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mutex.cr
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mutex.cr
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require "crystal/spin_lock"
# A fiber-safe mutex.
#
# Provides deadlock protection by default. Attempting to re-lock the mutex from
# the same fiber will raise an exception. Trying to unlock an unlocked mutex, or
# a mutex locked by another fiber will raise an exception.
#
# The reentrant protection maintains a lock count. Attempting to re-lock the
# mutex from the same fiber will increment the lock count. Attempting to unlock
# the counter from the same fiber will decrement the lock count, eventually
# releasing the lock when the lock count returns to 0. Attempting to unlock an
# unlocked mutex, or a mutex locked by another fiber will raise an exception.
#
# You also disable all protections with `unchecked`. Attempting to re-lock the
# mutex from the same fiber will deadlock. Any fiber can unlock the mutex, even
# if it wasn't previously locked.
class Mutex
@state = Atomic(Int32).new(0)
@mutex_fiber : Fiber?
@lock_count = 0
@queue = Deque(Fiber).new
@queue_count = Atomic(Int32).new(0)
@lock = Crystal::SpinLock.new
enum Protection
Checked
Reentrant
Unchecked
end
def initialize(@protection : Protection = :checked)
end
@[AlwaysInline]
def lock
if @state.swap(1) == 0
@mutex_fiber = Fiber.current unless @protection.unchecked?
return
end
if !@protection.unchecked? && @mutex_fiber == Fiber.current
if @protection.reentrant?
@lock_count += 1
return
else
raise "Attempt to lock a mutex recursively (deadlock)"
end
end
lock_slow
nil
end
@[NoInline]
private def lock_slow
loop do
break if try_lock
@lock.sync do
@queue_count.add(1)
if @state.get == 0
if @state.swap(1) == 0
@queue_count.add(-1)
@mutex_fiber = Fiber.current
return
end
end
@queue.push Fiber.current
end
Crystal::Scheduler.reschedule
end
@mutex_fiber = Fiber.current
nil
end
private def try_lock
i = 1000
while @state.swap(1) != 0
while @state.get != 0
Intrinsics.pause
i &-= 1
return false if i == 0
end
end
true
end
def unlock
unless @protection.unchecked?
if mutex_fiber = @mutex_fiber
unless mutex_fiber == Fiber.current
raise "Attempt to unlock a mutex locked by another fiber"
end
else
raise "Attempt to unlock a mutex which is not locked"
end
if @protection.reentrant? && @lock_count > 0
@lock_count -= 1
return
end
@mutex_fiber = nil
end
@state.lazy_set(0)
if @queue_count.get == 0
return
end
fiber = nil
@lock.sync do
if @queue_count.get == 0
return
end
if fiber = @queue.shift?
@queue_count.add(-1)
end
end
fiber.enqueue if fiber
nil
end
def synchronize
lock
begin
yield
ensure
unlock
end
end
end