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// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:nil -*-
// vim: ts=8 sw=2 smarttab
#pragma once
#include <seastar/core/future.hh>
#include <seastar/core/circular_buffer.hh>
class read_lock {
public:
seastar::future<> lock();
void unlock();
};
class write_lock {
public:
seastar::future<> lock();
void unlock();
};
class excl_lock {
public:
seastar::future<> lock();
void unlock();
};
// promote from read to excl
class excl_lock_from_read {
public:
seastar::future<> lock();
void unlock();
};
// promote from write to excl
class excl_lock_from_write {
public:
seastar::future<> lock();
void unlock();
};
// promote from excl to excl
class excl_lock_from_excl {
public:
seastar::future<> lock();
void unlock();
};
/// shared/exclusive mutual exclusion
///
/// this lock design uses reader and writer is entirely and completely
/// independent of the conventional reader/writer lock usage. Here, what we
/// mean is that we can pipeline reads, and we can pipeline writes, but we
/// cannot allow a read while writes are in progress or a write while reads are
/// in progress. Any rmw operation is therefore exclusive.
///
/// tri_mutex is based on seastar::shared_mutex, but instead of two kinds of
/// waiters, tri_mutex keeps track of three kinds of lock users:
/// - readers
/// - writers
/// - exclusive users
class tri_mutex : private read_lock,
write_lock,
excl_lock,
excl_lock_from_read,
excl_lock_from_write,
excl_lock_from_excl
{
public:
tri_mutex() = default;
~tri_mutex();
read_lock& for_read() {
return *this;
}
write_lock& for_write() {
return *this;
}
excl_lock& for_excl() {
return *this;
}
excl_lock_from_read& excl_from_read() {
return *this;
}
excl_lock_from_write& excl_from_write() {
return *this;
}
excl_lock_from_excl& excl_from_excl() {
return *this;
}
// for shared readers
seastar::future<> lock_for_read();
bool try_lock_for_read() noexcept;
void unlock_for_read();
void promote_from_read();
void demote_to_read();
unsigned get_readers() const {
return readers;
}
// for shared writers
seastar::future<> lock_for_write(bool greedy);
bool try_lock_for_write(bool greedy) noexcept;
void unlock_for_write();
void promote_from_write();
void demote_to_write();
unsigned get_writers() const {
return writers;
}
// for exclusive users
seastar::future<> lock_for_excl();
bool try_lock_for_excl() noexcept;
void unlock_for_excl();
bool is_excl_acquired() const {
return exclusively_used;
}
bool is_acquired() const;
/// pass the provided exception to any waiting waiters
template<typename Exception>
void abort(Exception ex) {
while (!waiters.empty()) {
auto& waiter = waiters.front();
waiter.pr.set_exception(std::make_exception_ptr(ex));
waiters.pop_front();
}
}
private:
void wake();
unsigned readers = 0;
unsigned writers = 0;
bool exclusively_used = false;
enum class type_t : uint8_t {
read,
write,
exclusive,
none,
};
struct waiter_t {
waiter_t(seastar::promise<>&& pr, type_t type)
: pr(std::move(pr)), type(type)
{}
seastar::promise<> pr;
type_t type;
};
seastar::circular_buffer<waiter_t> waiters;
friend class read_lock;
friend class write_lock;
friend class excl_lock;
friend class excl_lock_from_read;
friend class excl_lock_from_write;
friend class excl_lock_from_excl;
};
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