+  if (!oldItem.isInMMContainer() || oldItem.isOnlyMoving()) {
     return false;
   }
 
@@ -1226,7 +1234,7 @@ bool CacheAllocator<CacheTrait>::moveChainedItem(ChainedItem& oldItem,
   // parent's chain and the MMContainer.
   auto oldItemHandle =
       replaceChainedItemLocked(oldItem, std::move(newItemHdl), *parentHandle);
-  XDCHECK(oldItemHandle->isExclusive());
+  XDCHECK(oldItemHandle->isMoving());
   XDCHECK(!oldItemHandle->isInMMContainer());
 
   return true;
@@ -1255,7 +1263,7 @@ CacheAllocator<CacheTrait>::findEviction(PoolId pid, ClassId cid) {
             : toRecycle;
 
     // make sure no other thead is evicting the item
-    if (candidate->getRefCount() != 0 || !candidate->markExclusive()) {
+    if (candidate->getRefCount() != 0 || !candidate->markMoving()) {
       ++itr;
       continue;
     }
@@ -1270,11 +1278,11 @@ CacheAllocator<CacheTrait>::findEviction(PoolId pid, ClassId cid) {
               ? advanceIteratorAndTryEvictChainedItem(itr)
               : advanceIteratorAndTryEvictRegularItem(mmContainer, itr);
       evictionSuccessful = toReleaseHandle != nullptr;
-      // destroy toReleseHandle. The item won't be released to allocator
-      // since we marked it as exclusive.
+      // destroy toReleaseHandle. The item won't be released to allocator
+      // since we marked for eviction.
     }
 
-    const auto ref = candidate->unmarkExclusive();
+    const auto ref = candidate->unmarkMoving();
     if (ref == 0u) {
       // Invalidate iterator since later on we may use this mmContainer
       // again, which cannot be done unless we drop this iterator
@@ -2361,7 +2369,7 @@ void CacheAllocator<CacheTrait>::releaseSlabImpl(
     // Need to mark an item for release before proceeding
     // If we can't mark as moving, it means the item is already freed
     const bool isAlreadyFreed =
-        !markExclusiveForSlabRelease(releaseContext, alloc, throttler);
+        !markMovingForSlabRelease(releaseContext, alloc, throttler);
     if (isAlreadyFreed) {
       continue;
     }
@@ -2406,8 +2414,8 @@ bool CacheAllocator<CacheTrait>::moveForSlabRelease(
     stats_.numMoveAttempts.inc();
 
     // Nothing to move and the key is likely also bogus for chained items.
-    if (oldItem.isOnlyExclusive()) {
-      oldItem.unmarkExclusive();
+    if (oldItem.isOnlyMoving()) {
+      oldItem.unmarkMoving();
       const auto res =
           releaseBackToAllocator(oldItem, RemoveContext::kNormal, false);
       XDCHECK(res == ReleaseRes::kReleased);
@@ -2446,7 +2454,7 @@ bool CacheAllocator<CacheTrait>::moveForSlabRelease(
   // that's identical to this one to replace it. Here we just need to wait
   // until all users have dropped the item handles before we can proceed.
   startTime = util::getCurrentTimeSec();
-  while (!oldItem.isOnlyExclusive()) {
+  while (!oldItem.isOnlyMoving()) {
     throttleWith(throttler, [&] {
       XLOGF(WARN,
             "Spent {} seconds, slab release still waiting for refcount to "
@@ -2500,8 +2508,8 @@ CacheAllocator<CacheTrait>::allocateNewItemForOldItem(const Item& oldItem) {
       return {};
     }
 
-    // Set up the destination for the move. Since oldChainedItem would have
-    // the exclusive bit set, it won't be picked for eviction.
+    // Set up the destination for the move. Since oldChainedItem would be
+    // marked as moving, it won't be picked for eviction.
     auto newItemHdl =
         allocateChainedItemInternal(parentHandle, oldChainedItem.getSize());
     if (!newItemHdl) {
@@ -2553,7 +2561,7 @@ bool CacheAllocator<CacheTrait>::tryMovingForSlabRelease(
       // item is still valid.
       const std::string parentKey =
           oldItem.asChainedItem().getParentItem(compressor_).getKey().str();
-      if (oldItem.isOnlyExclusive()) {
+      if (oldItem.isOnlyMoving()) {
         // If chained item no longer has a refcount, its parent is already
         // being released, so we abort this try to moving.
         return false;
@@ -2583,10 +2591,10 @@ void CacheAllocator<CacheTrait>::evictForSlabRelease(
     stats_.numEvictionAttempts.inc();
 
     // if the item is already in a state where only the exclusive bit is set,
-    // nothing needs to be done. We simply need to unmark exclusive bit and free
+    // nothing needs to be done. We simply need to call unmarkMoving and free
     // the item.
-    if (item.isOnlyExclusive()) {
-      item.unmarkExclusive();
+    if (item.isOnlyMoving()) {
+      item.unmarkMoving();
       const auto res =
           releaseBackToAllocator(item, RemoveContext::kNormal, false);
       XDCHECK(ReleaseRes::kReleased == res);
@@ -2617,7 +2625,7 @@ void CacheAllocator<CacheTrait>::evictForSlabRelease(
       stats_.numEvictionSuccesses.inc();
 
       // we have the last handle. no longer need to hold on to the exclusive bit
-      item.unmarkExclusive();
+      item.unmarkMoving();
 
       // manually decrement the refcount to call releaseBackToAllocator
       const auto ref = decRef(*owningHandle);
@@ -2629,7 +2637,7 @@ void CacheAllocator<CacheTrait>::evictForSlabRelease(
     }
 
     if (shutDownInProgress_) {
-      item.unmarkExclusive();
+      item.unmarkMoving();
       allocator_->abortSlabRelease(ctx);
       throw exception::SlabReleaseAborted(
           folly::sformat("Slab Release aborted while trying to evict"
@@ -2775,9 +2783,9 @@ CacheAllocator<CacheTrait>::advanceIteratorAndTryEvictChainedItem(
 template <typename CacheTrait>
 typename CacheAllocator<CacheTrait>::WriteHandle
 CacheAllocator<CacheTrait>::evictNormalItemForSlabRelease(Item& item) {
-  XDCHECK(item.isExclusive());
+  XDCHECK(item.isMoving());
 
-  if (item.isOnlyExclusive()) {
+  if (item.isOnlyMoving()) {
     return WriteHandle{};
   }
 
@@ -2789,7 +2797,7 @@ CacheAllocator<CacheTrait>::evictNormalItemForSlabRelease(Item& item) {
 
   // We remove the item from both access and mm containers. It doesn't matter
   // if someone else calls remove on the item at this moment, the item cannot
-  // be freed as long as we have the exclusive bit set.
+  // be freed as long as it's marked for eviction.
   auto handle = accessContainer_->removeIf(item, std::move(predicate));
 
   if (!handle) {
@@ -2813,7 +2821,7 @@ CacheAllocator<CacheTrait>::evictNormalItemForSlabRelease(Item& item) {
 template <typename CacheTrait>
 typename CacheAllocator<CacheTrait>::WriteHandle
 CacheAllocator<CacheTrait>::evictChainedItemForSlabRelease(ChainedItem& child) {
-  XDCHECK(child.isExclusive());
+  XDCHECK(child.isMoving());
 
   // We have the child marked as moving, but dont know anything about the
   // state of the parent. Unlike the case of regular eviction where we are
@@ -2835,7 +2843,7 @@ CacheAllocator<CacheTrait>::evictChainedItemForSlabRelease(ChainedItem& child) {
   // check if the child is still in mmContainer and the expected parent is
   // valid under the chained item lock.
   if (expectedParent.getKey() != parentKey || !child.isInMMContainer() ||
-      child.isOnlyExclusive() ||
+      child.isOnlyMoving() ||
       &expectedParent != &child.getParentItem(compressor_) ||
       !expectedParent.isAccessible() || !expectedParent.hasChainedItem()) {
     return {};
@@ -2890,14 +2898,14 @@ CacheAllocator<CacheTrait>::evictChainedItemForSlabRelease(ChainedItem& child) {
 
   // In case someone else had removed this chained item from its parent by now
   // So we check again to see if it has been unlinked from its parent
-  if (!child.isInMMContainer() || child.isOnlyExclusive()) {
+  if (!child.isInMMContainer() || child.isOnlyMoving()) {
     return {};
   }
 
   // check after removing from the MMContainer that the parent is still not
   // being marked as moving. If parent is moving, it will release the child
   // item and we will wait for that.
-  if (parentHandle->isExclusive()) {
+  if (parentHandle->isMoving()) {
     return {};
   }
 
@@ -2930,7 +2938,7 @@ bool CacheAllocator<CacheTrait>::removeIfExpired(const ReadHandle& handle) {
 }
 
 template <typename CacheTrait>
-bool CacheAllocator<CacheTrait>::markExclusiveForSlabRelease(
+bool CacheAllocator<CacheTrait>::markMovingForSlabRelease(
     const SlabReleaseContext& ctx, void* alloc, util::Throttler& throttler) {
   // MemoryAllocator::processAllocForRelease will execute the callback
   // if the item is not already free. So there are three outcomes here:
@@ -2949,7 +2957,7 @@ bool CacheAllocator<CacheTrait>::markExclusiveForSlabRelease(
     // Since this callback is executed, the item is not yet freed
     itemFreed = false;
     Item* item = static_cast<Item*>(memory);
-    if (item->markExclusive()) {
+    if (item->markMoving()) {
       markedMoving = true;
     }
   };

cachelib/allocator/CacheAllocator.h

@@ -1308,7 +1308,7 @@ class CacheAllocator : public CacheBase {
 
  private:
   // wrapper around Item's refcount and active handle tracking
-  FOLLY_ALWAYS_INLINE void incRef(Item& it);
+  FOLLY_ALWAYS_INLINE bool incRef(Item& it);
   FOLLY_ALWAYS_INLINE RefcountWithFlags::Value decRef(Item& it);
 
   // drops the refcount and if needed, frees the allocation back to the memory
@@ -1756,9 +1756,9 @@ class CacheAllocator : public CacheBase {
 
   // @return  true when successfully marked as moving,
   //          fasle when this item has already been freed
-  bool markExclusiveForSlabRelease(const SlabReleaseContext& ctx,
-                                   void* alloc,
-                                   util::Throttler& throttler);
+  bool markMovingForSlabRelease(const SlabReleaseContext& ctx,
+                                void* alloc,
+                                util::Throttler& throttler);
 
   // "Move" (by copying) the content in this item to another memory
   // location by invoking the move callback.
@@ -1937,7 +1937,7 @@ class CacheAllocator : public CacheBase {
   }
 
   static bool parentEvictForSlabReleasePredicate(const Item& item) {
-    return item.getRefCount() == 1 && !item.isExclusive();
+    return item.getRefCount() == 1 && !item.isMoving();
   }
 
   std::unique_ptr<Deserializer> createDeserializer();