[tracing] Simplify logic of MemoryDumpManager

base/trace_event/memory_dump_manager.cc

 // Copyright 2015 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 "base/trace_event/memory_dump_manager.h"
 
 #include <algorithm>
 #include <utility>
 
 #include "base/atomic_sequence_num.h"
@@ skipping 104 matching lines @@
   return Singleton<MemoryDumpManager,
                    LeakySingletonTraits<MemoryDumpManager>>::get();
 }
 
 // static
 void MemoryDumpManager::SetInstanceForTesting(MemoryDumpManager* instance) {
   g_instance_for_testing = instance;
 }
 
 MemoryDumpManager::MemoryDumpManager()
-    : delegate_(nullptr),
+    : dump_providers_sequence_number_(0),
+      delegate_(nullptr),
       is_coordinator_(false),
       memory_tracing_enabled_(0),
       tracing_process_id_(kInvalidTracingProcessId),
       dumper_registrations_ignored_for_testing_(false) {
   g_next_guid.GetNext();  // Make sure that first guid is not zero.
 
   heap_profiling_enabled_ = CommandLine::InitializedForCurrentProcess()
                                 ? CommandLine::ForCurrentProcess()->HasSwitch(
                                       switches::kEnableHeapProfiling)
                                 : false;
@@ skipping 50 matching lines @@
 }
 
 void MemoryDumpManager::RegisterDumpProvider(
     MemoryDumpProvider* mdp,
     const char* name,
     const scoped_refptr<SingleThreadTaskRunner>& task_runner,
     const MemoryDumpProvider::Options& options) {
   if (dumper_registrations_ignored_for_testing_)
     return;
 
-  MemoryDumpProviderInfo mdp_info(mdp, name, task_runner, options);
-  AutoLock lock(lock_);
-  auto iter_new = dump_providers_.insert(mdp_info);
-
-  // If there was a previous entry, replace it with the new one. This is to deal
-  // with the case where a dump provider unregisters itself and then re-
-  // registers before a memory dump happens, so its entry was still in the
-  // collection but flagged |unregistered|.
-  if (!iter_new.second) {
-    dump_providers_.erase(iter_new.first);
-    dump_providers_.insert(mdp_info);
+  {
+    AutoLock lock(lock_);
+    dump_providers_sequence_number_++;
+    scoped_refptr<MemoryDumpProviderInfo> mdpinfo = new MemoryDumpProviderInfo(
+        mdp, name, dump_providers_sequence_number_, task_runner, options);
+    dump_providers_.insert(mdpinfo);
   }
 
   if (heap_profiling_enabled_)
     mdp->OnHeapProfilingEnabled(true);
 }
 
 void MemoryDumpManager::RegisterDumpProvider(
     MemoryDumpProvider* mdp,
     const char* name,
     const scoped_refptr<SingleThreadTaskRunner>& task_runner) {
   RegisterDumpProvider(mdp, name, task_runner, MemoryDumpProvider::Options());
 }
 
 void MemoryDumpManager::UnregisterDumpProvider(MemoryDumpProvider* mdp) {
   AutoLock lock(lock_);
 
   auto mdp_iter = dump_providers_.begin();
   for (; mdp_iter != dump_providers_.end(); ++mdp_iter) {
-    if (mdp_iter->dump_provider == mdp)
+    if ((*mdp_iter)->dump_provider == mdp)
       break;
   }
 
   if (mdp_iter == dump_providers_.end())
     return;
 
   // Unregistration of a MemoryDumpProvider while tracing is ongoing is safe
   // only if the MDP has specified a thread affinity (via task_runner()) AND
   // the unregistration happens on the same thread (so the MDP cannot unregister
   // and OnMemoryDump() at the same time).
   // Otherwise, it is not possible to guarantee that its unregistration is
   // race-free. If you hit this DCHECK, your MDP has a bug.
   DCHECK(!subtle::NoBarrier_Load(&memory_tracing_enabled_) ||
-         (mdp_iter->task_runner &&
-          mdp_iter->task_runner->BelongsToCurrentThread()))
-      << "MemoryDumpProvider \"" << mdp_iter->name << "\" attempted to "
+         ((*mdp_iter)->task_runner &&
+          (*mdp_iter)->task_runner->BelongsToCurrentThread()))
+      << "MemoryDumpProvider \"" << (*mdp_iter)->name << "\" attempted to "
       << "unregister itself in a racy way. Please file a crbug.";
 
-  mdp_iter->unregistered = true;
+  // The MDPInfo instance can still be referenced by the
+  // |ProcessMemoryDumpAsyncState.pending_dump_providers|. For this reason
+  // the MDPInfo is flagged as disabled. It will cause ContinueAsyncProcessDump
+  // to just skip it, without actually invoking the |mdp|, which might be
+  // destroyed by the caller soon after this method returns.
+  (*mdp_iter)->disabled = true;
+  dump_providers_.erase(mdp_iter);
 }
 
 void MemoryDumpManager::RequestGlobalDump(
     MemoryDumpType dump_type,
     MemoryDumpLevelOfDetail level_of_detail,
     const MemoryDumpCallback& callback) {
   // Bail out immediately if tracing is not enabled at all.
   if (!UNLIKELY(subtle::NoBarrier_Load(&memory_tracing_enabled_))) {
     if (!callback.is_null())
       callback.Run(0u /* guid */, false /* success */);
@@ skipping 34 matching lines @@
 }
 
 void MemoryDumpManager::CreateProcessDump(const MemoryDumpRequestArgs& args,
                                           const MemoryDumpCallback& callback) {
   TRACE_EVENT_NESTABLE_ASYNC_BEGIN0(kTraceCategory, "ProcessMemoryDump",
                                     TRACE_ID_MANGLE(args.dump_guid));
 
   scoped_ptr<ProcessMemoryDumpAsyncState> pmd_async_state;
   {
     AutoLock lock(lock_);
-    pmd_async_state.reset(new ProcessMemoryDumpAsyncState(
-        args, dump_providers_.begin(), session_state_, callback,
-        dump_thread_->task_runner()));
+    pmd_async_state.reset(
+        new ProcessMemoryDumpAsyncState(args, dump_providers_, session_state_,
+                                        callback, dump_thread_->task_runner()));
   }
 
   TRACE_EVENT_WITH_FLOW0(kTraceCategory, "MemoryDumpManager::CreateProcessDump",
                          TRACE_ID_MANGLE(args.dump_guid),
                          TRACE_EVENT_FLAG_FLOW_OUT);
 
   // Start the thread hop. |dump_providers_| are kept sorted by thread, so
   // ContinueAsyncProcessDump will hop at most once per thread (w.r.t. thread
   // affinity specified by the MemoryDumpProvider(s) in RegisterDumpProvider()).
-  ContinueAsyncProcessDump(std::move(pmd_async_state));
+  ContinueAsyncProcessDump(pmd_async_state.release());
 }
 
 // At most one ContinueAsyncProcessDump() can be active at any time for a given
 // PMD, regardless of status of the |lock_|. |lock_| is used here purely to
 // ensure consistency w.r.t. (un)registrations of |dump_providers_|.
 // The linearization of dump providers' OnMemoryDump invocations is achieved by
 // means of subsequent PostTask(s).
 //
 // 1) Prologue:
+//   - If this was the last hop, create a trace event, add it to the trace
+//     and finalize (invoke callback).
+//   - Check if we are on the right thread. If not hop and continue there.
 //   - Check if the dump provider is disabled, if so skip the dump.
-//   - Check if we are on the right thread. If not hop and continue there.
 // 2) Invoke the dump provider's OnMemoryDump() (unless skipped).
 // 3) Epilogue:
-//  - Unregister the dump provider if it failed too many times consecutively.
-//  - Advance the |next_dump_provider| iterator to the next dump provider.
-//  - If this was the last hop, create a trace event, add it to the trace
-//    and finalize (invoke callback).
-
+//   - Unregister the dump provider if it failed too many times consecutively.
+//   - Pop() the MDP from the |pending_dump_providers| list, eventually
+//     destroying the MDPInfo if that was unregistered in the meantime.
 void MemoryDumpManager::ContinueAsyncProcessDump(
-    scoped_ptr<ProcessMemoryDumpAsyncState> pmd_async_state) {
+    ProcessMemoryDumpAsyncState* owned_pmd_async_state) {
   // Initalizes the ThreadLocalEventBuffer to guarantee that the TRACE_EVENTs
   // in the PostTask below don't end up registering their own dump providers
   // (for discounting trace memory overhead) while holding the |lock_|.
   TraceLog::GetInstance()->InitializeThreadLocalEventBufferIfSupported();
 
-  const uint64_t dump_guid = pmd_async_state->req_args.dump_guid;
-  const char* dump_provider_name = nullptr;
+  // In theory |owned_pmd_async_state| should be a scoped_ptr. The only reason
+  // why it isn't is because of the corner case logic of |did_post_task| below,
+  // which needs to take back the ownership of the |pmd_async_state| when a
+  // thread goes away and consequently the PostTask() fails.
+  // Unfortunately, PostTask() destroys the scoped_ptr arguments upon failure
+  // to prevent accidental leaks. Using a scoped_ptr would prevent us to to
+  // skip the hop and move on. Hence the manual naked -> scoped ptr juggling.
+  auto pmd_async_state = make_scoped_ptr(owned_pmd_async_state);
+  owned_pmd_async_state = nullptr;
 
-  // Pid of the target process being dumped. Often kNullProcessId (= current
-  // process), non-zero when the coordinator process creates dumps on behalf
-  // of child processes (see crbug.com/461788).
-  ProcessId pid;
+  if (pmd_async_state->pending_dump_providers.empty())
+    return FinalizeDumpAndAddToTrace(std::move(pmd_async_state));
 
-  // DO NOT put any LOG() statement in the locked sections, as in some contexts
-  // (GPU process) LOG() ends up performing PostTask/IPCs.
-  MemoryDumpProvider* mdp;
-  bool skip_dump = false;
-  {
-    AutoLock lock(lock_);
+  // Read MemoryDumpProviderInfo thread safety considerations in
+  // memory_dump_manager.h when accessing |mdpinfo| fields.
+  MemoryDumpProviderInfo* mdpinfo =
+      pmd_async_state->pending_dump_providers.back().get();
 
-    auto mdp_info = pmd_async_state->next_dump_provider;
-    mdp = mdp_info->dump_provider;
-    dump_provider_name = mdp_info->name;
-    pid = mdp_info->options.target_pid;
+  // If the dump provider did not specify a thread affinity, dump on
+  // |dump_thread_|. Note that |dump_thread_| might have been Stop()-ed at this
+  // point (if tracing was disabled in the meanwhile). In such case the
+  // PostTask() below will fail, but |task_runner| should always be non-null.
+  SingleThreadTaskRunner* task_runner = mdpinfo->task_runner.get();
+  if (!task_runner)
+    task_runner = pmd_async_state->dump_thread_task_runner.get();
 
-    // If the dump provider did not specify a thread affinity, dump on
-    // |dump_thread_|.
-    SingleThreadTaskRunner* task_runner = mdp_info->task_runner.get();
-    if (!task_runner)
-      task_runner = pmd_async_state->dump_thread_task_runner.get();
+  if (!task_runner->BelongsToCurrentThread()) {
+    // It's time to hop onto another thread.
+    const bool did_post_task = task_runner->PostTask(
+        FROM_HERE, Bind(&MemoryDumpManager::ContinueAsyncProcessDump,
+                        Unretained(this), Unretained(pmd_async_state.get())));
+    if (did_post_task) {
+      // Ownership is tranferred to the next ContinueAsyncProcessDump().
+      ignore_result(pmd_async_state.release());
+      return;
+    }
+    // The thread is gone. Skip the dump provider and keep going.
+    mdpinfo->disabled = true;

[Ruud van Asseldonk, 2015/12/18 12:40:43]

Now that this is no longer inside the lock, is it possible for other threads to
read stale data?

[Primiano Tucci (use gerrit), 2015/12/18 14:02:11]

This is precisely why I wrote the large block below.
THere is only one instance, and adding a lock wouldn't really solve anything.

+  }
 
-    // |dump_thread_| might have been Stop()-ed at this point (if tracing was
-    // disabled in the meanwhile). In such case the PostTask() below will fail.
-    // |task_runner|, however, should always be non-null.
-    DCHECK(task_runner);
+  // We are now on the right thread to read non-const |mdpinfo| fields.
+  // Tehcnically there is a data race here while accessing |mdpinfo.disabled|

[Ruud van Asseldonk, 2015/12/18 12:40:42]

/s/Tehcnically/Technically/

[Primiano Tucci (use gerrit), 2015/12/18 14:02:11]

Done.

+  // that is: if tracing is disabled and immediately re-enabled while a dump is
+  // in progress, the OnTraceLogEnabled() would clear-up the |disabled| and
+  // |consecutive_failures| fields while we concurrently access them here.
+  // This is an extremely unluckily case, not worth a lock (which wouldn't
+  // really solve the problem). In the worst case, we can end up with a spurious
+  // invocation of the dump provider.
 
-    if (mdp_info->disabled || mdp_info->unregistered) {
-      skip_dump = true;
-    } else if (!task_runner->BelongsToCurrentThread()) {
-      // It's time to hop onto another thread.
-
-      // Copy the callback + arguments just for the unlikley case in which
-      // PostTask fails. In such case the Bind helper will destroy the
-      // pmd_async_state and we must keep a copy of the fields to notify the
-      // abort.
-      MemoryDumpCallback callback = pmd_async_state->callback;
-      scoped_refptr<SingleThreadTaskRunner> callback_task_runner =
-          pmd_async_state->callback_task_runner;
-
-      const bool did_post_task = task_runner->PostTask(
-          FROM_HERE, Bind(&MemoryDumpManager::ContinueAsyncProcessDump,
-                          Unretained(this), Passed(&pmd_async_state)));
-      if (did_post_task)
-        return;
-
-      // The thread is gone. At this point the best thing we can do is to
-      // disable the dump provider and abort this dump.
-      mdp_info->disabled = true;
-      return AbortDumpLocked(callback, callback_task_runner, dump_guid);
-    }
-  }  // AutoLock(lock_)
-
-  // Invoke the dump provider without holding the |lock_|.
-  bool finalize = false;
-  bool dump_successful = false;
-
-  if (!skip_dump) {
+  if (!mdpinfo->disabled) {

[Ruud van Asseldonk, 2015/12/18 12:40:42]

No, you need to keep the lock. (Or add a lock per MDPInfo.)

Scenario:

1. This threads loads |mdpinfo->disabled|, sees it is not disabled,
   and takes the branch.
2. Somewhere during |TRACE_EVENT_WITH_FLOW1| this thread gets preempted.
3. On a different thread, the MDProvider is unsubscribed and deleted.
   The MDPInfo is kept alive properly, but the dump provider is gone.
4. The dumping thread resumes and uses the MDProvider after free.

[Primiano Tucci (use gerrit), 2015/12/18 14:02:11]

This cannot happen.
A MDProvider can be unsubcribed only on the same task_runner where it lives.
ANd you cannot be pre-empted and unregistered by the same thread.

+    // Invoke the dump provider.
     TRACE_EVENT_WITH_FLOW1(kTraceCategory,
                            "MemoryDumpManager::ContinueAsyncProcessDump",
-                           TRACE_ID_MANGLE(dump_guid),
+                           TRACE_ID_MANGLE(pmd_async_state->req_args.dump_guid),
                            TRACE_EVENT_FLAG_FLOW_IN | TRACE_EVENT_FLAG_FLOW_OUT,
-                           "dump_provider.name", dump_provider_name);
+                           "dump_provider.name", mdpinfo->name);
+
+    // Pid of the target process being dumped. Often kNullProcessId (= current
+    // process), non-zero when the coordinator process creates dumps on behalf
+    // of child processes (see crbug.com/461788).
+    ProcessId target_pid = mdpinfo->options.target_pid;
+    ProcessMemoryDump* pmd =
+        pmd_async_state->GetOrCreateMemoryDumpContainerForProcess(target_pid);
     MemoryDumpArgs args = {pmd_async_state->req_args.level_of_detail};
-    ProcessMemoryDump* process_memory_dump =
-        pmd_async_state->GetOrCreateMemoryDumpContainerForProcess(pid);
-    dump_successful = mdp->OnMemoryDump(args, process_memory_dump);
-  }
+    bool dump_successful = mdpinfo->dump_provider->OnMemoryDump(args, pmd);
 
-  {
-    AutoLock lock(lock_);
-    auto mdp_info = pmd_async_state->next_dump_provider;
     if (dump_successful) {
-      mdp_info->consecutive_failures = 0;
-    } else if (!skip_dump) {
-      ++mdp_info->consecutive_failures;
-      if (mdp_info->consecutive_failures >= kMaxConsecutiveFailuresCount) {
-        mdp_info->disabled = true;
+      mdpinfo->consecutive_failures = 0;
+    } else {
+      ++mdpinfo->consecutive_failures;
+      if (mdpinfo->consecutive_failures >= kMaxConsecutiveFailuresCount) {
+        mdpinfo->disabled = true;
+        LOG(ERROR) << "MemoryDumpProvider \"" << mdpinfo->name << "\" failed, "
+                   << "possibly due to sandboxing (crbug.com/461788)."
+                   << "Disabling dumper for current process. Try --no-sandbox.";
       }
     }
-    ++pmd_async_state->next_dump_provider;
-    finalize = pmd_async_state->next_dump_provider == dump_providers_.end();
+  }  // if (!mdpinfo->disabled)
 
-    if (mdp_info->unregistered)
-      dump_providers_.erase(mdp_info);
-  }
-
-  if (!skip_dump && !dump_successful) {
-    LOG(ERROR) << "MemoryDumpProvider \"" << dump_provider_name << "\" failed, "
-               << "possibly due to sandboxing (crbug.com/461788)."
-               << "Disabling dumper for current process. Try --no-sandbox.";
-  }
-
-  if (finalize)
-    return FinalizeDumpAndAddToTrace(std::move(pmd_async_state));
-
-  ContinueAsyncProcessDump(std::move(pmd_async_state));
+  pmd_async_state->pending_dump_providers.pop_back();
+  ContinueAsyncProcessDump(pmd_async_state.release());
 }
 
 // static
 void MemoryDumpManager::FinalizeDumpAndAddToTrace(
     scoped_ptr<ProcessMemoryDumpAsyncState> pmd_async_state) {
+  DCHECK(pmd_async_state->pending_dump_providers.empty());
   const uint64_t dump_guid = pmd_async_state->req_args.dump_guid;
   if (!pmd_async_state->callback_task_runner->BelongsToCurrentThread()) {
     scoped_refptr<SingleThreadTaskRunner> callback_task_runner =
         pmd_async_state->callback_task_runner;
     callback_task_runner->PostTask(
         FROM_HERE, Bind(&MemoryDumpManager::FinalizeDumpAndAddToTrace,
                         Passed(&pmd_async_state)));
     return;
   }
 
@@ skipping 23 matching lines @@
 
   if (!pmd_async_state->callback.is_null()) {
     pmd_async_state->callback.Run(dump_guid, true /* success */);
     pmd_async_state->callback.Reset();
   }
 
   TRACE_EVENT_NESTABLE_ASYNC_END0(kTraceCategory, "ProcessMemoryDump",
                                   TRACE_ID_MANGLE(dump_guid));
 }
 
-// static
-void MemoryDumpManager::AbortDumpLocked(
-    MemoryDumpCallback callback,
-    scoped_refptr<SingleThreadTaskRunner> task_runner,
-    uint64_t dump_guid) {
-  if (callback.is_null())
-    return;  // There is nothing to NACK.
-
-  // Post the callback even if we are already on the right thread to avoid
-  // invoking the callback while holding the lock_.
-  task_runner->PostTask(FROM_HERE,
-                        Bind(callback, dump_guid, false /* success */));
-}
-
 void MemoryDumpManager::OnTraceLogEnabled() {
   bool enabled;
   TRACE_EVENT_CATEGORY_GROUP_ENABLED(kTraceCategory, &enabled);
   if (!enabled)
     return;
 
   // Initialize the TraceLog for the current thread. This is to avoid that the
   // TraceLog memory dump provider is registered lazily in the PostTask() below
   // while the |lock_| is taken;
   TraceLog::GetInstance()->InitializeThreadLocalEventBufferIfSupported();
@@ skipping 24 matching lines @@
     TRACE_EVENT_API_ADD_METADATA_EVENT(
         "typeNames", "typeNames",
         scoped_refptr<ConvertableToTraceFormat>(type_name_deduplicator));
   }
 
   DCHECK(!dump_thread_);
   dump_thread_ = std::move(dump_thread);
   session_state_ = new MemoryDumpSessionState(stack_frame_deduplicator,
                                               type_name_deduplicator);
 
-  for (auto it = dump_providers_.begin(); it != dump_providers_.end(); ++it) {
-    it->disabled = false;
-    it->consecutive_failures = 0;
+  for (const scoped_refptr<MemoryDumpProviderInfo>& mdpinfo : dump_providers_) {
+    mdpinfo->disabled = false;
+    mdpinfo->consecutive_failures = 0;
   }
 
   subtle::NoBarrier_Store(&memory_tracing_enabled_, 1);
 
   // TODO(primiano): This is a temporary hack to disable periodic memory dumps
   // when running memory benchmarks until telemetry uses TraceConfig to
   // enable/disable periodic dumps. See crbug.com/529184 .
   if (!is_coordinator_ ||
       CommandLine::ForCurrentProcess()->HasSwitch(
           "enable-memory-benchmarking")) {
@@ skipping 46 matching lines @@
     dump_thread->Stop();
 }
 
 uint64_t MemoryDumpManager::GetTracingProcessId() const {
   return delegate_->GetTracingProcessId();
 }
 
 MemoryDumpManager::MemoryDumpProviderInfo::MemoryDumpProviderInfo(
     MemoryDumpProvider* dump_provider,
     const char* name,
+    uint32_t registration_order,
     const scoped_refptr<SingleThreadTaskRunner>& task_runner,
     const MemoryDumpProvider::Options& options)
     : dump_provider(dump_provider),
       name(name),
+      registration_order(registration_order),
       task_runner(task_runner),
       options(options),
       consecutive_failures(0),
-      disabled(false),
-      unregistered(false) {}
+      disabled(false) {}
 
 MemoryDumpManager::MemoryDumpProviderInfo::~MemoryDumpProviderInfo() {}
 
-bool MemoryDumpManager::MemoryDumpProviderInfo::operator<(
-    const MemoryDumpProviderInfo& other) const {
-  if (task_runner == other.task_runner)
-    return dump_provider < other.dump_provider;
+bool MemoryDumpManager::MemoryDumpProviderInfo::Comparator::operator()(
+    const scoped_refptr<MemoryDumpManager::MemoryDumpProviderInfo>& a,
+    const scoped_refptr<MemoryDumpManager::MemoryDumpProviderInfo>& b) const {
+  if (!a || !b)
+    return a.get() < b.get();
   // Ensure that unbound providers (task_runner == nullptr) always run last.
-  return !(task_runner < other.task_runner);
+  // Rationale: some unbound dump providers are known to be slow, keep them last
+  // to avoid skewing timings of the other dump providers.
+  return std::tie(a->task_runner, a->registration_order) >
+         std::tie(b->task_runner, b->registration_order);

[Ruud van Asseldonk, 2015/12/18 12:40:42]

Looks much nicer than before :D

[Primiano Tucci (use gerrit), 2015/12/18 14:02:11]

Acknowledged.

 }
 
 MemoryDumpManager::ProcessMemoryDumpAsyncState::ProcessMemoryDumpAsyncState(
     MemoryDumpRequestArgs req_args,
-    MemoryDumpProviderInfoSet::iterator next_dump_provider,
+    const MemoryDumpProviderInfo::OrderedSet& dump_providers,
     const scoped_refptr<MemoryDumpSessionState>& session_state,
     MemoryDumpCallback callback,
     const scoped_refptr<SingleThreadTaskRunner>& dump_thread_task_runner)
     : req_args(req_args),
-      next_dump_provider(next_dump_provider),
       session_state(session_state),
       callback(callback),
       callback_task_runner(MessageLoop::current()->task_runner()),
-      dump_thread_task_runner(dump_thread_task_runner) {}
+      dump_thread_task_runner(dump_thread_task_runner) {
+  pending_dump_providers.reserve(dump_providers.size());
+  pending_dump_providers.assign(dump_providers.rbegin(), dump_providers.rend());
+}
 
 MemoryDumpManager::ProcessMemoryDumpAsyncState::~ProcessMemoryDumpAsyncState() {
 }
 
 ProcessMemoryDump* MemoryDumpManager::ProcessMemoryDumpAsyncState::
     GetOrCreateMemoryDumpContainerForProcess(ProcessId pid) {
   auto iter = process_dumps.find(pid);
   if (iter == process_dumps.end()) {
     scoped_ptr<ProcessMemoryDump> new_pmd(new ProcessMemoryDump(session_state));
     iter = process_dumps.insert(std::make_pair(pid, std::move(new_pmd))).first;
   }
   return iter->second.get();
 }
 
 }  // namespace trace_event
 }  // namespace base