sync.cpp

// Copyright (c) 2011-2021 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#if defined(HAVE_CONFIG_H)
#include <config/bitcoin-config.h>
#endif

#include <sync.h>

#include <logging.h>
#include <tinyformat.h>
#include <util/strencodings.h>
#include <util/threadnames.h>

#include <map>
#include <mutex>
#include <set>
#include <system_error>
#include <thread>
#include <type_traits>
#include <unordered_map>
#include <utility>
#include <vector>

#ifdef DEBUG_LOCKORDER
//
// Early deadlock detection.
// Problem being solved:
//    Thread 1 locks A, then B, then C
//    Thread 2 locks D, then C, then A
//     --> may result in deadlock between the two threads, depending on when they run.
// Solution implemented here:
// Keep track of pairs of locks: (A before B), (A before C), etc.
// Complain if any thread tries to lock in a different order.
//

struct CLockLocation {
    CLockLocation(
        const char* pszName,
        const char* pszFile,
        int nLine,
        bool fTryIn,
        const std::string& thread_name)
        : fTry(fTryIn),
          mutexName(pszName),
          sourceFile(pszFile),
          m_thread_name(thread_name),
          sourceLine(nLine) {}

    std::string ToString() const
    {
        return strprintf(
            "'%s' in %s:%s%s (in thread '%s')",
            mutexName, sourceFile, sourceLine, (fTry ? " (TRY)" : ""), m_thread_name);
    }

    std::string Name() const
    {
        return mutexName;
    }

private:
    bool fTry;
    std::string mutexName;
    std::string sourceFile;
    const std::string& m_thread_name;
    int sourceLine;
};

using LockStackItem = std::pair<void*, CLockLocation>;
using LockStack = std::vector<LockStackItem>;
using LockStacks = std::unordered_map<std::thread::id, LockStack>;

using LockPair = std::pair<void*, void*>;
using LockOrders = std::map<LockPair, LockStack>;
using InvLockOrders = std::set<LockPair>;

struct LockData {
    LockStacks m_lock_stacks;
    LockOrders lockorders;
    InvLockOrders invlockorders;
    std::mutex dd_mutex;
};

LockData& GetLockData() {
    // This approach guarantees that the object is not destroyed until after its last use.
    // The operating system automatically reclaims all the memory in a program's heap when that program exits.
    // Since the ~LockData() destructor is never called, the LockData class and all
    // its subclasses must have implicitly-defined destructors.
    static LockData& lock_data = *new LockData();
    return lock_data;
}

static void potential_deadlock_detected(const LockPair& mismatch, const LockStack& s1, const LockStack& s2)
{
    LogPrintf("POTENTIAL DEADLOCK DETECTED\n");
    LogPrintf("Previous lock order was:\n");
    for (const LockStackItem& i : s1) {
        std::string prefix{};
        if (i.first == mismatch.first) {
            prefix = " (1)";
        }
        if (i.first == mismatch.second) {
            prefix = " (2)";
        }
        LogPrintf("%s %s\n", prefix, i.second.ToString());
    }

    std::string mutex_a, mutex_b;
    LogPrintf("Current lock order is:\n");
    for (const LockStackItem& i : s2) {
        std::string prefix{};
        if (i.first == mismatch.first) {
            prefix = " (1)";
            mutex_a = i.second.Name();
        }
        if (i.first == mismatch.second) {
            prefix = " (2)";
            mutex_b = i.second.Name();
        }
        LogPrintf("%s %s\n", prefix, i.second.ToString());
    }
    if (g_debug_lockorder_abort) {
        tfm::format(std::cerr, "Assertion failed: detected inconsistent lock order for %s, details in debug log.\n", s2.back().second.ToString());
        abort();
    }
    throw std::logic_error(strprintf("potential deadlock detected: %s -> %s -> %s", mutex_b, mutex_a, mutex_b));
}

static void double_lock_detected(const void* mutex, const LockStack& lock_stack)
{
    LogPrintf("DOUBLE LOCK DETECTED\n");
    LogPrintf("Lock order:\n");
    for (const LockStackItem& i : lock_stack) {
        std::string prefix{};
        if (i.first == mutex) {
            prefix = " (*)";
        }
        LogPrintf("%s %s\n", prefix, i.second.ToString());
    }
    if (g_debug_lockorder_abort) {
        tfm::format(std::cerr,
                    "Assertion failed: detected double lock for %s, details in debug log.\n",
                    lock_stack.back().second.ToString());
        abort();
    }
    throw std::logic_error("double lock detected");
}

template <typename MutexType>
static void push_lock(MutexType* c, const CLockLocation& locklocation)
{
    constexpr bool is_recursive_mutex =
        std::is_base_of<RecursiveMutex, MutexType>::value ||
        std::is_base_of<std::recursive_mutex, MutexType>::value;

    LockData& lockdata = GetLockData();
    std::lock_guard<std::mutex> lock(lockdata.dd_mutex);

    LockStack& lock_stack = lockdata.m_lock_stacks[std::this_thread::get_id()];
    lock_stack.emplace_back(c, locklocation);
    for (size_t j = 0; j < lock_stack.size() - 1; ++j) {
        const LockStackItem& i = lock_stack[j];
        if (i.first == c) {
            if (is_recursive_mutex) {
                break;
            }
            // It is not a recursive mutex and it appears in the stack two times:
            // at position `j` and at the end (which we added just before this loop).
            // Can't allow locking the same (non-recursive) mutex two times from the
            // same thread as that results in an undefined behavior.
            auto lock_stack_copy = lock_stack;
            lock_stack.pop_back();
            double_lock_detected(c, lock_stack_copy);
            // double_lock_detected() does not return.
        }

        const LockPair p1 = std::make_pair(i.first, c);
        if (lockdata.lockorders.count(p1))
            continue;

        const LockPair p2 = std::make_pair(c, i.first);
        if (lockdata.lockorders.count(p2)) {
            auto lock_stack_copy = lock_stack;
            lock_stack.pop_back();
            potential_deadlock_detected(p1, lockdata.lockorders[p2], lock_stack_copy);
            // potential_deadlock_detected() does not return.
        }

        lockdata.lockorders.emplace(p1, lock_stack);
        lockdata.invlockorders.insert(p2);
    }
}

static void pop_lock()
{
    LockData& lockdata = GetLockData();
    std::lock_guard<std::mutex> lock(lockdata.dd_mutex);

    LockStack& lock_stack = lockdata.m_lock_stacks[std::this_thread::get_id()];
    lock_stack.pop_back();
    if (lock_stack.empty()) {
        lockdata.m_lock_stacks.erase(std::this_thread::get_id());
    }
}

template <typename MutexType>
void EnterCritical(const char* pszName, const char* pszFile, int nLine, MutexType* cs, bool fTry)
{
    push_lock(cs, CLockLocation(pszName, pszFile, nLine, fTry, util::ThreadGetInternalName()));
}
template void EnterCritical(const char*, const char*, int, Mutex*, bool);
template void EnterCritical(const char*, const char*, int, RecursiveMutex*, bool);
template void EnterCritical(const char*, const char*, int, std::mutex*, bool);
template void EnterCritical(const char*, const char*, int, std::recursive_mutex*, bool);

void CheckLastCritical(void* cs, std::string& lockname, const char* guardname, const char* file, int line)
{
    LockData& lockdata = GetLockData();
    std::lock_guard<std::mutex> lock(lockdata.dd_mutex);

    const LockStack& lock_stack = lockdata.m_lock_stacks[std::this_thread::get_id()];
    if (!lock_stack.empty()) {
        const auto& lastlock = lock_stack.back();
        if (lastlock.first == cs) {
            lockname = lastlock.second.Name();
            return;
        }
    }

    LogPrintf("INCONSISTENT LOCK ORDER DETECTED\n");
    LogPrintf("Current lock order (least recent first) is:\n");
    for (const LockStackItem& i : lock_stack) {
        LogPrintf(" %s\n", i.second.ToString());
    }
    if (g_debug_lockorder_abort) {
        tfm::format(std::cerr, "%s:%s %s was not most recent critical section locked, details in debug log.\n", file, line, guardname);
        abort();
    }
    throw std::logic_error(strprintf("%s was not most recent critical section locked", guardname));
}

void LeaveCritical()
{
    pop_lock();
}

static std::string LocksHeld()
{
    LockData& lockdata = GetLockData();
    std::lock_guard<std::mutex> lock(lockdata.dd_mutex);

    const LockStack& lock_stack = lockdata.m_lock_stacks[std::this_thread::get_id()];
    std::string result;
    for (const LockStackItem& i : lock_stack)
        result += i.second.ToString() + std::string("\n");
    return result;
}

static bool LockHeld(void* mutex)
{
    LockData& lockdata = GetLockData();
    std::lock_guard<std::mutex> lock(lockdata.dd_mutex);

    const LockStack& lock_stack = lockdata.m_lock_stacks[std::this_thread::get_id()];
    for (const LockStackItem& i : lock_stack) {
        if (i.first == mutex) return true;
    }

    return false;
}

template <typename MutexType>
void AssertLockHeldInternal(const char* pszName, const char* pszFile, int nLine, MutexType* cs)
{
    if (LockHeld(cs)) return;
    tfm::format(std::cerr, "Assertion failed: lock %s not held in %s:%i; locks held:\n%s", pszName, pszFile, nLine, LocksHeld());
    abort();
}
template void AssertLockHeldInternal(const char*, const char*, int, Mutex*);
template void AssertLockHeldInternal(const char*, const char*, int, RecursiveMutex*);

template <typename MutexType>
void AssertLockNotHeldInternal(const char* pszName, const char* pszFile, int nLine, MutexType* cs)
{
    if (!LockHeld(cs)) return;
    tfm::format(std::cerr, "Assertion failed: lock %s held in %s:%i; locks held:\n%s", pszName, pszFile, nLine, LocksHeld());
    abort();
}
template void AssertLockNotHeldInternal(const char*, const char*, int, Mutex*);
template void AssertLockNotHeldInternal(const char*, const char*, int, RecursiveMutex*);

void DeleteLock(void* cs)
{
    LockData& lockdata = GetLockData();
    std::lock_guard<std::mutex> lock(lockdata.dd_mutex);
    const LockPair item = std::make_pair(cs, nullptr);
    LockOrders::iterator it = lockdata.lockorders.lower_bound(item);
    while (it != lockdata.lockorders.end() && it->first.first == cs) {
        const LockPair invitem = std::make_pair(it->first.second, it->first.first);
        lockdata.invlockorders.erase(invitem);
        lockdata.lockorders.erase(it++);
    }
    InvLockOrders::iterator invit = lockdata.invlockorders.lower_bound(item);
    while (invit != lockdata.invlockorders.end() && invit->first == cs) {
        const LockPair invinvitem = std::make_pair(invit->second, invit->first);
        lockdata.lockorders.erase(invinvitem);
        lockdata.invlockorders.erase(invit++);
    }
}

bool LockStackEmpty()
{
    LockData& lockdata = GetLockData();
    std::lock_guard<std::mutex> lock(lockdata.dd_mutex);
    const auto it = lockdata.m_lock_stacks.find(std::this_thread::get_id());
    if (it == lockdata.m_lock_stacks.end()) {
        return true;
    }
    return it->second.empty();
}

bool g_debug_lockorder_abort = true;

#endif /* DEBUG_LOCKORDER */