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* Copyright (C) 2013 Cloudius Systems, Ltd.
* This work is open source software, licensed under the terms of the
* BSD license as described in the LICENSE file in the top-level directory.
// A lock-free mutex implementation, based on the combination of two basic
// techniques:
// 1. Our lock-free multi-producer single-consumer queue technique
// (see lockfree/queue-mpsc.hh)
// 2. The "responsibility hand-off" (RHO) protocol described in the 2007 paper
// "Blocking without Locking or LFTHREADS: A lock-free thread library"
// by Anders Gidenstam and Marina Papatriantafilou.
// The operation and correctness of the RHO protocol is discussed in the
// aforementioned G&P 2007 paper, so we will avoid lengthy comments about it
// below, except where we differ from G&P.
// One especially important issue that we do need to justify is:
// Our lockfree queue implementation assumes that there cannot be two
// concurrent pop()s. We claim that this is true in the RHO protocol because:
// 1. We have pop() calls at two places:
// (A) In unlock(), after decrementing count and outside a handoff (=null)
// (B) in lock(), after picking up a handoff.
// 2. We can't have two threads at (A) at the same time, because one thread
// at (A) means another thread thread was just in lock() (because count>0),
// but currently running lock()s cannot complete (and get to unlock and A)
// until somebody will wake them it (and this is what we're trying to show
// is impossible), and news lock()s will likewise wait because the waiting
// lock() is keeping count>0.
// 3. While one lock() is at (B), we cannot have another thread at (A) or (B):
// This is because in (B) we only pop() after picking a handoff, so other
// lock()s cannot reach (B) (they did not pick the handoff, we did), and
// unlock cannot be at (A) because it only reaches (A) before making the
// handoff of after taking it back - and we know it didn't because we took
// the handoff.
// Another difference from our implementation from G&P is the content of the
// handoff token. G&P use the processor ID, but remark that it is not enough
// because of the ABA problem (it is possible while a CPU running lock() is
// paused, another one finishes unlock(), and then succeeds in another lock()
// and then comes a different unlock() with its unrelated handoff) and suggest
// to add a per-processor sequence number. Instead, we just used a per-mutex
// sequence number. As long as one CPU does not pause for a long enough
// duration for our (currently 32-bit) sequence number to wrap, we won't have
// a problem. A per-mutex sequence number is slower than a per-cpu one, but
// I doubt this will make a practical difference.
#include <atomic>
#include <lockfree/queue-mpsc.hh>
// we don't want to include <sched.hh> because that includes a bunch of things
// which eventually, recursively, use mutexes.
// We also can't include <osv/wait_record.hh>, as that includes <sched.hh>.
namespace sched {
class thread;
struct wait_record;
namespace lockfree {
class mutex {
std::atomic<int> count;
// "owner" and "depth" are only used for implementing a recursive mutex.
// "depth" is not an atomic variable - only the lock-owning thread sets
// and reads its own depth. "owner" is atomic - one thread doing lock()
// needs to read the current owner possibly set by another thread - but
// it can be accessed with relaxed memory ordering.
unsigned int depth;
std::atomic<sched::thread *> owner;
queue_mpsc<wait_record> waitqueue;
std::atomic<unsigned int> handoff;
unsigned int sequence;
// Note: mutex's constructor just initializes the whole structure to
// zero, and its destructor does nothing. This is useful to know when
// allocating a mutex in C.
constexpr mutex() : count(0), depth(0), owner(nullptr), waitqueue(), handoff(0), sequence(0) { }
~mutex() { /*assert(count==0);*/ }
void lock();
bool try_lock();
void unlock();
bool owned() const;
// getdepth() should only be used by the thread holding the lock
inline unsigned int getdepth() const { return depth; }
// For wait morphing. Do not use unless you know what you are doing :-)
void send_lock(wait_record *wr);
bool send_lock_unless_already_waiting(wait_record *wr);
void receive_lock();