- 16 Oct, 2023 1 commit
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meiravgri authored
Using heap allocation during signal handlers is unsafe. This PR purpose is to replace all the heap allocations done within the signal handlers raised upon server crash and assertions. These were added in #12453. writeStacktraces(): allocates the stacktraces output array on the calling thread's stack and assigns the address to a global variable. It calls `ThreadsManager_runOnThreads()` that invokes `collect_stacktrace_data()` by each thread: each thread writes to a different location in the above array to allow sync writes. get_ready_to_signal_threads_tids(): instead of allocating the `tids` array, it receives it as a fixed size array parameter, allocated on on the stack of the calling function, and returns the number of valid threads. The array size is hard-coded to 50. `ThreadsManager_runOnThreads():` To avoid the outputs array allocation, the **callback signature** was changed. Now it should return void. This function return type has also changed to int - returns 1 if successful, and 0 otherwise. Other unsafe calls will be handled in following PRs
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- 02 Oct, 2023 1 commit
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meiravgri authored
## Crash fix ### Current behavior We might crash if we fail to collect some of the threads' output. If it exceeds timeout for example. The threads mngr API guarantees that the output array length will be `tids_len`, however, some indices can be NULL, in case it fails to collect some of the threads' outputs. When we use the threads mngr to collect the threads' stacktraces, we rely on this and skip NULL entries. Since the output array was allocated with malloc, instead of NULL, it contained garbage, so we got a segmentation fault when trying to read this garbage. (in debug.c:writeStacktraces() ) ### fix Allocate the global output array with zcalloc. ### To reproduce the bug, you'll have to change the code: **in threadsmngr:ThreadsManager_runOnThreads():** make sure the g_output_array allocation is initialized with garbage and not 0s (add `memset(g_output_array, 2, sizeof(void*) * tids_len);` below the allocation). Force one of the threads to write to the array: add a global var: `static redisAtomic size_t return_now = 0;` add to `invoke_callback()` before writing to the output array: ``` size_t i_return; atomicGetIncr(return_now, i_return, 1); if(i_return == 1) return; ``` compile, start the server with `--enable-debug-command local` and run `redis-cli debug assert` The assertion triggers the the stacktrace collection. Expect to get 2 prints of the stack trace - since we get the segmentation fault after we return from the threads mngr, it can be safely triggered again. ## Added global variables r/w lock in ThreadsManager To avoid a situation where the main thread runs `ThreadsManager_cleanups` while threads are still invoking the signal handler, we use a r/w lock. For cleanups, we will acquire the write lock. The threads will acquire the read lock to enable them to write simultaneously. If we fail to acquire the read lock, it means cleanups are in progress and we return immediately. After acquiring the lock we can safely check that the global output array wasn't nullified and proceed to write to it. This way we ensure the threads are not modifying the global variables/ trying to write to the output array after they were zeroed/nullified/destroyed(the semaphore). ## other minor logging change 1. removed logging if the semaphore times out because the threads can still write to the output array after this check. Instead, we print the total number of printed stacktraces compared to the exacted number (len_tids). 2. use noinline attribute to make sure the uplevel number of ignored stack trace entries stays correct. 3. improve testing Co-authored-by:Oran Agra <oran@redislabs.com>
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- 24 Sep, 2023 1 commit
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meiravgri authored
In this PR we are adding the functionality to collect all the process's threads' backtraces. ## Changes made in this PR ### **introduce threads mngr API** The **threads mngr API** which has 2 abilities: * `ThreadsManager_init() `- register to SIGUSR2. called on the server start-up. * ` ThreadsManager_runOnThreads()` - receives a list of a pid_t and a callback, tells every thread in the list to invoke the callback, and returns the output collected by each invocation. **Elaborating atomicvar API** * `atomicIncrGet(var,newvalue_var,count) `-- Increment and get the atomic counter new value * `atomicFlagGetSet` -- Get and set the atomic counter value to 1 ### **Always set SIGALRM handler** SIGALRM handler prints the process's stacktrace to the log file. Up until now, it was set only if the `server.watchdog_period` > 0. This can be also useful if debugging is needed. However, in situations where the server can't get requests, (a deadlock, for example) we weren't able to change the signal handler. To make it available at run time we set SIGALRM handler on server startup. The signal handler name was changed to a more general `sigalrmSignalHandler`. ### **Print all the process' threads' stacktraces** `logStackTrace()` now calls `writeStacktraces()`, instead of logging the current thread stacktrace. `writeStacktraces()`: * On Linux systems we use the threads manager API to collect the backtraces of all the process' threads. To get the `tids` list (threads ids) we read the `/proc/<redis-server-pid>/tasks` file which includes a list of directories. Each directory name corresponds to one tid (including the main thread). For each thread, we also need to check if it can get the signal from the threads manager (meaning it is not blocking/ignoring that signal). We send the threads manager this tids list and `collect_stacktrace_data()` callback, which collects the thread's backtrace addresses, its name, and tid. * On other systems, the behavior remained as it was (writing only the current thread stacktrace to the log file). ## compatibility notes 1. **The threads mngr API is only supported in linux.** 2. glibc earlier than 2.3 We use `syscall(SYS_gettid)` and `syscall(SYS_tgkill...)` because their dedicated alternatives (`gettid()` and `tgkill`) were added in glibc 2.3. ## Output example Each thread backtrace will have the following format: `<tid> <thread_name> [additional_info]` * **tid**: as read from the `/proc/<redis-server-pid>/tasks` file * **thread_name**: the tread name as it is registered in the os/ * **additional_info**: Sometimes we want to add specific information about one of the threads. currently. it is only used to mark the thread that handles the backtraces collection by adding "*". In case of crash - this also indicates which thread caused the crash. The handling thread in won't necessarily appear first. ``` ------ STACK TRACE ------ EIP: /lib/aarch64-linux-gnu/libc.so.6(epoll_pwait+0x9c)[0xffffb9295ebc] 67089 redis-server * linux-vdso.so.1(__kernel_rt_sigreturn+0x0)[0xffffb9437790] /lib/aarch64-linux-gnu/libc.so.6(epoll_pwait+0x9c)[0xffffb9295ebc] redis-server *:6379(+0x75e0c)[0xaaaac2fe5e0c] redis-server *:6379(aeProcessEvents+0x18c)[0xaaaac2fe6c00] redis-server *:6379(aeMain+0x24)[0xaaaac2fe7038] redis-server *:6379(main+0xe0c)[0xaaaac3001afc] /lib/aarch64-linux-gnu/libc.so.6(+0x273fc)[0xffffb91d73fc] /lib/aarch64-linux-gnu/libc.so.6(__libc_start_main+0x98)[0xffffb91d74cc] redis-server *:6379(_start+0x30)[0xaaaac2fe0370] 67093 bio_lazy_free /lib/aarch64-linux-gnu/libc.so.6(+0x79dfc)[0xffffb9229dfc] /lib/aarch64-linux-gnu/libc.so.6(pthread_cond_wait+0x208)[0xffffb922c8fc] redis-server *:6379(bioProcessBackgroundJobs+0x174)[0xaaaac30976e8] /lib/aarch64-linux-gnu/libc.so.6(+0x7d5c8)[0xffffb922d5c8] /lib/aarch64-linux-gnu/libc.so.6(+0xe5d1c)[0xffffb9295d1c] 67091 bio_close_file /lib/aarch64-linux-gnu/libc.so.6(+0x79dfc)[0xffffb9229dfc] /lib/aarch64-linux-gnu/libc.so.6(pthread_cond_wait+0x208)[0xffffb922c8fc] redis-server *:6379(bioProcessBackgroundJobs+0x174)[0xaaaac30976e8] /lib/aarch64-linux-gnu/libc.so.6(+0x7d5c8)[0xffffb922d5c8] /lib/aarch64-linux-gnu/libc.so.6(+0xe5d1c)[0xffffb9295d1c] 67092 bio_aof /lib/aarch64-linux-gnu/libc.so.6(+0x79dfc)[0xffffb9229dfc] /lib/aarch64-linux-gnu/libc.so.6(pthread_cond_wait+0x208)[0xffffb922c8fc] redis-server *:6379(bioProcessBackgroundJobs+0x174)[0xaaaac30976e8] /lib/aarch64-linux-gnu/libc.so.6(+0x7d5c8)[0xffffb922d5c8] /lib/aarch64-linux-gnu/libc.so.6(+0xe5d1c)[0xffffb9295d1c] 67089:signal-handler (1693824528) -------- ```
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