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/**
* Copyright (C) 2014 MongoDB Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License, version 3,
* as published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the GNU Affero General Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <climits> // For UINT_MAX
#include <vector>
#include "mongo/db/concurrency/lock_manager.h"
#include "mongo/db/concurrency/lock_stats.h"
#include "mongo/db/operation_context.h"
#include "mongo/stdx/thread.h"
namespace mongo {
/**
* Interface for acquiring locks. One of those objects will have to be instantiated for each
* request (transaction).
*
* Lock/unlock methods must always be called from a single thread.
*/
class Locker {
MONGO_DISALLOW_COPYING(Locker);
friend class UninterruptibleLockGuard;
public:
virtual ~Locker() {}
/**
* Returns true if this is an instance of LockerNoop. Because LockerNoop doesn't implement many
* methods, some users may need to check this first to find out what is safe to call. LockerNoop
* is only used in unittests and for a brief period at startup, so you can assume you hold the
* equivalent of a MODE_X lock when using it.
*
* TODO get rid of this once we kill LockerNoop.
*/
virtual bool isNoop() const {
return false;
}
/**
* Require global lock attempts to obtain tickets from 'reading' (for MODE_S and MODE_IS),
* and from 'writing' (for MODE_IX), which must have static lifetimes. There is no throttling
* for MODE_X, as there can only ever be a single locker using this mode. The throttling is
* intended to defend against arge drops in throughput under high load due to too much
* concurrency.
*/
static void setGlobalThrottling(class TicketHolder* reading, class TicketHolder* writing);
/**
* State for reporting the number of active and queued reader and writer clients.
*/
enum ClientState { kInactive, kActiveReader, kActiveWriter, kQueuedReader, kQueuedWriter };
/**
* Return whether client is holding any locks (active), or is queued on any locks or waiting
* for a ticket (throttled).
*/
virtual ClientState getClientState() const = 0;
virtual LockerId getId() const = 0;
/**
* Get a platform-specific thread identifier of the thread which owns the this locker for
* tracing purposes.
*/
virtual stdx::thread::id getThreadId() const = 0;
/**
* Indicate that shared locks should participate in two-phase locking for this Locker instance.
*/
virtual void setSharedLocksShouldTwoPhaseLock(bool sharedLocksShouldTwoPhaseLock) = 0;
/**
* This should be the first method invoked for a particular Locker object. It acquires the
* Global lock in the specified mode and effectively indicates the mode of the operation.
* This is what the lock modes on the global lock mean:
*
* IX - Regular write operation
* IS - Regular read operation
* S - Stops all *write* activity. Used for administrative operations (repl, etc).
* X - Stops all activity. Used for administrative operations (repl state changes,
* shutdown, etc).
*
* This method can be called recursively, but each call to lockGlobal must be accompanied
* by a call to unlockGlobal.
*
* @param opCtx OperationContext used to interrupt the lock waiting, if provided.
* @param mode Mode in which the global lock should be acquired. Also indicates the intent
* of the operation.
*
* @return LOCK_OK, if the global lock (and the flush lock, for the MMAP V1 engine) were
* acquired within the specified time bound. Otherwise, the respective failure
* code and neither lock will be acquired.
*/
virtual LockResult lockGlobal(OperationContext* opCtx, LockMode mode) = 0;
virtual LockResult lockGlobal(LockMode mode) = 0;
/**
* Requests the global lock to be acquired in the specified mode.
*
* See the comments for lockBegin/Complete for more information on the semantics.
* The deadline indicates the absolute time point when this lock acquisition will time out, if
* not yet granted. The lockGlobalBegin
* method has a deadline for use with the TicketHolder, if there is one.
*/
virtual LockResult lockGlobalBegin(OperationContext* opCtx, LockMode mode, Date_t deadline) = 0;
virtual LockResult lockGlobalBegin(LockMode mode, Date_t deadline) = 0;
/**
* Calling lockGlobalComplete without an OperationContext does not allow the lock acquisition
* to be interrupted.
*/
virtual LockResult lockGlobalComplete(OperationContext* opCtx, Date_t deadline) = 0;
virtual LockResult lockGlobalComplete(Date_t deadline) = 0;
/**
* This method is used only in the MMAP V1 storage engine, otherwise it is a no-op. See the
* comments in the implementation for more details on how MMAP V1 journaling works.
*/
virtual void lockMMAPV1Flush() = 0;
/**
* Decrements the reference count on the global lock. If the reference count on the
* global lock hits zero, the transaction is over, and unlockGlobal unlocks all other locks
* except for RESOURCE_MUTEX locks.
*
* @return true if this is the last endTransaction call (i.e., the global lock was
* released); false if there are still references on the global lock. This value
* should not be relied on and is only used for assertion purposes.
*
* @return false if the global lock is still held.
*/
virtual bool unlockGlobal() = 0;
/**
* This is only necessary for the MMAP V1 engine and in particular, the fsyncLock command
* which needs to first acquire the global lock in X-mode for truncating the journal and
* then downgrade to S before it blocks.
*
* The downgrade is necessary in order to be nice and not block readers while under
* fsyncLock.
*/
virtual void downgradeGlobalXtoSForMMAPV1() = 0;
/**
* beginWriteUnitOfWork/endWriteUnitOfWork are called at the start and end of WriteUnitOfWorks.
* They can be used to implement two-phase locking.
*/
virtual void beginWriteUnitOfWork() = 0;
virtual void endWriteUnitOfWork() = 0;
virtual bool inAWriteUnitOfWork() const = 0;
/**
* Acquires lock on the specified resource in the specified mode and returns the outcome
* of the operation. See the details for LockResult for more information on what the
* different results mean.
*
* Each successful acquisition of a lock on a given resource increments the reference count
* of the lock. Therefore, each call, which returns LOCK_OK must be matched with a
* corresponding call to unlock.
*
* @param opCtx If provided, will be used to interrupt a LOCK_WAITING state.
* @param resId Id of the resource to be locked.
* @param mode Mode in which the resource should be locked. Lock upgrades are allowed.
* @param deadline How long to wait for the lock to be granted, before
* returning LOCK_TIMEOUT. This parameter defaults to an infinite deadline.
* If Milliseconds(0) is passed, the request will return immediately, if
* the request could not be granted right away.
* @param checkDeadlock Whether to enable deadlock detection for this acquisition. This
* parameter is put in place until we can handle deadlocks at all places,
* which acquire locks.
*
* @return All LockResults except for LOCK_WAITING, because it blocks.
*/
virtual LockResult lock(OperationContext* opCtx,
ResourceId resId,
LockMode mode,
Date_t deadline = Date_t::max(),
bool checkDeadlock = false) = 0;
/**
* Calling lock without an OperationContext does not allow LOCK_WAITING states to be
* interrupted.
*/
virtual LockResult lock(ResourceId resId,
LockMode mode,
Date_t deadline = Date_t::max(),
bool checkDeadlock = false) = 0;
/**
* Downgrades the specified resource's lock mode without changing the reference count.
*/
virtual void downgrade(ResourceId resId, LockMode newMode) = 0;
/**
* Releases a lock previously acquired through a lock call. It is an error to try to
* release lock which has not been previously acquired (invariant violation).
*
* @return true if the lock was actually released; false if only the reference count was
* decremented, but the lock is still held.
*/
virtual bool unlock(ResourceId resId) = 0;
/**
* Retrieves the mode in which a lock is held or checks whether the lock held for a
* particular resource covers the specified mode.
*
* For example isLockHeldForMode will return true for MODE_S, if MODE_X is already held,
* because MODE_X covers MODE_S.
*/
virtual LockMode getLockMode(ResourceId resId) const = 0;
virtual bool isLockHeldForMode(ResourceId resId, LockMode mode) const = 0;
// These are shortcut methods for the above calls. They however check that the entire
// hierarchy is properly locked and because of this they are very expensive to call.
// Do not use them in performance critical code paths.
virtual bool isDbLockedForMode(StringData dbName, LockMode mode) const = 0;
virtual bool isCollectionLockedForMode(StringData ns, LockMode mode) const = 0;
/**
* Returns the resource that this locker is waiting/blocked on (if any). If the locker is
* not waiting for a resource the returned value will be invalid (isValid() == false).
*/
virtual ResourceId getWaitingResource() const = 0;
/**
* Describes a single lock acquisition for reporting/serialization purposes.
*/
struct OneLock {
// What lock resource is held?
ResourceId resourceId;
// In what mode is it held?
LockMode mode;
// Reporting/serialization order is by resourceId, which is the canonical locking order
bool operator<(const OneLock& rhs) const {
return resourceId < rhs.resourceId;
}
};
/**
* Returns information and locking statistics for this instance of the locker. Used to
* support the db.currentOp view. This structure is not thread-safe and ideally should
* be used only for obtaining the necessary information and then discarded instead of
* reused.
*/
struct LockerInfo {
// List of high-level locks held by this locker, sorted by ResourceId
std::vector<OneLock> locks;
// If isValid(), then what lock this particular locker is sleeping on
ResourceId waitingResource;
// Lock timing statistics
SingleThreadedLockStats stats;
};
virtual void getLockerInfo(LockerInfo* lockerInfo) const = 0;
/**
* LockSnapshot captures the state of all resources that are locked, what modes they're
* locked in, and how many times they've been locked in that mode.
*/
struct LockSnapshot {
// The global lock is handled differently from all other locks.
LockMode globalMode;
// The non-global non-flush locks held, sorted by granularity. That is, locks[i] is
// coarser or as coarse as locks[i + 1].
std::vector<OneLock> locks;
};
/**
* Retrieves all locks held by this transaction, other than RESOURCE_MUTEX locks, and what mode
* they're held in.
* Stores these locks in 'stateOut', destroying any previous state. Unlocks all locks
* held by this transaction. This functionality is used for yielding, which is
* voluntary/cooperative lock release and reacquisition in order to allow for interleaving
* of otherwise conflicting long-running operations.
*
* This functionality is also used for releasing locks on databases and collections
* when cursors are dormant and waiting for a getMore request.
*
* Returns true if locks are released. It is expected that restoreLockerImpl will be called
* in the future.
*
* Returns false if locks are not released. restoreLockState(...) does not need to be
* called in this case.
*/
virtual bool saveLockStateAndUnlock(LockSnapshot* stateOut) = 0;
/**
* Re-locks all locks whose state was stored in 'stateToRestore'.
* @param opCtx An operation context that enables the restoration to be interrupted.
*/
virtual void restoreLockState(OperationContext* opCtx, const LockSnapshot& stateToRestore) = 0;
virtual void restoreLockState(const LockSnapshot& stateToRestore) = 0;
/**
* Releases the ticket associated with the Locker. This allows locks to be held without
* contributing to reader/writer throttling.
*/
virtual void releaseTicket() = 0;
/**
* Reacquires a ticket for the Locker. This must only be called after releaseTicket(). It
* restores the ticket under its previous LockMode.
* An OperationContext is required to interrupt the ticket acquisition to prevent deadlocks.
* A dead lock is possible when a ticket is reacquired while holding a lock.
*/
virtual void reacquireTicket(OperationContext* opCtx) = 0;
//
// These methods are legacy from LockerImpl and will eventually go away or be converted to
// calls into the Locker methods
//
virtual void dump() const = 0;
virtual bool isW() const = 0;
virtual bool isR() const = 0;
virtual bool isLocked() const = 0;
virtual bool isWriteLocked() const = 0;
virtual bool isReadLocked() const = 0;
virtual bool isGlobalLockedRecursively() = 0;
/**
* Pending means we are currently trying to get a lock (could be the parallel batch writer
* lock).
*/
virtual bool hasLockPending() const = 0;
/**
* If set to false, this opts out of conflicting with replication's use of the
* ParallelBatchWriterMode lock. Code that opts-out must be ok with seeing an inconsistent view
* of data because within a batch, secondaries apply operations in a different order than on the
* primary. User operations should *never* opt out.
*/
void setShouldConflictWithSecondaryBatchApplication(bool newValue) {
_shouldConflictWithSecondaryBatchApplication = newValue;
}
bool shouldConflictWithSecondaryBatchApplication() const {
return _shouldConflictWithSecondaryBatchApplication;
}
/**
* If set to false, this opts out of the ticket mechanism. This should be used sparingly
* for special purpose threads, such as FTDC.
*/
void setShouldAcquireTicket(bool newValue) {
invariant(!isLocked());
_shouldAcquireTicket = newValue;
}
bool shouldAcquireTicket() const {
return _shouldAcquireTicket;
}
protected:
Locker() {}
/**
* The number of callers that are guarding from lock interruptions.
* When 0, all lock acquisitions are interruptible. When positive, no lock acquisitions
* are interruptible. This is only true for database and global locks. Collection locks are
* never interruptible.
*/
int _uninterruptibleLocksRequested = 0;
private:
bool _shouldConflictWithSecondaryBatchApplication = true;
bool _shouldAcquireTicket = true;
};
/**
* This class prevents lock acquisitions from being interrupted when it is in scope.
* The default behavior of acquisitions depends on the type of lock that is being requested.
* Use this in the unlikely case that waiting for a lock can't be interrupted.
*
* Lock acquisitions can still return LOCK_TIMEOUT, just not if the parent operation
* context is killed first.
*
* It is possible that multiple callers are requesting uninterruptible behavior, so the guard
* increments a counter on the Locker class to indicate how may guards are active.
*/
class UninterruptibleLockGuard {
public:
/*
* Accepts a Locker, and increments the _uninterruptibleLocksRequested. Decrements the
* counter when destoyed.
*/
explicit UninterruptibleLockGuard(Locker* locker) : _locker(locker) {
invariant(_locker);
invariant(_locker->_uninterruptibleLocksRequested >= 0);
invariant(_locker->_uninterruptibleLocksRequested < std::numeric_limits<int>::max());
_locker->_uninterruptibleLocksRequested += 1;
}
~UninterruptibleLockGuard() {
invariant(_locker->_uninterruptibleLocksRequested > 0);
_locker->_uninterruptibleLocksRequested -= 1;
}
private:
Locker* const _locker;
};
/**
* RAII-style class to opt out of replication's use of ParallelBatchWriterMode.
*/
class ShouldNotConflictWithSecondaryBatchApplicationBlock {
MONGO_DISALLOW_COPYING(ShouldNotConflictWithSecondaryBatchApplicationBlock);
public:
explicit ShouldNotConflictWithSecondaryBatchApplicationBlock(Locker* lockState)
: _lockState(lockState),
_originalShouldConflict(_lockState->shouldConflictWithSecondaryBatchApplication()) {
_lockState->setShouldConflictWithSecondaryBatchApplication(false);
}
~ShouldNotConflictWithSecondaryBatchApplicationBlock() {
_lockState->setShouldConflictWithSecondaryBatchApplication(_originalShouldConflict);
}
private:
Locker* const _lockState;
const bool _originalShouldConflict;
};
} // namespace mongo
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