Debugging
A guide to debugging Redis server processes
Redis is developed with an emphasis on stability. We do our best with every release to make sure you'll experience a stable product with no crashes. However, if you ever need to debug the Redis process itself, read on.
When Redis crashes, it produces a detailed report of what happened. However, sometimes looking at the crash report is not enough, nor is it possible for the Redis core team to reproduce the issue independently. In this scenario, we need help from the user who can reproduce the issue.
This guide shows how to use GDB to provide the information the Redis developers will need to track the bug more easily.
What is GDB?
GDB is the Gnu Debugger: a program that is able to inspect the internal state of another program. Usually tracking and fixing a bug is an exercise in gathering more information about the state of the program at the moment the bug happens, so GDB is an extremely useful tool.
GDB can be used in two ways:
- It can attach to a running program and inspect the state of it at runtime.
- It can inspect the state of a program that already terminated using what is called a core file, that is, the image of the memory at the time the program was running.
From the point of view of investigating Redis bugs we need to use both of these
GDB modes. The user able to reproduce the bug attaches GDB to their running Redis
instance, and when the crash happens, they create the core file that in turn
the developer will use to inspect the Redis internals at the time of the crash.
This way the developer can perform all the inspections in his or her computer without the help of the user, and the user is free to restart Redis in their production environment.
Compiling Redis without optimizations
By default Redis is compiled with the -O2 switch, this means that compiler
optimizations are enabled. This makes the Redis executable faster, but at the
same time it makes Redis (like any other program) harder to inspect using GDB.
It is better to attach GDB to Redis compiled without optimizations using the
make noopt command (instead of just using the plain make command). However,
if you have an already running Redis in production there is no need to recompile
and restart it if this is going to create problems on your side. GDB still works
against executables compiled with optimizations.
You should not be overly concerned at the loss of performance from compiling Redis without optimizations. It is unlikely that this will cause problems in your environment as Redis is not very CPU-bound.
Attaching GDB to a running process
If you have an already running Redis server, you can attach GDB to it, so that
if Redis crashes it will be possible to both inspect the internals and generate
a core dump file.
After you attach GDB to the Redis process it will continue running as usual without any loss of performance, so this is not a dangerous procedure.
In order to attach GDB the first thing you need is the process ID of the running
Redis instance (the pid of the process). You can easily obtain it using
redis-cli:
$ redis-cli info | grep process_id
process_id:58414
In the above example the process ID is 58414.
Login into your Redis server.
(Optional but recommended) Start screen or tmux or any other program that will make sure that your GDB session will not be closed if your ssh connection times out. You can learn more about screen in this article.
Attach GDB to the running Redis server by typing:
$ gdb <path-to-redis-executable> <pid>
For example:
$ gdb /usr/local/bin/redis-server 58414
GDB will start and will attach to the running server printing something like the following:
Reading symbols for shared libraries + done
0x00007fff8d4797e6 in epoll_wait ()
(gdb)
At this point GDB is attached but your Redis instance is blocked by GDB. In order to let the Redis instance continue the execution just type continue at the GDB prompt, and press enter.
(gdb) continue
Continuing.
Done! Now your Redis instance has GDB attached. Now you can wait for the next crash. :)
Now it's time to detach your screen/tmux session, if you are running GDB using it, by pressing Ctrl-a a key combination.
After the crash
Redis has a command to simulate a segmentation fault (in other words a bad crash) using
the DEBUG SEGFAULT command (don't use it against a real production instance of course!
So I'll use this command to crash my instance to show what happens in the GDB side:
(gdb) continue
Continuing.
Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0xffffffffffffffff
debugCommand (c=0x7ffc32005000) at debug.c:220
220 *((char*)-1) = 'x';
As you can see GDB detected that Redis crashed, and was even able to show me the file name and line number causing the crash. This is already much better than the Redis crash report back trace (containing just function names and binary offsets).
Obtaining the stack trace
The first thing to do is to obtain a full stack trace with GDB. This is as simple as using the bt command:
(gdb) bt
#0 debugCommand (c=0x7ffc32005000) at debug.c:220
#1 0x000000010d246d63 in call (c=0x7ffc32005000) at redis.c:1163
#2 0x000000010d247290 in processCommand (c=0x7ffc32005000) at redis.c:1305
#3 0x000000010d251660 in processInputBuffer (c=0x7ffc32005000) at networking.c:959
#4 0x000000010d251872 in readQueryFromClient (el=0x0, fd=5, privdata=0x7fff76f1c0b0, mask=220924512) at networking.c:1021
#5 0x000000010d243523 in aeProcessEvents (eventLoop=0x7fff6ce408d0, flags=220829559) at ae.c:352
#6 0x000000010d24373b in aeMain (eventLoop=0x10d429ef0) at ae.c:397
#7 0x000000010d2494ff in main (argc=1, argv=0x10d2b2900) at redis.c:2046
This shows the backtrace, but we also want to dump the processor registers using the info registers command:
(gdb) info registers
rax 0x0 0
rbx 0x7ffc32005000 140721147367424
rcx 0x10d2b0a60 4515891808
rdx 0x7fff76f1c0b0 140735188943024
rsi 0x10d299777 4515796855
rdi 0x0 0
rbp 0x7fff6ce40730 0x7fff6ce40730
rsp 0x7fff6ce40650 0x7fff6ce40650
r8 0x4f26b3f7 1327936503
r9 0x7fff6ce40718 140735020271384
r10 0x81 129
r11 0x10d430398 4517462936
r12 0x4b7c04f8babc0 1327936503000000
r13 0x10d3350a0 4516434080
r14 0x10d42d9f0 4517452272
r15 0x10d430398 4517462936
rip 0x10d26cfd4 0x10d26cfd4 <debugCommand+68>
eflags 0x10246 66118
cs 0x2b 43
ss 0x0 0
ds 0x0 0
es 0x0 0
fs 0x0 0
gs 0x0 0
Please make sure to include both of these outputs in your bug report.
Obtaining the core file
The next step is to generate the core dump, that is the image of the memory of the running Redis process. This is done using the gcore command:
(gdb) gcore
Saved corefile core.58414
Now you have the core dump to send to the Redis developer, but it is important to understand that this happens to contain all the data that was inside the Redis instance at the time of the crash; Redis developers will make sure not to share the content with anyone else, and will delete the file as soon as it is no longer used for debugging purposes, but you are warned that by sending the core file you are sending your data.
What to send to developers
Finally you can send everything to the Redis core team:
- The Redis executable you are using.
- The stack trace produced by the bt command, and the registers dump.
- The core file you generated with gdb.
- Information about the operating system and GCC version, and Redis version you are using.
Thank you
Your help is extremely important! Many issues can only be tracked this way. So thanks!