[tracing] Simplify and improve thread-hopping logic of memory-infra

base/trace_event/memory_dump_manager.cc

Index: base/trace_event/memory_dump_manager.cc
diff --git a/base/trace_event/memory_dump_manager.cc b/base/trace_event/memory_dump_manager.cc
index 8a8e34fe4e27bca1bca4ae8f434f52428bf52a88..00f1d177512d5abb0ecf253add298ec0bc6cad76 100644
--- a/base/trace_event/memory_dump_manager.cc
+++ b/base/trace_event/memory_dump_manager.cc
@@ -54,69 +54,6 @@ uint32 g_periodic_dumps_count = 0;
 MemoryDumpManager* g_instance_for_testing = nullptr;
 MemoryDumpProvider* g_mmaps_dump_provider = nullptr;
 
-// Internal class used to hold details about ProcessMemoryDump requests for the
-// current process.
-class ProcessMemoryDumpHolder
-    : public RefCountedThreadSafe<ProcessMemoryDumpHolder> {
- public:
-  ProcessMemoryDumpHolder(
-      MemoryDumpRequestArgs req_args,
-      const scoped_refptr<MemoryDumpSessionState>& session_state,
-      MemoryDumpCallback callback)
-      : process_memory_dump(session_state),
-        req_args(req_args),
-        callback(callback),
-        task_runner(MessageLoop::current()->task_runner()),
-        num_pending_async_requests(0) {}
-
-  ProcessMemoryDump process_memory_dump;
-  const MemoryDumpRequestArgs req_args;
-
-  // Callback passed to the initial call to CreateProcessDump().
-  MemoryDumpCallback callback;
-
-  // Thread on which FinalizeDumpAndAddToTrace() should be called, which is the
-  // same that invoked the initial CreateProcessDump().
-  const scoped_refptr<SingleThreadTaskRunner> task_runner;
-
-  // Number of pending ContinueAsyncProcessDump() calls.
-  int num_pending_async_requests;
-
- private:
-  friend class RefCountedThreadSafe<ProcessMemoryDumpHolder>;
-  virtual ~ProcessMemoryDumpHolder() {}
-  DISALLOW_COPY_AND_ASSIGN(ProcessMemoryDumpHolder);
-};
-
-void FinalizeDumpAndAddToTrace(
-    const scoped_refptr<ProcessMemoryDumpHolder>& pmd_holder) {
-  DCHECK_EQ(0, pmd_holder->num_pending_async_requests);
-
-  if (!pmd_holder->task_runner->BelongsToCurrentThread()) {
-    pmd_holder->task_runner->PostTask(
-        FROM_HERE, Bind(&FinalizeDumpAndAddToTrace, pmd_holder));
-    return;
-  }
-
-  scoped_refptr<ConvertableToTraceFormat> event_value(new TracedValue());
-  pmd_holder->process_memory_dump.AsValueInto(
-      static_cast<TracedValue*>(event_value.get()));
-  const char* const event_name =
-      MemoryDumpTypeToString(pmd_holder->req_args.dump_type);
-
-  TRACE_EVENT_API_ADD_TRACE_EVENT(
-      TRACE_EVENT_PHASE_MEMORY_DUMP,
-      TraceLog::GetCategoryGroupEnabled(kTraceCategory), event_name,
-      pmd_holder->req_args.dump_guid, kTraceEventNumArgs, kTraceEventArgNames,
-      kTraceEventArgTypes, nullptr /* arg_values */, &event_value,
-      TRACE_EVENT_FLAG_HAS_ID);
-
-  if (!pmd_holder->callback.is_null()) {
-    pmd_holder->callback.Run(pmd_holder->req_args.dump_guid, true);
-    pmd_holder->callback.Reset();
-  }
-}
-
 void RequestPeriodicGlobalDump() {
   MemoryDumpType dump_type = g_periodic_dumps_count == 0
                                  ? MemoryDumpType::PERIODIC_INTERVAL_WITH_MMAPS
@@ -127,10 +64,6 @@ void RequestPeriodicGlobalDump() {
   MemoryDumpManager::GetInstance()->RequestGlobalDump(dump_type);
 }
 
-void InitializeThreadLocalEventBufferIfSupported() {
-  TraceLog::GetInstance()->InitializeThreadLocalEventBufferIfSupported();
-}
-
 }  // namespace
 
 // static
@@ -159,7 +92,8 @@ void MemoryDumpManager::SetInstanceForTesting(MemoryDumpManager* instance) {
 }
 
 MemoryDumpManager::MemoryDumpManager()
-    : delegate_(nullptr),
+    : did_unregister_dump_provider_(false),
+      delegate_(nullptr),
       memory_tracing_enabled_(0),
       tracing_process_id_(kInvalidTracingProcessId),
       skip_core_dumpers_auto_registration_for_testing_(false) {
@@ -206,9 +140,9 @@ void MemoryDumpManager::SetDelegate(MemoryDumpManagerDelegate* delegate) {
 void MemoryDumpManager::RegisterDumpProvider(
     MemoryDumpProvider* mdp,
     const scoped_refptr<SingleThreadTaskRunner>& task_runner) {
-  MemoryDumpProviderInfo mdp_info(task_runner);
+  MemoryDumpProviderInfo mdp_info(mdp, task_runner);
   AutoLock lock(lock_);
-  dump_providers_.insert(std::make_pair(mdp, mdp_info));
+  dump_providers_.insert(mdp_info);
 }
 
 void MemoryDumpManager::RegisterDumpProvider(MemoryDumpProvider* mdp) {
@@ -218,11 +152,15 @@ void MemoryDumpManager::RegisterDumpProvider(MemoryDumpProvider* mdp) {
 void MemoryDumpManager::UnregisterDumpProvider(MemoryDumpProvider* mdp) {
   AutoLock lock(lock_);
 
-  auto it = dump_providers_.find(mdp);
-  if (it == dump_providers_.end())
+  auto mdp_iter = dump_providers_.begin();
+  for (; mdp_iter != dump_providers_.end(); ++mdp_iter) {
+    if (mdp_iter->dump_provider == mdp)
+      break;
+  }
+
+  if (mdp_iter == dump_providers_.end())
     return;
 
-  const MemoryDumpProviderInfo& mdp_info = it->second;
   // 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
@@ -231,13 +169,14 @@ void MemoryDumpManager::UnregisterDumpProvider(MemoryDumpProvider* mdp) {
   // race-free. If you hit this DCHECK, your MDP has a bug.
   DCHECK_IMPLIES(
       subtle::NoBarrier_Load(&memory_tracing_enabled_),
-      mdp_info.task_runner && mdp_info.task_runner->BelongsToCurrentThread())
+      mdp_iter->task_runner && mdp_iter->task_runner->BelongsToCurrentThread())
       << "The MemoryDumpProvider attempted to unregister itself in a racy way. "
       << " Please file a crbug.";

[petrcermak, 2015/07/03 11:04:15]

nit: Double space here (not your patch but I thought I'd point it out anyway)

[Primiano Tucci (use gerrit), 2015/07/03 15:05:43]

Done.

 
   // Remove from the enabled providers list. This is to deal with the case that
   // UnregisterDumpProvider is called while the trace is enabled.

[petrcermak, 2015/07/03 11:04:16]

nit: It seems to me that this comment is outdated. Even if the trace is enabled,
the unregistration needs to happen, right?

[Primiano Tucci (use gerrit), 2015/07/03 15:05:44]

Correct, there is only one list now. Thanks :)

-  dump_providers_.erase(it);
+  dump_providers_.erase(mdp_iter);
+  did_unregister_dump_provider_ = true;
 }
 
 void MemoryDumpManager::RequestGlobalDump(
@@ -272,119 +211,184 @@ void MemoryDumpManager::RequestGlobalDump(MemoryDumpType dump_type) {
   RequestGlobalDump(dump_type, MemoryDumpCallback());
 }
 
-// Creates a memory dump for the current process and appends it to the trace.
 void MemoryDumpManager::CreateProcessDump(const MemoryDumpRequestArgs& args,
                                           const MemoryDumpCallback& callback) {
-  scoped_refptr<ProcessMemoryDumpHolder> pmd_holder(
-      new ProcessMemoryDumpHolder(args, session_state_, callback));
-  ProcessMemoryDump* pmd = &pmd_holder->process_memory_dump;
-  bool did_any_provider_dump = false;
-  bool did_post_any_async_task = false;
-
-  // Initalizes the ThreadLocalEventBuffer for the syncrhonous dump providers
-  // that will be invoked in this thread without other posts. The initialization
-  // for the asynchronous providers, instead, is handled in OnTraceLogEnabled().
-  InitializeThreadLocalEventBufferIfSupported();
-
-  // Iterate over the active dump providers and invoke OnMemoryDump(pmd).
-  // The MDM guarantees linearity (at most one MDP is active within one
-  // process) and thread-safety (MDM enforces the right locking when entering /
-  // leaving the MDP.OnMemoryDump() call). This is to simplify the clients'
-  // design
-  // and not let the MDPs worry about locking.
-  // As regards thread affinity, depending on the MDP configuration (see
-  // memory_dump_provider.h), the OnMemoryDump() invocation can happen:
-  //  - Synchronousy on the MDM thread, when MDP.task_runner() is not set.
-  //  - Posted on MDP.task_runner(), when MDP.task_runner() is set.
+  scoped_ptr<ProcessMemoryDumpAsyncState> pmd_async_state;
   {
     AutoLock lock(lock_);
-    for (auto it = dump_providers_.begin(); it != dump_providers_.end(); ++it) {
-      MemoryDumpProvider* mdp = it->first;
-      MemoryDumpProviderInfo* mdp_info = &it->second;
+    did_unregister_dump_provider_ = false;
+    pmd_async_state.reset(new ProcessMemoryDumpAsyncState(
+        args, dump_providers_.begin(), session_state_, callback));
+  }
+
+  // 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(pmd_async_state.Pass());
+}
+
+// At most one ContinueAsyncProcessDump() can be active at any time for a given
+// PMD, regardless of status of the |lock_|. |lock_| here is used purely to

[petrcermak, 2015/07/03 11:04:16]

nit: s/here is used/is used here/

[Primiano Tucci (use gerrit), 2015/07/03 15:05:44]

Done.

+// 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:
+//   - 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() (% skip).

[petrcermak, 2015/07/03 11:04:16]

nit: I'd say "unless skipped" instead of "% skip" just to make it a little
clearer.

[Primiano Tucci (use gerrit), 2015/07/03 15:05:44]

Done.

+// 3) Epilogue:
+//  - unregister the dump provider if failed too many times consecutively.

[petrcermak, 2015/07/03 11:04:16]

nit: "if IT failed"

[Primiano Tucci (use gerrit), 2015/07/03 15:05:44]

Done.

+//  - Advance the |next_dump_provider| iterator to the next dump provider.
+//  - If this was the last hop add to the trace and finalize (invoke callback).

[petrcermak, 2015/07/03 11:04:16]

nit2: I think you should say *what* is added to the trace.

[petrcermak, 2015/07/03 11:04:16]

nit1: there should be a comma after "If this was the last hop"

[Primiano Tucci (use gerrit), 2015/07/03 15:05:44]

Done.

[Primiano Tucci (use gerrit), 2015/07/03 15:05:44]

Done.

+
+void MemoryDumpManager::ContinueAsyncProcessDump(
+    scoped_ptr<ProcessMemoryDumpAsyncState> pmd_async_state) {
+  // Initalizes the ThreadLocalEventBuffer, to guarantee that the TRACE_EVENTs

[petrcermak, 2015/07/03 11:04:15]

nit: No need for a comma here.

[Primiano Tucci (use gerrit), 2015/07/03 15:05:45]

Done.

+  // 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();
+
+  // 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;
+  enum { NONE, THREAD_GONE, DUMPER_FAILED } log_warning = NONE;

[petrcermak, 2015/07/03 11:04:15]

I think that this definition should be moved to line 310. This way, it looks
like it belongs to the block below, but it never touches it.

[Primiano Tucci (use gerrit), 2015/07/03 15:05:44]

actually don't need it anymore at all. Just need one warning.

+  {
+    AutoLock lock(lock_);
+    // In the unlikely event that a dump provider was unregister while dumping

[petrcermak, 2015/07/03 11:04:15]

s/unregister/unregistered/

[petrcermak, 2015/07/03 11:04:16]

nit: There should be a comma before "abort the dump" as well.

[Primiano Tucci (use gerrit), 2015/07/03 15:05:45]

Done.

+    // abort the dump, as that would make the |next_dump_provider| invalid.
+    // Registration of a new MDP, instead, is safe as per std::set<> contract.

[petrcermak, 2015/07/03 11:04:16]

nit: I think that "on the other hand" would work better than "instead" here.

[Primiano Tucci (use gerrit), 2015/07/03 15:05:44]

Done.

+    if (did_unregister_dump_provider_) {
+      return AbortDumpLocked(pmd_async_state->callback,
+                             pmd_async_state->task_runner,
+                             pmd_async_state->req_args.dump_guid);
+    }
+
+    auto* mdp_info = &*pmd_async_state->next_dump_provider;
+    mdp = mdp_info->dump_provider;
+    if (mdp_info->disabled) {
+      skip_dump = true;
+    } else if (mdp == g_mmaps_dump_provider &&
+               pmd_async_state->req_args.dump_type !=
+                   MemoryDumpType::PERIODIC_INTERVAL_WITH_MMAPS) {
       // Mmaps dumping is very heavyweight and cannot be performed at the same
       // rate of other dumps. TODO(primiano): this is a hack and should be
       // cleaned up as part of crbug.com/499731.
-      if (mdp == g_mmaps_dump_provider &&
-          args.dump_type != MemoryDumpType::PERIODIC_INTERVAL_WITH_MMAPS) {
-        continue;
-      }
-      if (mdp_info->disabled)
-        continue;
-      if (mdp_info->task_runner) {
-        // The OnMemoryDump() call must be posted.
-        bool did_post_async_task = mdp_info->task_runner->PostTask(
-            FROM_HERE, Bind(&MemoryDumpManager::ContinueAsyncProcessDump,
-                            Unretained(this), Unretained(mdp), pmd_holder));
-        // The thread underlying the TaskRunner might have gone away.
-        if (did_post_async_task) {
-          ++pmd_holder->num_pending_async_requests;
-          did_post_any_async_task = true;
-        }
-      } else {
-        // Invoke the dump provider synchronously.
-        did_any_provider_dump |= InvokeDumpProviderLocked(mdp, pmd);
-      }
+      skip_dump = true;
+    } else if (mdp_info->task_runner &&
+               !mdp_info->task_runner->BelongsToCurrentThread()) {
+      // It's time to hop on another thread.

[petrcermak, 2015/07/03 11:04:16]

nit: s/on/onto/

[Primiano Tucci (use gerrit), 2015/07/03 15:05:44]

Done.

+
+      // 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->task_runner;
+      const uint64 dump_guid = pmd_async_state->req_args.dump_guid;
+
+      const bool did_post_task = mdp_info->task_runner->PostTask(
+          FROM_HERE, Bind(&MemoryDumpManager::ContinueAsyncProcessDump,

[petrcermak, 2015/07/03 11:04:16]

Just a thought: Could you store the Bind in a local variable so that it wouldn't
destroy the arguments (and you wouldn't need to copy them):

auto bind = Bind(...)
const bool did_post_task = mdp_info->task_runner->PostTask(FROM_HERE, bind);

[Primiano Tucci (use gerrit), 2015/07/03 15:05:44]

Discussed this offline. Can't. THe problem is that the Bind takes ownership of
the state, which includes the task runner

+                          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 disabling

[petrcermak, 2015/07/03 11:04:16]

nit: you say "is disablING ... and abort ...". It should probably be "is TO
disable ... and abort ...".

[Primiano Tucci (use gerrit), 2015/07/03 15:05:44]

Done.

+      // the dump provider and abort this dump.
+      mdp_info->disabled = true;
+      return AbortDumpLocked(callback, callback_task_runner, dump_guid);
     }
-  }  // AutoLock
+  }  // AutoLock(lock_)
 
-  // If at least one synchronous provider did dump and there are no pending
-  // asynchronous requests, add the dump to the trace and invoke the callback
-  // straight away (FinalizeDumpAndAddToTrace() takes care of the callback).
-  if (did_any_provider_dump && !did_post_any_async_task)
-    FinalizeDumpAndAddToTrace(pmd_holder);
-}
+  // Invoke the dump provider without holding the |lock_|.
+  bool finalize = false;
+  bool dump_successful = false;
+  if (!skip_dump)
+    dump_successful = mdp->OnMemoryDump(&pmd_async_state->process_memory_dump);
 
-// Invokes the MemoryDumpProvider.OnMemoryDump(), taking care of the fail-safe
-// logic which disables the dumper when failing (crbug.com/461788).
-bool MemoryDumpManager::InvokeDumpProviderLocked(MemoryDumpProvider* mdp,
-                                                 ProcessMemoryDump* pmd) {
-  lock_.AssertAcquired();
-  bool dump_successful = mdp->OnMemoryDump(pmd);
-  MemoryDumpProviderInfo* mdp_info = &dump_providers_.find(mdp)->second;
-  if (dump_successful) {
-    mdp_info->consecutive_failures = 0;
-  } else {
-    // Disable the MDP if it fails kMaxConsecutiveFailuresCount times
-    // consecutively.
-    mdp_info->consecutive_failures++;
-    if (mdp_info->consecutive_failures >= kMaxConsecutiveFailuresCount) {
-      mdp_info->disabled = true;
-      LOG(ERROR) << "The memory dumper failed, possibly due to sandboxing "
-                    "(crbug.com/461788), disabling it for current process. Try "
-                    "restarting chrome with the --no-sandbox switch.";
+  {
+    AutoLock lock(lock_);
+    if (did_unregister_dump_provider_) {
+      return AbortDumpLocked(pmd_async_state->callback,
+                             pmd_async_state->task_runner,
+                             pmd_async_state->req_args.dump_guid);
+    }
+    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;
+        log_warning = DUMPER_FAILED;
+      }
     }
+    ++pmd_async_state->next_dump_provider;
+    finalize = pmd_async_state->next_dump_provider == dump_providers_.end();
+  }
+
+  if (log_warning == THREAD_GONE) {
+    LOG(WARNING) << "A memory dump provider specified a thread affinity but "
+                    "its thread is gone";

[petrcermak, 2015/07/03 11:04:16]

nit: missing full stop at the end of the message.

[Primiano Tucci (use gerrit), 2015/07/03 15:05:44]

removed the entire warning.

+  } else if (log_warning == DUMPER_FAILED) {
+    LOG(ERROR) << "The memory dumper failed, possibly due to sandboxing "

[petrcermak, 2015/07/03 11:04:15]

nit: "The memory dumper" -> "A memory dump provider" (to have consistent LOG
messages)

[Primiano Tucci (use gerrit), 2015/07/03 15:05:44]

Done.

+                  "(crbug.com/461788), disabling it for current process. "
+                  "Try restarting chrome with the --no-sandbox switch.";
   }
-  return dump_successful;
+
+  if (finalize) {
+    FinalizeDumpAndAddToTrace(pmd_async_state.Pass());

[petrcermak, 2015/07/03 11:04:16]

I think you could do "return FinalizeDumpAndAddToTrace(pmd_async_state.Pass());"
to match the style of line 263.

[Primiano Tucci (use gerrit), 2015/07/03 15:05:44]

Indeed. Thanks!

+    return;
+  }
+
+  ContinueAsyncProcessDump(pmd_async_state.Pass());
 }
 
-// This is posted to arbitrary threads as a continuation of CreateProcessDump(),
-// when one or more MemoryDumpProvider(s) require the OnMemoryDump() call to
-// happen on a different thread.
-void MemoryDumpManager::ContinueAsyncProcessDump(
-    MemoryDumpProvider* mdp,
-    scoped_refptr<ProcessMemoryDumpHolder> pmd_holder) {
-  bool should_finalize_dump = false;
-  {
-    // The lock here is to guarantee that different asynchronous dumps on
-    // different threads are still serialized, so that the MemoryDumpProvider
-    // has a consistent view of the |pmd| argument passed.
-    AutoLock lock(lock_);
-    ProcessMemoryDump* pmd = &pmd_holder->process_memory_dump;
-
-    // Check if the MemoryDumpProvider is still there. It might have been
-    // destroyed and unregistered while hopping threads.
-    if (dump_providers_.count(mdp))
-      InvokeDumpProviderLocked(mdp, pmd);
-
-    // Finalize the dump appending it to the trace if this was the last
-    // asynchronous request pending.
-    --pmd_holder->num_pending_async_requests;
-    if (pmd_holder->num_pending_async_requests == 0)
-      should_finalize_dump = true;
-  }  // AutoLock(lock_)
+// static
+void MemoryDumpManager::FinalizeDumpAndAddToTrace(
+    scoped_ptr<ProcessMemoryDumpAsyncState> pmd_async_state) {
+  if (!pmd_async_state->task_runner->BelongsToCurrentThread()) {
+    scoped_refptr<SingleThreadTaskRunner> task_runner =
+        pmd_async_state->task_runner;
+    task_runner->PostTask(
+        FROM_HERE, Bind(&MemoryDumpManager::FinalizeDumpAndAddToTrace,
+                        Passed(&pmd_async_state)));
+    return;
+  }
+
+  scoped_refptr<ConvertableToTraceFormat> event_value(new TracedValue());
+  pmd_async_state->process_memory_dump.AsValueInto(
+      static_cast<TracedValue*>(event_value.get()));
+  const char* const event_name =
+      MemoryDumpTypeToString(pmd_async_state->req_args.dump_type);
+
+  TRACE_EVENT_API_ADD_TRACE_EVENT(
+      TRACE_EVENT_PHASE_MEMORY_DUMP,
+      TraceLog::GetCategoryGroupEnabled(kTraceCategory), event_name,
+      pmd_async_state->req_args.dump_guid, kTraceEventNumArgs,
+      kTraceEventArgNames, kTraceEventArgTypes, nullptr /* arg_values */,
+      &event_value, TRACE_EVENT_FLAG_HAS_ID);
 
-  if (should_finalize_dump)
-    FinalizeDumpAndAddToTrace(pmd_holder);
+  if (!pmd_async_state->callback.is_null()) {
+    pmd_async_state->callback.Run(pmd_async_state->req_args.dump_guid, true);

[petrcermak, 2015/07/03 11:04:15]

nit: Could you please add "/* success */" after "true" like on line 391?

[Primiano Tucci (use gerrit), 2015/07/03 15:05:44]

NOOOOOOOOOOOOOO an extra new line.
Pfff ok. Done

+    pmd_async_state->callback.Reset();
+  }
+}
+
+// static
+void MemoryDumpManager::AbortDumpLocked(
+    MemoryDumpCallback callback,
+    scoped_refptr<SingleThreadTaskRunner> task_runner,
+    uint64 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 holing the lock_.

[petrcermak, 2015/07/03 11:04:15]

s/holing/holding/

[Primiano Tucci (use gerrit), 2015/07/03 15:05:44]

Holy lock! Done

+  task_runner->PostTask(FROM_HERE,
+                        Bind(callback, dump_guid, false /* success */));
 }
 
 void MemoryDumpManager::OnTraceLogEnabled() {
@@ -397,7 +401,7 @@ void MemoryDumpManager::OnTraceLogEnabled() {
   // 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;
-  InitializeThreadLocalEventBufferIfSupported();
+  TraceLog::GetInstance()->InitializeThreadLocalEventBufferIfSupported();
 
   AutoLock lock(lock_);
 
@@ -405,24 +409,14 @@ void MemoryDumpManager::OnTraceLogEnabled() {
   if (!enabled || !delegate_) {
     // Disable all the providers.
     for (auto it = dump_providers_.begin(); it != dump_providers_.end(); ++it)
-      it->second.disabled = true;
+      it->disabled = true;
     return;
   }
 
   session_state_ = new MemoryDumpSessionState();
   for (auto it = dump_providers_.begin(); it != dump_providers_.end(); ++it) {
-    MemoryDumpProviderInfo& mdp_info = it->second;
-    mdp_info.disabled = false;
-    mdp_info.consecutive_failures = 0;
-    if (mdp_info.task_runner) {
-      // The thread local event buffer must be initialized at this point as it
-      // registers its own dump provider (for tracing overhead acounting).
-      // The registration cannot happen lazily during the first TRACE_EVENT*
-      // as it might end up registering the ThreadLocalEventBuffer while
-      // in onMemoryDump(), which will deadlock.
-      mdp_info.task_runner->PostTask(
-          FROM_HERE, Bind(&InitializeThreadLocalEventBufferIfSupported));
-    }
+    it->disabled = false;
+    it->consecutive_failures = 0;
   }
 
   subtle::NoBarrier_Store(&memory_tracing_enabled_, 1);
@@ -452,11 +446,38 @@ uint64 MemoryDumpManager::ChildProcessIdToTracingProcessId(
 }
 
 MemoryDumpManager::MemoryDumpProviderInfo::MemoryDumpProviderInfo(
+    MemoryDumpProvider* dump_provider,
     const scoped_refptr<SingleThreadTaskRunner>& task_runner)
-    : task_runner(task_runner), consecutive_failures(0), disabled(false) {
+    : dump_provider(dump_provider),
+      task_runner(task_runner),
+      consecutive_failures(0),
+      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;
+  return task_runner < other.task_runner;
+}
+
+MemoryDumpManager::ProcessMemoryDumpAsyncState::ProcessMemoryDumpAsyncState(
+    MemoryDumpRequestArgs req_args,
+    MemoryDumpProviderInfoSet::iterator next_dump_provider,
+    const scoped_refptr<MemoryDumpSessionState>& session_state,
+    MemoryDumpCallback callback)
+    : process_memory_dump(session_state),
+      req_args(req_args),
+      next_dump_provider(next_dump_provider),
+      callback(callback),
+      task_runner(MessageLoop::current()->task_runner()) {
+}
+
+MemoryDumpManager::ProcessMemoryDumpAsyncState::~ProcessMemoryDumpAsyncState() {
+}
+
 }  // namespace trace_event
 }  // namespace base