lock_manager_defs.h

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#pragma once

#include <cstdint>
#include <limits>
#include <string>

#include "mongo/base/static_assert.h"
#include "mongo/base/string_data.h"
#include "mongo/config.h"
#include "mongo/platform/hash_namespace.h"

namespace mongo {

class Locker;

struct LockHead;
struct PartitionedLockHead;

enum LockMode {
    MODE_NONE = 0,
    MODE_IS = 1,
    MODE_IX = 2,
    MODE_S = 3,
    MODE_X = 4,

    LockModesCount
};

const char* modeName(LockMode mode);

const char* legacyModeName(LockMode mode);

bool isModeCovered(LockMode mode, LockMode coveringMode);

inline bool isSharedLockMode(LockMode mode) {
    return (mode == MODE_IS || mode == MODE_S);
}


enum LockResult {

    LOCK_OK,

    LOCK_WAITING,

    LOCK_TIMEOUT,

    LOCK_DEADLOCK,

    LOCK_INVALID
};


enum ResourceType {
    RESOURCE_INVALID = 0,

    RESOURCE_GLOBAL,

    RESOURCE_MMAPV1_FLUSH,

    RESOURCE_DATABASE,
    RESOURCE_COLLECTION,
    RESOURCE_METADATA,

    RESOURCE_MUTEX,

    ResourceTypesCount
};

const char* resourceTypeName(ResourceType resourceType);

class ResourceId {
    // We only use 3 bits for the resource type in the ResourceId hash
    enum { resourceTypeBits = 3 };
    MONGO_STATIC_ASSERT(ResourceTypesCount <= (1 << resourceTypeBits));

public:
    enum SingletonHashIds {
        SINGLETON_INVALID = 0,
        SINGLETON_PARALLEL_BATCH_WRITER_MODE,
        SINGLETON_GLOBAL,
        SINGLETON_MMAPV1_FLUSH,
    };

    ResourceId() : _fullHash(0) {}
    ResourceId(ResourceType type, StringData ns);
    ResourceId(ResourceType type, const std::string& ns);
    ResourceId(ResourceType type, uint64_t hashId);

    bool isValid() const {
        return getType() != RESOURCE_INVALID;
    }

    operator uint64_t() const {
        return _fullHash;
    }

    // This defines the canonical locking order, first by type and then hash id
    bool operator<(const ResourceId& rhs) const {
        return _fullHash < rhs._fullHash;
    }

    ResourceType getType() const {
        return static_cast<ResourceType>(_fullHash >> (64 - resourceTypeBits));
    }

    uint64_t getHashId() const {
        return _fullHash & (std::numeric_limits<uint64_t>::max() >> resourceTypeBits);
    }

    std::string toString() const;

private:
    uint64_t _fullHash;

    static uint64_t fullHash(ResourceType type, uint64_t hashId);

#ifdef MONGO_CONFIG_DEBUG_BUILD
    // Keep the complete namespace name for debugging purposes (TODO: this will be
    // removed once we are confident in the robustness of the lock manager).
    std::string _nsCopy;
#endif
};

#ifndef MONGO_CONFIG_DEBUG_BUILD
// Treat the resource ids as 64-bit integers in release mode in order to ensure we do
// not spend too much time doing comparisons for hashing.
MONGO_STATIC_ASSERT(sizeof(ResourceId) == sizeof(uint64_t));
#endif


// Type to uniquely identify a given locker object
typedef uint64_t LockerId;

// Hardcoded resource id for the oplog collection, which is special-cased both for resource
// acquisition purposes and for statistics reporting.
extern const ResourceId resourceIdLocalDB;
extern const ResourceId resourceIdOplog;

// Hardcoded resource id for admin db. This is to ensure direct writes to auth collections
// are serialized (see SERVER-16092)
extern const ResourceId resourceIdAdminDB;

// Hardcoded resource id for ParallelBatchWriterMode. We use the same resource type
// as resourceIdGlobal. This will also ensure the waits are reported as global, which
// is appropriate. The lock will never be contended unless the parallel batch writers
// must stop all other accesses globally. This resource must be locked before all other
// resources (including resourceIdGlobal). Replication applier threads don't take this
// lock.
// TODO: Merge this with resourceIdGlobal
extern const ResourceId resourceIdParallelBatchWriterMode;

class LockGrantNotification {
public:
    virtual ~LockGrantNotification() {}

    virtual void notify(ResourceId resId, LockResult result) = 0;
};


struct LockRequest {
    enum Status {
        STATUS_NEW,
        STATUS_GRANTED,
        STATUS_WAITING,
        STATUS_CONVERTING,

        // Counts the rest. Always insert new status types above this entry.
        StatusCount
    };

    void initNew(Locker* locker, LockGrantNotification* notify);

    // This is the Locker, which created this LockRequest. Pointer is not owned, just referenced.
    // Must outlive the LockRequest.
    //
    // Written at construction time by Locker
    // Read by LockManager on any thread
    // No synchronization
    Locker* locker;

    // Notification to be invoked when the lock is granted. Pointer is not owned, just referenced.
    // If a request is in the WAITING or CONVERTING state, must live at least until
    // LockManager::unlock is cancelled or the notification has been invoked.
    //
    // Written at construction time by Locker
    // Read by LockManager
    // No synchronization
    LockGrantNotification* notify;

    // If the request cannot be granted right away, whether to put it at the front or at the end of
    // the queue. By default, requests are put at the back. If a request is requested to be put at
    // the front, this effectively bypasses fairness. Default is FALSE.
    //
    // Written at construction time by Locker
    // Read by LockManager on any thread
    // No synchronization
    bool enqueueAtFront;

    // When this request is granted and as long as it is on the granted queue, the particular
    // resource's policy will be changed to "compatibleFirst". This means that even if there are
    // pending requests on the conflict queue, if a compatible request comes in it will be granted
    // immediately. This effectively turns off fairness.
    //
    // Written at construction time by Locker
    // Read by LockManager on any thread
    // No synchronization
    bool compatibleFirst;

    // When set, an attempt is made to execute this request using partitioned lockheads. This speeds
    // up the common case where all requested locking modes are compatible with each other, at the
    // cost of extra overhead for conflicting modes.
    //
    // Written at construction time by LockManager
    // Read by LockManager on any thread
    // No synchronization
    bool partitioned;

    // How many times has LockManager::lock been called for this request. Locks are released when
    // their recursive count drops to zero.
    //
    // Written by LockManager on Locker thread
    // Read by LockManager on Locker thread
    // Read by Locker on Locker thread
    // No synchronization
    unsigned recursiveCount;

    // Pointer to the lock to which this request belongs, or null if this request has not yet been
    // assigned to a lock or if it belongs to the PartitionedLockHead for locker (in which case
    // partitionedLock must be set). The LockHead should be alive as long as there are LockRequests
    // on it, so it is safe to have this pointer hanging around.
    //
    // Written by LockManager on any thread
    // Read by LockManager on any thread
    // Protected by LockHead bucket's mutex
    LockHead* lock;

    // Pointer to the partitioned lock to which this request belongs, or null if it is not
    // partitioned. Only one of 'lock' and 'partitionedLock' is non-NULL, and a request can only
    // transition from 'partitionedLock' to 'lock', never the other way around.
    //
    // Written by LockManager on any thread
    // Read by LockManager on any thread
    // Protected by LockHead bucket's mutex
    PartitionedLockHead* partitionedLock;

    // The linked list chain on which this request hangs off the owning lock head. The reason
    // intrusive linked list is used instead of the std::list class is to allow for entries to be
    // removed from the middle of the list in O(1) time, if they are known instead of having to
    // search for them and we cannot persist iterators, because the list can be modified while an
    // iterator is held.
    //
    // Written by LockManager on any thread
    // Read by LockManager on any thread
    // Protected by LockHead bucket's mutex
    LockRequest* prev;
    LockRequest* next;

    // The current status of this request. Always starts at STATUS_NEW.
    //
    // Written by LockManager on any thread
    // Read by LockManager on any thread
    // Protected by LockHead bucket's mutex
    Status status;

    // If this request is not granted, the mode which has been requested for this lock. If granted,
    // the mode in which it is currently granted.
    //
    // Written by LockManager on any thread
    // Read by LockManager on any thread
    // Protected by LockHead bucket's mutex
    // Read by Locker on Locker thread
    // It is safe for the Locker to read this without taking the bucket mutex provided that the
    // LockRequest status is not WAITING or CONVERTING.
    LockMode mode;

    // This value is different from MODE_NONE only if a conversion is requested for a lock and that
    // conversion cannot be immediately granted.
    //
    // Written by LockManager on any thread
    // Read by LockManager on any thread
    // Protected by LockHead bucket's mutex
    LockMode convertMode;

    // This unsigned represents the number of pending unlocks for this LockRequest. It is greater
    // than 0 when the LockRequest is participating in two-phase lock and unlock() is called on it.
    // It can be greater than 1 if this lock is participating in two-phase-lock and has been
    // converted to a different mode that also participates in two-phase-lock. unlock() may be
    // called multiple times on the same resourceId within the same WriteUnitOfWork in this case, so
    // the number of unlocks() to execute at the end of this WUOW is tracked with this unsigned.
    //
    // Written by Locker on Locker thread
    // Read by Locker on Locker thread
    // No synchronization
    unsigned unlockPending = 0;
};

const char* lockRequestStatusName(LockRequest::Status status);

}  // namespace mongo


MONGO_HASH_NAMESPACE_START
template <>
struct hash<mongo::ResourceId> {
    size_t operator()(const mongo::ResourceId& resource) const {
        return resource;
    }
};
MONGO_HASH_NAMESPACE_END