[Lldb-commits] [PATCH] D116372: [lldb-server/linux] Fix waitpid for multithreaded forks

[Pavel Labath via Phabricator via lldb-commits]

labath added inline comments.


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Comment at: lldb/source/Plugins/Process/Linux/NativeProcessLinux.cpp:1909
+             wait_status);
+    tid_events.try_emplace(thread_up->GetID(), wait_status);
+  }
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mgorny wrote:
> Hmm, I'm a bit confused here. Why would the key already exist? I mean, you start with empty `tid_events` and catch only one event for every thread, correct?
It doesn't exist. It's just how the api is named. c++ std::map offers both emplace and try_emplace (which, ironically, don't differ in "trying" -- they only differ in the way they construct the pair). DenseMap offers only try_emplace (maybe for that reason).


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Comment at: lldb/source/Plugins/Process/Linux/NativeProcessLinux.cpp:1918
+    } else {
+      // This can happen if one of the events is an main thread exit.
+      LLDB_LOG(log, "... but the thread has disappeared");
----------------
mgorny wrote:
> Are we talking about some kind of race here? Or some thread that appears in `m_threads` but is not returned by `GetThreadByID()`?
> 
> I was wondering if you could use thread pointers as keys.
The problem is when a thread disappears. This can happen in case of a main thread exit or an execve, in which case we remove all non-main threads from the list. However, we can still have some pending events for the other threads. Now, I haven't managed to reproduce this in my experiments, but the manpage is adamant that a SIGKILL should immediately terminate a process. In my (limited) tests the debugger always got a PTRACE_EVENT_EXIT stop for each threads (which is again something that the manpage says should not happen), so we theoretically (with careful management of thread lifetimes) might ensure that a thread with pending events does not disappear, but depending on it doesn't seem like a good idea.


Repository:
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  https://reviews.llvm.org/D116372/new/

https://reviews.llvm.org/D116372