diff options
Diffstat (limited to 'activesupport/lib/active_support/concurrency')
-rw-r--r-- | activesupport/lib/active_support/concurrency/share_lock.rb | 88 |
1 files changed, 66 insertions, 22 deletions
diff --git a/activesupport/lib/active_support/concurrency/share_lock.rb b/activesupport/lib/active_support/concurrency/share_lock.rb index ca48164c54..54244317e4 100644 --- a/activesupport/lib/active_support/concurrency/share_lock.rb +++ b/activesupport/lib/active_support/concurrency/share_lock.rb @@ -6,12 +6,6 @@ module ActiveSupport # A share/exclusive lock, otherwise known as a read/write lock. # # https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock - #-- - # Note that a pending Exclusive lock attempt does not block incoming - # Share requests (i.e., we are "read-preferring"). That seems - # consistent with the behavior of "loose" upgrades, but may be the - # wrong choice otherwise: it nominally reduces the possibility of - # deadlock by risking starvation instead. class ShareLock include MonitorMixin @@ -48,17 +42,11 @@ module ActiveSupport def start_exclusive(purpose: nil, compatible: [], no_wait: false) synchronize do unless @exclusive_thread == Thread.current - if busy?(purpose) + if busy_for_exclusive?(purpose) return false if no_wait - loose_shares = @sharing.delete(Thread.current) - @waiting[Thread.current] = compatible if loose_shares - - begin - @cv.wait_while { busy?(purpose) } - ensure - @waiting.delete Thread.current - @sharing[Thread.current] = loose_shares if loose_shares + yield_shares(purpose: purpose, compatible: compatible, block_share: true) do + @cv.wait_while { busy_for_exclusive?(purpose) } end end @exclusive_thread = Thread.current @@ -71,13 +59,19 @@ module ActiveSupport # Relinquish the exclusive lock. Must only be called by the thread # that called start_exclusive (and currently holds the lock). - def stop_exclusive + def stop_exclusive(compatible: []) synchronize do raise "invalid unlock" if @exclusive_thread != Thread.current @exclusive_depth -= 1 if @exclusive_depth == 0 @exclusive_thread = nil + + if eligible_waiters?(compatible) + yield_shares(compatible: compatible, block_share: true) do + @cv.wait_while { @exclusive_thread || eligible_waiters?(compatible) } + end + end @cv.broadcast end end @@ -85,8 +79,16 @@ module ActiveSupport def start_sharing synchronize do - if @exclusive_thread && @exclusive_thread != Thread.current + if @sharing[Thread.current] > 0 || @exclusive_thread == Thread.current + # We already hold a lock; nothing to wait for + elsif @waiting[Thread.current] + # We're nested inside a +yield_shares+ call: we'll resume as + # soon as there isn't an exclusive lock in our way @cv.wait_while { @exclusive_thread } + else + # This is an initial / outermost share call: any outstanding + # requests for an exclusive lock get to go first + @cv.wait_while { busy_for_sharing?(false) } end @sharing[Thread.current] += 1 end @@ -109,12 +111,12 @@ module ActiveSupport # the block. # # See +start_exclusive+ for other options. - def exclusive(purpose: nil, compatible: [], no_wait: false) + def exclusive(purpose: nil, compatible: [], after_compatible: [], no_wait: false) if start_exclusive(purpose: purpose, compatible: compatible, no_wait: no_wait) begin yield ensure - stop_exclusive + stop_exclusive(compatible: after_compatible) end end end @@ -129,14 +131,56 @@ module ActiveSupport end end + # Temporarily give up all held Share locks while executing the + # supplied block, allowing any +compatible+ exclusive lock request + # to proceed. + def yield_shares(purpose: nil, compatible: [], block_share: false) + loose_shares = previous_wait = nil + synchronize do + if loose_shares = @sharing.delete(Thread.current) + if previous_wait = @waiting[Thread.current] + purpose = nil unless purpose == previous_wait[0] + compatible &= previous_wait[1] + end + compatible |= [false] unless block_share + @waiting[Thread.current] = [purpose, compatible] + + @cv.broadcast + end + end + + begin + yield + ensure + synchronize do + @cv.wait_while { @exclusive_thread && @exclusive_thread != Thread.current } + + if previous_wait + @waiting[Thread.current] = previous_wait + else + @waiting.delete Thread.current + end + @sharing[Thread.current] = loose_shares if loose_shares + end + end + end + private # Must be called within synchronize - def busy?(purpose) - (@exclusive_thread && @exclusive_thread != Thread.current) || - @waiting.any? { |k, v| k != Thread.current && !v.include?(purpose) } || + def busy_for_exclusive?(purpose) + busy_for_sharing?(purpose) || @sharing.size > (@sharing[Thread.current] > 0 ? 1 : 0) end + + def busy_for_sharing?(purpose) + (@exclusive_thread && @exclusive_thread != Thread.current) || + @waiting.any? { |t, (_, c)| t != Thread.current && !c.include?(purpose) } + end + + def eligible_waiters?(compatible) + @waiting.any? { |t, (p, _)| compatible.include?(p) && @waiting.all? { |t2, (_, c2)| t == t2 || c2.include?(p) } } + end end end end |