RFR: 8323807: Async UL: Add a stalling mode to async UL [v10]

[Axel Boldt-Christmas]

On Thu, 23 Jan 2025 08:37:18 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

>> src/hotspot/share/logging/logAsyncWriter.cpp line 105:
>> 
>>> 103:   if (LogConfiguration::async_mode() == LogConfiguration::AsyncMode::Stall) {
>>> 104:     size_t size = Message::calc_size(msg_len);
>>> 105:     void* ptr = os::malloc(size, mtLogging);
>> 
>> Your mention of recursion made me check if we are logging from our `os:malloc/free` calls. I can find at least one path  do `log_warning` in debug builds. This will cause both recursive locking on our PlatformMonitor and cause infinite recursion if the second message does not fit in the buffer. 
>> 
>> Maybe this logging is unreachable from this specific context. But I find it a little scary nonetheless. Maybe this should be documented somewhere.
>
> Hi Axel,
> 
> I've already switched to having the consumer thread log anything it itself logs directly without going through the buffer so this won't be an issue. See `enqueue_if_locked` (I'll be removing that and only have `enqueue` soon, however, so check there if you can't find it).

Not sure I understand what you are trying to say. Maybe it will be clearer when you have pushed code.

But I am not talking about the consumer. I am talking about any thread which logs and calls into here, then fails to write to the buffer, and calls `os::malloc` which in turn may log, which calls back into this code. But maybe you are saying there are already guardrails around logging under the ProducerLock.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22770#discussion_r1926746176