[Lldb-commits] [lldb] [lldb] Don't unregister a listener that's being destroyed (PR #97300)
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Mon Jul 1 07:21:59 PDT 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-lldb
Author: Pavel Labath (labath)
<details>
<summary>Changes</summary>
It's not necessary because the broadcasters (and broadcast managers) hold a weak_ptr (*) to it, and will delete the weak_ptr next time they try to lock it. Doing this prevents recursion in RemoveListener, where the function can end up holding the only shared_ptr to a listener, and its destruction can trigger another call to RemoveListener -- which will mess up the state of the first instance.
This is the same bug that we've have fixed in
https://reviews.llvm.org/D23406, but it was effectively undone in https://reviews.llvm.org/D157556. With the addition of a primary listener, a fix like D23406 becomes unwieldy (and it has already shown itself to be fragile), which is why this patch attempts a different approach.
Like in 2016, I don't know a good way to unit test this bug, since it depends on precise timing, but the thing I like about this approach is that it enables us to change the broadcaster mutex into a non-recursive one. While that doesn't prevent the bug from happening again, it will make it much easier to spot in the future, as the code will hang with a smoking gun (instead of crashing a little while later). I'm going to attempt that in a separate patch to minimize disruption.
(*) Technically a broadcaster holds the *primary* listener as a shared_ptr. That's probably not a good idea, because it means the listener will never get destroyed (unless it is explicitly unregistered), but it also means that this change has no impact on primary listeners.
---
Full diff: https://github.com/llvm/llvm-project/pull/97300.diff
1 Files Affected:
- (modified) lldb/source/Utility/Listener.cpp (+1-1)
``````````diff
diff --git a/lldb/source/Utility/Listener.cpp b/lldb/source/Utility/Listener.cpp
index 6a74c530ad257..5aacb4104e1cf 100644
--- a/lldb/source/Utility/Listener.cpp
+++ b/lldb/source/Utility/Listener.cpp
@@ -30,7 +30,7 @@ Listener::Listener(const char *name)
Listener::~Listener() {
Log *log = GetLog(LLDBLog::Object);
- Clear();
+ // Don't call Clear() from here as that can cause races. See #96750.
LLDB_LOGF(log, "%p Listener::%s('%s')", static_cast<void *>(this),
__FUNCTION__, m_name.c_str());
``````````
</details>
https://github.com/llvm/llvm-project/pull/97300
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