Bounce all requests off the cache lock after 25ms.

net/http/http_cache_transaction.cc

Index: net/http/http_cache_transaction.cc
diff --git a/net/http/http_cache_transaction.cc b/net/http/http_cache_transaction.cc
index 8c19b9a592bd4fb8d491cfd2e5ea555b218b8082..9ae4da5c42502f1094349f2dcc6bf0fec8f64801 100644
--- a/net/http/http_cache_transaction.cc
+++ b/net/http/http_cache_transaction.cc
@@ -265,7 +265,6 @@ HttpCache::Transaction::Transaction(RequestPriority priority, HttpCache* cache)
       done_reading_(false),
       vary_mismatch_(false),
       couldnt_conditionalize_request_(false),
-      bypass_lock_for_test_(false),
       fail_conditionalization_for_test_(false),
       io_buf_len_(0),
       read_offset_(0),
@@ -1167,34 +1166,29 @@ int HttpCache::Transaction::DoAddToEntry() {
   entry_lock_waiting_since_ = TimeTicks::Now();
   int rv = cache_->AddTransactionToEntry(new_entry_, this);
   if (rv == ERR_IO_PENDING) {
-    if (bypass_lock_for_test_) {
-      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;
-      }
-      base::ThreadTaskRunnerHandle::Get()->PostDelayedTask(
-          FROM_HERE,
-          base::Bind(&HttpCache::Transaction::OnAddToEntryTimeout,
-                     weak_factory_.GetWeakPtr(), entry_lock_waiting_since_),
-          TimeDelta::FromMilliseconds(timeout_milliseconds));
-    }
+    // Quickly timeout and bypass the cache if the blocked by the
+    // reader/writer lock. Metrics indicate that only 0.89% of requests are
+    // blocked on the cache lock for longer than 25ms.
+    //
+    // 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(25));
   }
   return rv;
 }