path: root/lib/sanitizer_common/sanitizer_deadlock_detector.h

//===-- sanitizer_deadlock_detector.h ---------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file is a part of Sanitizer runtime.
// The deadlock detector maintains a directed graph of lock acquisitions.
// When a lock event happens, the detector checks if the locks already held by
// the current thread are reachable from the newly acquired lock.
//
// The detector can handle only a fixed amount of simultaneously live locks
// (a lock is alive if it has been locked at least once and has not been
// destroyed). When the maximal number of locks is reached the entire graph
// is flushed and the new lock epoch is started. The node ids from the old
// epochs can not be used with any of the detector methods except for
// nodeBelongsToCurrentEpoch().
//
// FIXME: this is work in progress, nothing really works yet.
//
//===----------------------------------------------------------------------===//

#ifndef SANITIZER_DEADLOCK_DETECTOR_H
#define SANITIZER_DEADLOCK_DETECTOR_H

#include "sanitizer_bvgraph.h"
#include "sanitizer_common.h"

namespace __sanitizer {

// Thread-local state for DeadlockDetector.
// It contains the locks currently held by the owning thread.
template <class BV>
class DeadlockDetectorTLS {
 public:
  // No CTOR.
  void clear() {
    bv_.clear();
    epoch_ = 0;
    n_recursive_locks = 0;
    n_all_locks_ = 0;
  }

  bool empty() const { return bv_.empty(); }

  void ensureCurrentEpoch(uptr current_epoch) {
    if (epoch_ == current_epoch) return;
    bv_.clear();
    epoch_ = current_epoch;
    n_recursive_locks = 0;
    n_all_locks_ = 0;
  }

  uptr getEpoch() const { return epoch_; }

  // Returns true if this is the first (non-recursive) acquisition of this lock.
  bool addLock(uptr lock_id, uptr current_epoch, u32 stk) {
    CHECK_EQ(epoch_, current_epoch);
    if (!bv_.setBit(lock_id)) {
      // The lock is already held by this thread, it must be recursive.
      CHECK_LT(n_recursive_locks, ARRAY_SIZE(recursive_locks));
      recursive_locks[n_recursive_locks++] = lock_id;
      return false;
    }
    CHECK_LT(n_all_locks_, ARRAY_SIZE(all_locks_with_contexts_));
    // lock_id < BV::kSize, can cast to a smaller int.
    u32 lock_id_short = static_cast<u32>(lock_id);
    LockWithContext l = {lock_id_short, stk};
    all_locks_with_contexts_[n_all_locks_++] = l;
    return true;
  }

  void removeLock(uptr lock_id) {
    if (n_recursive_locks) {
      for (sptr i = n_recursive_locks - 1; i >= 0; i--) {
        if (recursive_locks[i] == lock_id) {
          n_recursive_locks--;
          Swap(recursive_locks[i], recursive_locks[n_recursive_locks]);
          return;
        }
      }
    }
    if (!bv_.clearBit(lock_id))
      return;  // probably addLock happened before flush
    if (n_all_locks_) {
      for (sptr i = n_all_locks_ - 1; i >= 0; i--) {
        if (all_locks_with_contexts_[i].lock == static_cast<u32>(lock_id)) {
          Swap(all_locks_with_contexts_[i],
               all_locks_with_contexts_[n_all_locks_ - 1]);
          n_all_locks_--;
          break;
        }
      }
    }
  }

  u32 findLockContext(uptr lock_id) {
    for (uptr i = 0; i < n_all_locks_; i++)
      if (all_locks_with_contexts_[i].lock == static_cast<u32>(lock_id))
        return all_locks_with_contexts_[i].stk;
    return 0;
  }

  const BV &getLocks(uptr current_epoch) const {
    CHECK_EQ(epoch_, current_epoch);
    return bv_;
  }

  uptr getNumLocks() const { return n_all_locks_; }
  uptr getLock(uptr idx) const { return all_locks_with_contexts_[idx].lock; }

 private:
  BV bv_;
  uptr epoch_;
  uptr recursive_locks[64];
  uptr n_recursive_locks;
  struct LockWithContext {
    u32 lock;
    u32 stk;
  };
  LockWithContext all_locks_with_contexts_[64];
  uptr n_all_locks_;
};

// DeadlockDetector.
// For deadlock detection to work we need one global DeadlockDetector object
// and one DeadlockDetectorTLS object per evey thread.
// This class is not thread safe, all concurrent accesses should be guarded
// by an external lock.
// Most of the methods of this class are not thread-safe (i.e. should
// be protected by an external lock) unless explicitly told otherwise.
template <class BV>
class DeadlockDetector {
 public:
  typedef BV BitVector;

  uptr size() const { return g_.size(); }

  // No CTOR.
  void clear() {
    current_epoch_ = 0;
    available_nodes_.clear();
    recycled_nodes_.clear();
    g_.clear();
    n_edges_ = 0;
  }

  // Allocate new deadlock detector node.
  // If we are out of available nodes first try to recycle some.
  // If there is nothing to recycle, flush the graph and increment the epoch.
  // Associate 'data' (opaque user's object) with the new node.
  uptr newNode(uptr data) {
    if (!available_nodes_.empty())
      return getAvailableNode(data);
    if (!recycled_nodes_.empty()) {
      for (sptr i = n_edges_ - 1; i >= 0; i--) {
        if (recycled_nodes_.getBit(edges_[i].from) ||
            recycled_nodes_.getBit(edges_[i].to)) {
          Swap(edges_[i], edges_[n_edges_ - 1]);
          n_edges_--;
        }
      }
      CHECK(available_nodes_.empty());
      // removeEdgesFrom was called in removeNode.
      g_.removeEdgesTo(recycled_nodes_);
      available_nodes_.setUnion(recycled_nodes_);
      recycled_nodes_.clear();
      return getAvailableNode(data);
    }
    // We are out of vacant nodes. Flush and increment the current_epoch_.
    current_epoch_ += size();
    recycled_nodes_.clear();
    available_nodes_.setAll();
    g_.clear();
    n_edges_ = 0;
    return getAvailableNode(data);
  }

  // Get data associated with the node created by newNode().
  uptr getData(uptr node) const { return data_[nodeToIndex(node)]; }

  bool nodeBelongsToCurrentEpoch(uptr node) {
    return node && (node / size() * size()) == current_epoch_;
  }

  void removeNode(uptr node) {
    uptr idx = nodeToIndex(node);
    CHECK(!available_nodes_.getBit(idx));
    CHECK(recycled_nodes_.setBit(idx));
    g_.removeEdgesFrom(idx);
  }

  void ensureCurrentEpoch(DeadlockDetectorTLS<BV> *dtls) {
    dtls->ensureCurrentEpoch(current_epoch_);
  }

  // Returns true if there is a cycle in the graph after this lock event.
  // Ideally should be called before the lock is acquired so that we can
  // report a deadlock before a real deadlock happens.
  bool onLockBefore(DeadlockDetectorTLS<BV> *dtls, uptr cur_node) {
    ensureCurrentEpoch(dtls);
    uptr cur_idx = nodeToIndex(cur_node);
    return g_.isReachable(cur_idx, dtls->getLocks(current_epoch_));
  }

  u32 findLockContext(DeadlockDetectorTLS<BV> *dtls, uptr node) {
    return dtls->findLockContext(nodeToIndex(node));
  }

  // Add cur_node to the set of locks held currently by dtls.
  void onLockAfter(DeadlockDetectorTLS<BV> *dtls, uptr cur_node, u32 stk = 0) {
    ensureCurrentEpoch(dtls);
    uptr cur_idx = nodeToIndex(cur_node);
    dtls->addLock(cur_idx, current_epoch_, stk);
  }

  // Experimental *racy* fast path function.
  // Returns true if all edges from the currently held locks to cur_node exist.
  bool hasAllEdges(DeadlockDetectorTLS<BV> *dtls, uptr cur_node) {
    uptr local_epoch = dtls->getEpoch();
    // Read from current_epoch_ is racy.
    if (cur_node && local_epoch == current_epoch_ &&
        local_epoch == nodeToEpoch(cur_node)) {
      uptr cur_idx = nodeToIndexUnchecked(cur_node);
      for (uptr i = 0, n = dtls->getNumLocks(); i < n; i++) {
        if (!g_.hasEdge(dtls->getLock(i), cur_idx))
          return false;
      }
      return true;
    }
    return false;
  }

  // Adds edges from currently held locks to cur_node,
  // returns the number of added edges, and puts the sources of added edges
  // into added_edges[].
  // Should be called before onLockAfter.
  uptr addEdges(DeadlockDetectorTLS<BV> *dtls, uptr cur_node, u32 stk,
                int unique_tid) {
    ensureCurrentEpoch(dtls);
    uptr cur_idx = nodeToIndex(cur_node);
    uptr added_edges[40];
    uptr n_added_edges = g_.addEdges(dtls->getLocks(current_epoch_), cur_idx,
                                     added_edges, ARRAY_SIZE(added_edges));
    for (uptr i = 0; i < n_added_edges; i++) {
      if (n_edges_ < ARRAY_SIZE(edges_)) {
        Edge e = {(u16)added_edges[i], (u16)cur_idx,
                  dtls->findLockContext(added_edges[i]), stk,
                  unique_tid};
        edges_[n_edges_++] = e;
      }
    }
    return n_added_edges;
  }

  bool findEdge(uptr from_node, uptr to_node, u32 *stk_from, u32 *stk_to,
                int *unique_tid) {
    uptr from_idx = nodeToIndex(from_node);
    uptr to_idx = nodeToIndex(to_node);
    for (uptr i = 0; i < n_edges_; i++) {
      if (edges_[i].from == from_idx && edges_[i].to == to_idx) {
        *stk_from = edges_[i].stk_from;
        *stk_to = edges_[i].stk_to;
        *unique_tid = edges_[i].unique_tid;
        return true;
      }
    }
    return false;
  }

  // Test-only function. Handles the before/after lock events,
  // returns true if there is a cycle.
  bool onLock(DeadlockDetectorTLS<BV> *dtls, uptr cur_node, u32 stk = 0) {
    ensureCurrentEpoch(dtls);
    bool is_reachable = !isHeld(dtls, cur_node) && onLockBefore(dtls, cur_node);
    addEdges(dtls, cur_node, stk, 0);
    onLockAfter(dtls, cur_node, stk);
    return is_reachable;
  }

  // Handles the try_lock event, returns false.
  // When a try_lock event happens (i.e. a try_lock call succeeds) we need
  // to add this lock to the currently held locks, but we should not try to
  // change the lock graph or to detect a cycle.  We may want to investigate
  // whether a more aggressive strategy is possible for try_lock.
  bool onTryLock(DeadlockDetectorTLS<BV> *dtls, uptr cur_node, u32 stk = 0) {
    ensureCurrentEpoch(dtls);
    uptr cur_idx = nodeToIndex(cur_node);
    dtls->addLock(cur_idx, current_epoch_, stk);
    return false;
  }

  // Returns true iff dtls is empty (no locks are currently held) and we can
  // add the node to the currently held locks w/o chanding the global state.
  // This operation is thread-safe as it only touches the dtls.
  bool onFirstLock(DeadlockDetectorTLS<BV> *dtls, uptr node, u32 stk = 0) {
    if (!dtls->empty()) return false;
    if (dtls->getEpoch() && dtls->getEpoch() == nodeToEpoch(node)) {
      dtls->addLock(nodeToIndexUnchecked(node), nodeToEpoch(node), stk);
      return true;
    }
    return false;
  }

  // Finds a path between the lock 'cur_node' (currently not held in dtls)
  // and some currently held lock, returns the length of the path
  // or 0 on failure.
  uptr findPathToLock(DeadlockDetectorTLS<BV> *dtls, uptr cur_node, uptr *path,
                      uptr path_size) {
    tmp_bv_.copyFrom(dtls->getLocks(current_epoch_));
    uptr idx = nodeToIndex(cur_node);
    CHECK(!tmp_bv_.getBit(idx));
    uptr res = g_.findShortestPath(idx, tmp_bv_, path, path_size);
    for (uptr i = 0; i < res; i++)
      path[i] = indexToNode(path[i]);
    if (res)
      CHECK_EQ(path[0], cur_node);
    return res;
  }

  // Handle the unlock event.
  // This operation is thread-safe as it only touches the dtls.
  void onUnlock(DeadlockDetectorTLS<BV> *dtls, uptr node) {
    if (dtls->getEpoch() == nodeToEpoch(node))
      dtls->removeLock(nodeToIndexUnchecked(node));
  }

  // Tries to handle the lock event w/o writing to global state.
  // Returns true on success.
  // This operation is thread-safe as it only touches the dtls
  // (modulo racy nature of hasAllEdges).
  bool onLockFast(DeadlockDetectorTLS<BV> *dtls, uptr node, u32 stk = 0) {
    if (hasAllEdges(dtls, node)) {
      dtls->addLock(nodeToIndexUnchecked(node), nodeToEpoch(node), stk);
      return true;
    }
    return false;
  }

  bool isHeld(DeadlockDetectorTLS<BV> *dtls, uptr node) const {
    return dtls->getLocks(current_epoch_).getBit(nodeToIndex(node));
  }

  uptr testOnlyGetEpoch() const { return current_epoch_; }
  bool testOnlyHasEdge(uptr l1, uptr l2) {
    return g_.hasEdge(nodeToIndex(l1), nodeToIndex(l2));
  }
  // idx1 and idx2 are raw indices to g_, not lock IDs.
  bool testOnlyHasEdgeRaw(uptr idx1, uptr idx2) {
    return g_.hasEdge(idx1, idx2);
  }

  void Print() {
    for (uptr from = 0; from < size(); from++)
      for (uptr to = 0; to < size(); to++)
        if (g_.hasEdge(from, to))
          Printf("  %zx => %zx\n", from, to);
  }

 private:
  void check_idx(uptr idx) const { CHECK_LT(idx, size()); }

  void check_node(uptr node) const {
    CHECK_GE(node, size());
    CHECK_EQ(current_epoch_, nodeToEpoch(node));
  }

  uptr indexToNode(uptr idx) const {
    check_idx(idx);
    return idx + current_epoch_;
  }

  uptr nodeToIndexUnchecked(uptr node) const { return node % size(); }

  uptr nodeToIndex(uptr node) const {
    check_node(node);
    return nodeToIndexUnchecked(node);
  }

  uptr nodeToEpoch(uptr node) const { return node / size() * size(); }

  uptr getAvailableNode(uptr data) {
    uptr idx = available_nodes_.getAndClearFirstOne();
    data_[idx] = data;
    return indexToNode(idx);
  }

  struct Edge {
    u16 from;
    u16 to;
    u32 stk_from;
    u32 stk_to;
    int unique_tid;
  };

  uptr current_epoch_;
  BV available_nodes_;
  BV recycled_nodes_;
  BV tmp_bv_;
  BVGraph<BV> g_;
  uptr data_[BV::kSize];
  Edge edges_[BV::kSize * 32];
  uptr n_edges_;
};

} // namespace __sanitizer

#endif // SANITIZER_DEADLOCK_DETECTOR_H