.NET 9 Server Freeze: Deep Dive Into CPU 0 Deadlock

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.NET 9 Server Freeze/Deadlock Investigation: A Deep Dive

Hey guys! Ever hit a wall with your .NET server app? We recently wrestled with a nasty production issue: a complete server freeze on our .NET 9 application. This was a real head-scratcher, so let's break down what happened, what we observed, and how we're (hopefully) getting to the bottom of it. Our main goal is to help you if you ever hit the same problem, so you can solve it ASAP. We'll cover everything from the symptoms to the debugging steps, and hopefully, provide some useful insights. Let's dive in!

The Core Problem: .NET 9 Server Freeze and Deadlock

The central issue was our .NET 9 server application grinding to a halt in production. This wasn't just a performance hiccup; the entire server froze, becoming totally unresponsive. No more API calls went through, and logging stopped dead. Timers got delayed, and the CPU usage tanked. Using tools like htop showed the CPU at 0%, and even dotnet-stack report couldn't get a reading. It was a complete stall. We were seeing a production server freeze/deadlock on .NET 9.0.7, and this meant no happy customers! The core of the problem seemed to be a deadlock, with multiple threads involved. The key clue? A function called __GI_cfsetispeed showing up in the gdb stack traces. This, combined with idle worker threads, pointed to something deep inside the system causing the stall.

The Environment Setup

Before we go further, it's important to know our environment. We were running on:

  • .NET Runtime Version: 9.0.7. This is the specific version of the .NET runtime we were using. The issue seems to be directly linked to this version.
  • Operating System: Linux Ubuntu 24.04. The server was running on Ubuntu. Linux's behavior is critical to understand the behavior of our server.
  • Architecture: x64. We're on a 64-bit system, which influences the memory and how the software runs.
  • GC Mode: Server GC. The server GC is designed for high-throughput scenarios, and we suspect this may have a role in the issue. The settings influence how the garbage collector operates, which can sometimes interact in unexpected ways.
  • Environment Variable: DOTNET_GCName=libclrgc.so. This configuration affects the garbage collection implementation, something we have to remember when we try to debug this issue.

Symptoms of the .NET Freeze

When this freeze struck, we saw a specific set of symptoms. Identifying these was crucial for understanding the root cause. This information could save you time if your .NET 9 server freezes.

  1. No Response: HTTP API endpoints became completely unresponsive. No calls could get through.
  2. No Logs: The application stopped writing new logs. This makes it difficult to trace the issue.
  3. Delayed Timers: System.Threading.Timer instances triggered much later than scheduled, signaling the whole process was stalled.
  4. Correlation with GC: The freeze frequently happened during periods of high GC activity, with reported pause times of over 2 seconds before the incident.
  5. CPU Usage Drop: htop or top showed the process's CPU usage dropping to 0, which suggests that the application has stopped processing.
  6. Thread Pool: The thread pool queue length fell to almost zero, even though new requests were still coming in.
  7. Diagnostics Fail: Attempts to use dotnet-stack report on the hung process also hung, providing no output. It was like the diagnostics tools themselves were also frozen.

The gdb Stack Traces: The Smoking Gun

We successfully used gdb to debug the hung process. The stack traces provided some very important clues. Let's see how our engineers used gdb to debug this issue!

  1. Suspicious Stacks (Potential Deadlock)

    Two threads caught our attention. One was the .NET SynchManag thread, and the other was another thread, both stuck in __GI_cfsetispeed. This pointed to a potential deadlock deep within the system. The __GI_cfsetispeed function is related to setting serial port speeds in Linux, which seemed odd. What's even stranger is that it showed up in two different threads. This suggested a very low-level issue, possibly involving how the runtime interacts with the operating system or its libraries, a very suspicious stack indeed!

    Thread 2 (Thread 0x7fb6b2fb56c0 (LWP 7) ".NET SynchManag"):
#0  0x00007fb6b38c121f in __GI_cfsetispeed (termios_p=0x7fb6b2fb4d78, speed=1) at ../sysdeps/unix/sysv/linux/speed.c:96
#1  0x00007fb6b2fb4db0 in ?? ()
#2  0x00007fb6b2fb4d78 in ?? ()
#3  0x0000000000000001 in ?? ()
#4  0xffffffff00000000 in ?? ()
#5  0x000055a8504b0f58 in ?? ()
#6  0x00007fb6b3628fa0 in ?? () from /usr/share/dotnet/shared/Microsoft.NETCore.App/9.0.7/libcoreclr.so
#7  0x00007fb6b3628603 in ?? () from /usr/share/dotnet/shared/Microsoft.NETCore.App/9.0.7/libcoreclr.so
#8  0x00007fb6b363214e in ?? () from /usr/share/dotnet/shared/Microsoft.NETCore.App/9.0.7/libcoreclr.so
#9  0x00007fb6b384e1f5 in __pthread_create_2_1 (newthread=<optimized out>, attr=<optimized out>, start_routine=<optimized out>, arg=<optimized out>) at ./nptl/pthread_create.c:846
#10 0x0000000000000000 in ?? ()

Thread 181 (Thread 0x7e83927fc6c0 (LWP 255) "rdk:broker10265"):
#0  0x00007fb6b38c121f in __GI_cfsetispeed (termios_p=0x7e83740049b8, speed=2) at ../sysdeps/unix/sysv/linux/speed.c:96
#1# 0x0000000000000000 in ?? ()

  2. Idle Worker Threads

    The other threads weren't doing much. Most .NET TP Worker threads were idle, waiting for work. They were stuck in __pthread_attr_extension. This implied the thread pool was unable to handle incoming requests, further contributing to the freeze.

    Thread 194 (Thread 0x7e8651ffb6c0 (LWP 311668) ".NET TP Worker"):
#0  0x00007fb6b384af16 in __pthread_attr_extension (attr=0x0) at ./nptl/pthread_attr_extension.c:30
#1  0x0000000000000189 in ?? ()
#2  0x000055a8504e83b8 in ?? ()
#3  0x0000000000000000 in ?? ()

Thread 193 (Thread 0x7e84fdffb6c0 (LWP 311660) ".NET TP Worker"):
#0  0x00007fb6b384af16 in __pthread_attr_extension (attr=0x0) at ./nptl/pthread_attr_extension.c:30
#1  0x000055a800000189 in ?? ()
#2  0x000055a8504e83b8 in ?? ()
#3  0x0000000000000000 in ?? ()

Thread 192 (Thread 0x7e851effd6c0 (LWP 311650) ".NET TP Worker"):
#0  0x00007fb6b384af16 in __pthread_attr_extension (attr=0x0) at ./nptl/pthread_attr_extension.c:30
#1  0x0000000000000189 in ?? ()
#2  0x000055a8504e83b8 in ?? ()
#3  0x0000000000000000 in ?? ()

Summary of the Findings

The __GI_cfsetispeed function in the stack traces, particularly in the .NET SynchManag thread, suggests a native-level deadlock. This implies the issue is likely within the interaction between the .NET runtime and the underlying operating system or its libraries, such as glibc. This stall prevents managed code, including timers and the thread pool, from making any progress. The fact that diagnostic tools like dotnet-stack also failed further reinforces this, indicating a very low-level lock or stall.

Steps to Reproduce (and why it's tricky)

Reproducing this type of freeze is proving difficult. It only happened a few times in our production environment. This makes it hard to pin down the exact sequence of events. Finding the perfect sequence to reproduce the error can take some time. However, by knowing the context of this issue, the debugging process is more efficient.

What We Expected vs. What Happened

We expected our application to keep running without interruptions, handling requests promptly. Instead, it froze. This meant our clients got bad service. Understanding this is key to getting back to normal.

Current Status and Known Workarounds

Currently, we don't have a reliable workaround. We're still actively investigating. This is an ongoing effort, and we'll update as we learn more.

Configuration Details

Here's the configuration again, just to be thorough:

  • .NET Runtime Version: 9.0.7
  • Operating System: Linux (inferred from stack trace)
  • Architecture: x64 (inferred from stack trace)
  • GC Mode: Server GC
  • Environment Variable: DOTNET_GCName=libclrgc.so

Other Relevant Information

We don't have other information right now, but we will add it if we figure it out! We will provide all of the details as soon as possible, so stay tuned!

Conclusion: Looking Ahead

This .NET 9 server freeze has been a challenge. The gdb stack traces provided some crucial insights, pointing to a potential deadlock in the interaction between the runtime and the underlying system. Even with the challenge of reproduction, the environment details and stack trace analysis help us work toward a solution. We will continue to investigate, and we hope to provide updates and a resolution soon. Keep checking back for more details as we uncover them! Thanks for reading. Let us know if you have any insights or have experienced something similar!