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Always lock chunks first to avoid deadlock #1721

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merged 4 commits into from Oct 23, 2017
Merged

Always lock chunks first to avoid deadlock #1721

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e845897
Refactoring: Remove unnecessary conditions
abicky Oct 16, 2017
ed9f921
Always lock chunks first to avoid deadlock
abicky Oct 20, 2017
1a6f16b
Process chunks with buffer lock in enqueue_chunk
abicky Oct 20, 2017
2261428
[ci skip] Add a comment
abicky Oct 23, 2017
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41 changes: 21 additions & 20 deletions lib/fluent/plugin/buffer.rb
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Expand Up @@ -313,22 +313,24 @@ def queued?(metadata=nil)

def enqueue_chunk(metadata)
log.trace "enqueueing chunk", instance: self.object_id, metadata: metadata
synchronize do
chunk = @stage.delete(metadata)
return nil unless chunk
chunk = synchronize do
@stage.delete(metadata)
end
return nil unless chunk

chunk.synchronize do
chunk.synchronize do
synchronize do
if chunk.empty?
chunk.close
else
@queue << chunk
@queued_num[metadata] = @queued_num.fetch(metadata, 0) + 1
chunk.enqueued! if chunk.respond_to?(:enqueued!)
chunk.enqueued!
end
bytesize = chunk.bytesize
@stage_size -= bytesize
@queue_size += bytesize
end
bytesize = chunk.bytesize
@stage_size -= bytesize
@queue_size += bytesize
end
nil
end
Expand All @@ -340,25 +342,24 @@ def enqueue_unstaged_chunk(chunk)
metadata = chunk.metadata
@queue << chunk
@queued_num[metadata] = @queued_num.fetch(metadata, 0) + 1
chunk.enqueued! if chunk.respond_to?(:enqueued!)
chunk.enqueued!
end
@queue_size += chunk.bytesize
end
end

def enqueue_all
log.trace "enqueueing all chunks in buffer", instance: self.object_id
synchronize do
if block_given?
@stage.keys.each do |metadata|
chunk = @stage[metadata]
v = yield metadata, chunk
enqueue_chunk(metadata) if v
end
else
@stage.keys.each do |metadata|
enqueue_chunk(metadata)
end
if block_given?
synchronize{ @stage.keys }.each do |metadata|
chunk = @stage[metadata]
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No need synchronized guard here?

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Hmm... deleting metadata from @stage happens in only enqueue_chunk so it seems safe in CRuby.

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I think data race doesn't occur in CRuby thanks to GIL and synchronized is not necessary, but I'll change the code like below for other Ruby implementations:

diff --git a/lib/fluent/plugin/buffer.rb b/lib/fluent/plugin/buffer.rb
index 337b3b79..61a904ed 100644
--- a/lib/fluent/plugin/buffer.rb
+++ b/lib/fluent/plugin/buffer.rb
@@ -351,11 +351,15 @@ module Fluent
       def enqueue_all
         log.trace "enqueueing all chunks in buffer", instance: self.object_id
         if block_given?
-          synchronize{ @stage.keys }.each do |metadata|
-            chunk = @stage[metadata]
-            next unless chunk
-            v = yield metadata, chunk
-            enqueue_chunk(metadata) if v
+          metadata_array = []
+          synchronize do
+            @stage.each do |metadata, chunk|
+              v = yield metadata, chunk
+              metadata_array << metadata if v
+            end
+          end
+          metadata_array.each do |metadata|
+            enqueue_chunk(metadata)
           end
         else
           synchronize{ @stage.keys }.each do |metadata|

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Yes and currently fluentd focus on CRuby so I think code change is not needed for now.
But adding comment is better for the future.

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Okey, I added a comment 2261428

next unless chunk
v = yield metadata, chunk
enqueue_chunk(metadata) if v
end
else
synchronize{ @stage.keys }.each do |metadata|
enqueue_chunk(metadata)
end
end
end
Expand Down