rocksdb/utilities/transactions/transaction_lock_mgr.cc

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// Copyright (c) 2015, Facebook, Inc. All rights reserved.
// This source code is licensed under the BSD-style license found in the
// LICENSE file in the root directory of this source tree. An additional grant
// of patent rights can be found in the PATENTS file in the same directory.
#ifndef ROCKSDB_LITE
#ifndef __STDC_FORMAT_MACROS
#define __STDC_FORMAT_MACROS
#endif
#include "utilities/transactions/transaction_lock_mgr.h"
#include <inttypes.h>
#include <algorithm>
#include <condition_variable>
#include <functional>
#include <mutex>
#include <string>
#include <vector>
#include "rocksdb/slice.h"
#include "util/autovector.h"
#include "util/murmurhash.h"
#include "util/thread_local.h"
namespace rocksdb {
struct LockInfo {
TransactionID txn_id;
uint64_t
expiration_time; // Transaction locks are not valid after this time in ms
LockInfo(TransactionID id, uint64_t time)
: txn_id(id), expiration_time(time) {}
LockInfo(const LockInfo& lock_info)
: txn_id(lock_info.txn_id), expiration_time(lock_info.expiration_time) {}
};
struct LockMapStripe {
// Mutex must be held before modifying keys map
std::timed_mutex stripe_mutex;
// Condition Variable per stripe for waiting on a lock
std::condition_variable_any stripe_cv;
// Locked keys mapped to the info about the transactions that locked them.
// TODO(agiardullo): Explore performance of other data structures.
std::unordered_map<std::string, LockInfo> keys;
};
// Map of #num_stripes LockMapStripes
struct LockMap {
explicit LockMap(size_t num_stripes)
: num_stripes_(num_stripes), lock_map_stripes_(num_stripes) {}
LockMap(const LockMap& lock_map)
: num_stripes_(lock_map.num_stripes_), lock_map_stripes_(num_stripes_) {}
// Number of sepearate LockMapStripes to create, each with their own Mutex
const size_t num_stripes_;
// Count of keys that are currently locked in this column family.
// (Only maintained if TransactionLockMgr::max_num_locks_ is positive.)
std::atomic<int64_t> lock_cnt{0};
std::vector<LockMapStripe> lock_map_stripes_;
size_t GetStripe(const std::string& key) const;
};
namespace {
void UnrefLockMapsCache(void* ptr) {
// Called when a thread exits or a ThreadLocalPtr gets destroyed.
auto lock_maps_cache =
static_cast<std::unordered_map<uint32_t, std::shared_ptr<LockMap>>*>(ptr);
delete lock_maps_cache;
}
} // anonymous namespace
TransactionLockMgr::TransactionLockMgr(size_t default_num_stripes,
int64_t max_num_locks)
: default_num_stripes_(default_num_stripes),
max_num_locks_(max_num_locks),
lock_maps_cache_(new ThreadLocalPtr(&UnrefLockMapsCache)) {}
TransactionLockMgr::~TransactionLockMgr() {}
size_t LockMap::GetStripe(const std::string& key) const {
assert(num_stripes_ > 0);
static murmur_hash hash;
size_t stripe = hash(key) % num_stripes_;
return stripe;
}
void TransactionLockMgr::AddColumnFamily(uint32_t column_family_id) {
InstrumentedMutexLock l(&lock_map_mutex_);
if (lock_maps_.find(column_family_id) == lock_maps_.end()) {
lock_maps_.emplace(
column_family_id,
std::shared_ptr<LockMap>(new LockMap(default_num_stripes_)));
} else {
// column_family already exists in lock map
assert(false);
}
}
void TransactionLockMgr::RemoveColumnFamily(uint32_t column_family_id) {
// Remove lock_map for this column family. Since the lock map is stored
// as a shared ptr, concurrent transactions can still keep keep using it
// until they release their reference to it.
{
InstrumentedMutexLock l(&lock_map_mutex_);
auto lock_maps_iter = lock_maps_.find(column_family_id);
assert(lock_maps_iter != lock_maps_.end());
lock_maps_.erase(lock_maps_iter);
} // lock_map_mutex_
// Clear all thread-local caches
autovector<void*> local_caches;
lock_maps_cache_->Scrape(&local_caches, nullptr);
for (auto cache : local_caches) {
delete static_cast<LockMaps*>(cache);
}
}
// Look up the LockMap shared_ptr for a given column_family_id.
// Note: The LockMap is only valid as long as the caller is still holding on
// to the returned shared_ptr.
std::shared_ptr<LockMap> TransactionLockMgr::GetLockMap(
uint32_t column_family_id) {
// First check thread-local cache
if (lock_maps_cache_->Get() == nullptr) {
lock_maps_cache_->Reset(new LockMaps());
}
auto lock_maps_cache = static_cast<LockMaps*>(lock_maps_cache_->Get());
auto lock_map_iter = lock_maps_cache->find(column_family_id);
if (lock_map_iter != lock_maps_cache->end()) {
// Found lock map for this column family.
return lock_map_iter->second;
}
// Not found in local cache, grab mutex and check shared LockMaps
InstrumentedMutexLock l(&lock_map_mutex_);
lock_map_iter = lock_maps_.find(column_family_id);
if (lock_map_iter == lock_maps_.end()) {
return std::shared_ptr<LockMap>(nullptr);
} else {
// Found lock map. Store in thread-local cache and return.
std::shared_ptr<LockMap>& lock_map = lock_map_iter->second;
lock_maps_cache->insert({column_family_id, lock_map});
return lock_map;
}
}
// Returns true if this lock has expired and can be acquired by another
// transaction.
// If false, returns the number of microseconds until expiration in
// *wait_time_us, or 0 if no expiration.
bool TransactionLockMgr::IsLockExpired(const LockInfo& lock_info, Env* env,
uint64_t* wait_time_us) {
auto now = env->NowMicros();
bool expired = (lock_info.expiration_time > 0 &&
lock_info.expiration_time * 1000 <= now);
if (!expired && lock_info.expiration_time > 0 && wait_time_us != nullptr) {
// return how many microseconds until lock will be expired
*wait_time_us = (lock_info.expiration_time * 1000 - now);
}
return expired;
}
Status TransactionLockMgr::TryLock(const TransactionImpl* txn,
uint32_t column_family_id,
const std::string& key, Env* env) {
// Lookup lock map for this column family id
std::shared_ptr<LockMap> lock_map_ptr = GetLockMap(column_family_id);
LockMap* lock_map = lock_map_ptr.get();
if (lock_map == nullptr) {
char msg[255];
snprintf(msg, sizeof(msg), "Column family id not found: %" PRIu32,
column_family_id);
return Status::InvalidArgument(msg);
}
// Need to lock the mutex for the stripe that this key hashes to
size_t stripe_num = lock_map->GetStripe(key);
assert(lock_map->lock_map_stripes_.size() > stripe_num);
LockMapStripe* stripe = &lock_map->lock_map_stripes_.at(stripe_num);
LockInfo lock_info(txn->GetTxnID(), txn->GetExpirationTime());
int64_t timeout = txn->GetLockTimeout();
return AcquireWithTimeout(lock_map, stripe, key, env, timeout, lock_info);
}
// Helper function for TryLock().
Status TransactionLockMgr::AcquireWithTimeout(LockMap* lock_map,
LockMapStripe* stripe,
const std::string& key, Env* env,
int64_t timeout,
const LockInfo& lock_info) {
std::chrono::system_clock::time_point end_time;
if (timeout > 0) {
end_time =
std::chrono::system_clock::now() + std::chrono::milliseconds(timeout);
}
bool locked = true;
if (timeout == 0) {
// If timeout is 0, we do not wait to acquire the lock if it is not
// available
locked = stripe->stripe_mutex.try_lock();
} else if (timeout < 0) {
// If timeout is negative, we wait indefinitely to acquire the lock
stripe->stripe_mutex.lock();
} else {
// If timeout is positive, we attempt to acquire the lock unless we timeout
locked = stripe->stripe_mutex.try_lock_until(end_time);
}
if (!locked) {
// timeout acquiring mutex
return Status::TimedOut("Timeout Acquiring Mutex");
}
// Acquire lock if we are able to
uint64_t wait_time_us = 0;
Status result =
AcquireLocked(lock_map, stripe, key, env, lock_info, &wait_time_us);
if (!result.ok() && timeout != 0) {
// If we weren't able to acquire the lock, we will keep retrying as long
// as the
// timeout allows.
bool timed_out = false;
do {
// Check to see if the lock expires sooner than our timeout.
std::chrono::system_clock::time_point wait_time_end;
if (wait_time_us > 0 &&
(timeout < 0 ||
wait_time_us < static_cast<uint64_t>(timeout * 1000))) {
wait_time_end = std::chrono::system_clock::now() +
std::chrono::microseconds(wait_time_us);
if (timeout > 0 && wait_time_end >= end_time) {
// lock expiration time is after our timeout.
wait_time_us = 0;
}
} else {
wait_time_us = 0;
}
if (wait_time_us > 0) {
// Wait up to the locks current expiration time
stripe->stripe_cv.wait_until(stripe->stripe_mutex, wait_time_end);
} else if (timeout > 0) {
// Wait until we timeout
auto cv_status =
stripe->stripe_cv.wait_until(stripe->stripe_mutex, end_time);
if (cv_status == std::cv_status::timeout) {
timed_out = true;
// Even though we timed out, we will still make one more attempt to
// acquire lock below (it is possible the lock expired and we
// were never signaled).
}
} else {
// No wait timeout.
stripe->stripe_cv.wait(stripe->stripe_mutex);
}
result =
AcquireLocked(lock_map, stripe, key, env, lock_info, &wait_time_us);
} while (!result.ok() && !timed_out);
}
stripe->stripe_mutex.unlock();
return result;
}
// Try to lock this key after we have acquired the mutex.
// Returns the number of microseconds until expiration in *wait_time_us,
// or 0 if no expiration.
// REQUIRED: Stripe mutex must be held.
Status TransactionLockMgr::AcquireLocked(LockMap* lock_map,
LockMapStripe* stripe,
const std::string& key, Env* env,
const LockInfo& txn_lock_info,
uint64_t* wait_time_us) {
Status result;
// Check if this key is already locked
if (stripe->keys.find(key) != stripe->keys.end()) {
// Lock already held
LockInfo& lock_info = stripe->keys.at(key);
if (lock_info.txn_id != txn_lock_info.txn_id) {
// locked by another txn. Check if it's expired
if (IsLockExpired(lock_info, env, wait_time_us)) {
// lock is expired, can steal it
lock_info.txn_id = txn_lock_info.txn_id;
lock_info.expiration_time = txn_lock_info.expiration_time;
// lock_cnt does not change
} else {
result = Status::TimedOut("lock held");
}
}
} else { // Lock not held.
// Check lock limit
if (max_num_locks_ > 0 &&
lock_map->lock_cnt.load(std::memory_order_acquire) >= max_num_locks_) {
result =
Status::Busy("Failed to acquire lock due to max_num_locks limit");
} else {
// acquire lock
stripe->keys.insert({key, txn_lock_info});
// Maintain lock count if there is a limit on the number of locks
if (max_num_locks_) {
lock_map->lock_cnt++;
}
}
}
return result;
}
void TransactionLockMgr::UnLock(TransactionImpl* txn, uint32_t column_family_id,
const std::string& key, Env* env) {
std::shared_ptr<LockMap> lock_map_ptr = GetLockMap(column_family_id);
LockMap* lock_map = lock_map_ptr.get();
if (lock_map == nullptr) {
// Column Family must have been dropped.
return;
}
// Lock the mutex for the stripe that this key hashes to
size_t stripe_num = lock_map->GetStripe(key);
assert(lock_map->lock_map_stripes_.size() > stripe_num);
LockMapStripe* stripe = &lock_map->lock_map_stripes_.at(stripe_num);
TransactionID txn_id = txn->GetTxnID();
{
std::lock_guard<std::timed_mutex> lock(stripe->stripe_mutex);
const auto& iter = stripe->keys.find(key);
if (iter != stripe->keys.end() && iter->second.txn_id == txn_id) {
// Found the key we locked. unlock it.
stripe->keys.erase(iter);
if (max_num_locks_ > 0) {
// Maintain lock count if there is a limit on the number of locks.
assert(lock_map->lock_cnt.load(std::memory_order_relaxed) > 0);
lock_map->lock_cnt--;
}
} else {
// This key is either not locked or locked by someone else. This should
// only happen if the unlocking transaction has expired.
assert(txn->GetExpirationTime() > 0 &&
txn->GetExpirationTime() * 1000 < env->NowMicros());
}
} // stripe_mutex unlocked
// Signal waiting threads to retry locking
stripe->stripe_cv.notify_all();
}
void TransactionLockMgr::UnLock(const TransactionImpl* txn,
const TransactionKeyMap* key_map, Env* env) {
TransactionID txn_id = txn->GetTxnID();
for (auto& key_map_iter : *key_map) {
uint32_t column_family_id = key_map_iter.first;
auto& keys = key_map_iter.second;
std::shared_ptr<LockMap> lock_map_ptr = GetLockMap(column_family_id);
LockMap* lock_map = lock_map_ptr.get();
if (lock_map == nullptr) {
// Column Family must have been dropped.
return;
}
// Bucket keys by lock_map_ stripe
std::unordered_map<size_t, std::vector<const std::string*>> keys_by_stripe(
std::max(keys.size(), lock_map->num_stripes_));
for (auto& key_iter : keys) {
const std::string& key = key_iter.first;
size_t stripe_num = lock_map->GetStripe(key);
keys_by_stripe[stripe_num].push_back(&key);
}
// For each stripe, grab the stripe mutex and unlock all keys in this stripe
for (auto& stripe_iter : keys_by_stripe) {
size_t stripe_num = stripe_iter.first;
auto& stripe_keys = stripe_iter.second;
assert(lock_map->lock_map_stripes_.size() > stripe_num);
LockMapStripe* stripe = &lock_map->lock_map_stripes_.at(stripe_num);
{
std::lock_guard<std::timed_mutex> lock(stripe->stripe_mutex);
for (const std::string* key : stripe_keys) {
const auto& iter = stripe->keys.find(*key);
if (iter != stripe->keys.end() && iter->second.txn_id == txn_id) {
// Found the key we locked. unlock it.
stripe->keys.erase(iter);
if (max_num_locks_ > 0) {
// Maintain lock count if there is a limit on the number of locks.
assert(lock_map->lock_cnt.load(std::memory_order_relaxed) > 0);
lock_map->lock_cnt--;
}
} else {
// This key is either not locked or locked by someone else. This
// should only
// happen if the unlocking transaction has expired.
assert(txn->GetExpirationTime() > 0 &&
txn->GetExpirationTime() * 1000 < env->NowMicros());
}
}
} // stripe_mutex unlocked
// Signal waiting threads to retry locking
stripe->stripe_cv.notify_all();
}
}
}
} // namespace rocksdb
#endif // ROCKSDB_LITE