Bounce all requests off the cache lock after 25ms.

net/http/http_cache_transaction.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/http/http_cache_transaction.h"
 
 #include "build/build_config.h"  // For OS_POSIX
 
 #if defined(OS_POSIX)
 #include <unistd.h>
@@ skipping 1149 matching lines @@
 
 int HttpCache::Transaction::DoAddToEntry() {
   DCHECK(new_entry_);
   cache_pending_ = true;
   next_state_ = STATE_ADD_TO_ENTRY_COMPLETE;
   net_log_.BeginEvent(NetLog::TYPE_HTTP_CACHE_ADD_TO_ENTRY);
   DCHECK(entry_lock_waiting_since_.is_null());
   entry_lock_waiting_since_ = TimeTicks::Now();
   int rv = cache_->AddTransactionToEntry(new_entry_, this);
   if (rv == ERR_IO_PENDING) {
     if (bypass_lock_for_test_) {

[mmenke, 2015/11/25 20:55:39]

I think we can get rid of this, and use the real timeout on that test now?  This
also seems a really bad way of simulating timeouts, anyways.

[davidben, 2015/11/25 21:27:38]

Done.

       OnAddToEntryTimeout(entry_lock_waiting_since_);
     } else {
-      int timeout_milliseconds = 20 * 1000;
-      if (partial_ && new_entry_->writer &&
-          new_entry_->writer->range_requested_) {
-        // Quickly timeout and bypass the cache if we're a range request and
-        // we're blocked by the reader/writer lock. Doing so eliminates a long
-        // running issue, http://crbug.com/31014, where two of the same media
-        // resources could not be played back simultaneously due to one locking
-        // the cache entry until the entire video was downloaded.
-        //
-        // Bypassing the cache is not ideal, as we are now ignoring the cache
-        // entirely for all range requests to a resource beyond the first. This
-        // is however a much more succinct solution than the alternatives, which
-        // would require somewhat significant changes to the http caching logic.
-        //
-        // Allow some timeout slack for the entry addition to complete in case
-        // the writer lock is imminently released; we want to avoid skipping
-        // the cache if at all possible. See http://crbug.com/408765
-        timeout_milliseconds = 25;
-      }
+      // Quickly timeout and bypass the cache if the blocked by the
+      // reader/writer lock. Metrics indicate that only 0.42% of requests are
+      // blocked on the cache lock for longer than 250ms.
+      //
+      // Bypassing the cache is not ideal, as we are now ignoring the cache
+      // entirely for requests which hit a resource in
+      // parallel. https://crbug.com/472740 tracks a better design for the
+      // system.
+      //
+      // In the meantime, having a cache lock causes numerous priority inversion
+      // problems and deadlocks, so it is better to ensure the system can make
+      // forward progress. (See https://crbug.com/31014,
+      // https://crbug.com/458620, https://crbug.com/535793, and
+      // https://crbug.com/6697.)
+      //
+      // Allow some timeout slack for the entry addition to complete in case the
+      // writer lock is imminently released; we want to avoid skipping the cache
+      // if at all possible. See http://crbug.com/408765
       base::ThreadTaskRunnerHandle::Get()->PostDelayedTask(
           FROM_HERE,
           base::Bind(&HttpCache::Transaction::OnAddToEntryTimeout,
                      weak_factory_.GetWeakPtr(), entry_lock_waiting_since_),
-          TimeDelta::FromMilliseconds(timeout_milliseconds));
+          TimeDelta::FromMilliseconds(25));

[mmenke, 2015/11/25 20:55:39]

The CL description and comment above say 250, this is 25.

[davidben, 2015/11/25 21:27:38]

Fixed to 25 since that's what range requests do. Updated description and
comment.

     }
   }
   return rv;
 }
 
 int HttpCache::Transaction::DoAddToEntryComplete(int result) {
   net_log_.EndEventWithNetErrorCode(NetLog::TYPE_HTTP_CACHE_ADD_TO_ENTRY,
                                     result);
   const TimeDelta entry_lock_wait =
       TimeTicks::Now() - entry_lock_waiting_since_;
@@ skipping 1730 matching lines @@
     default:
       NOTREACHED();
   }
 }
 
 void HttpCache::Transaction::OnIOComplete(int result) {
   DoLoop(result);
 }
 
 }  // namespace net