[uclibc-ng-devel] Bug: Deadlock in malloc on ARMv6 with uClibc-ng + Thumb1 + NPTL

Hello, We have noticed random deadlocks in malloc/free when using uClibc-ng + Thumb1 + NPTL compiled for ARMv6 in multi-threaded environment. Steps to reproduce 1. Build Buildroot 2019.02.9 SDK with the following configuration: BR2_arm=y BR2_arm1176jz_s=y BR2_ARM_INSTRUCTIONS_THUMB=y BR2_SHARED_STATIC_LIBS=y BR2_TOOLCHAIN_BUILDROOT_WCHAR=y BR2_TOOLCHAIN_BUILDROOT_USE_SSP=y BR2_TOOLCHAIN_BUILDROOT_CXX=y 2. Cross compile the sample (see below) application with the SDK using static linking: $ arm-buildroot-linux-uclibcgnueabi-gcc -static -g deadlock.c \ -lpthread -o deadlock 3. Run the application with at least two threads 3.1. Run the application using QEMU static emulation: $ qemu-arm-static ./deadlock 4 1000000 3.2. Run the application on a device with ARMv6 support (ex. any RPi with Raspbian image): $ ./deadlock 4 1000000 Expected results: The application should always exit cleanly. Actual results: The application often deadlocks in both cases. Notes: 1. The bug is also reproducible with latest uClibc-ng (v1.0.32) 2. According to our tests the endianness does not matter 3. The callstack when deadlock appears: (gdb) info threads Id Target Id Frame * 1 LWP 11653 "mutex" 0x000165b2 in pthread_join () 2 LWP 11656 "mutex" 0x00014cac in __lll_lock_wait () 3 LWP 11657 "mutex" 0x00014ce2 in __lll_lock_wait () 4 LWP 11658 "mutex" 0x00014cac in __lll_lock_wait () 5 LWP 11659 "mutex" 0x00014cac in __lll_lock_wait () (gdb) t 1 [Switching to thread 1 (LWP 11653)] #0 0x000165b2 in pthread_join () (gdb) bt #0 0x000165b2 in pthread_join () #1 0x00010396 in main (argc=3, argv=0x7efff784) at deadlock.c:42 (gdb) t 2 [Switching to thread 2 (LWP 11656)] #0 0x00014cac in __lll_lock_wait () (gdb) bt #0 0x00014cac in __lll_lock_wait () #1 0x00016b7e in pthread_mutex_lock () #2 0x0001199e in __libc_malloc () #3 0x00010278 in do_request (data=0x1) at deadlock.c:13 #4 0x00015b36 in start_thread () #5 0x000173f4 in clone () (gdb) t 3 [Switching to thread 3 (LWP 11657)] #0 0x00014ce2 in __lll_lock_wait () (gdb) bt #0 0x00014ce2 in __lll_lock_wait () #1 0x00016b7e in pthread_mutex_lock () #2 0x000117c2 in __libc_free () #3 0x00010284 in do_request (data=0x2) at deadlock.c:15 #4 0x00015b36 in start_thread () #5 0x000173f4 in clone () (gdb) t 4 [Switching to thread 4 (LWP 11658)] #0 0x00014cac in __lll_lock_wait () (gdb) bt #0 0x00014cac in __lll_lock_wait () #1 0x00016b7e in pthread_mutex_lock () #2 0x0001199e in __libc_malloc () #3 0x00010278 in do_request (data=0x3) at deadlock.c:13 #4 0x00015b36 in start_thread () #5 0x000173f4 in clone () (gdb) t 5 [Switching to thread 5 (LWP 11659)] #0 0x00014cac in __lll_lock_wait () (gdb) bt #0 0x00014cac in __lll_lock_wait () #1 0x00016b7e in pthread_mutex_lock () #2 0x0001199e in __libc_malloc () #3 0x00010278 in do_request (data=0x4) at deadlock.c:13 #4 0x00015b36 in start_thread () #5 0x000173f4 in clone () Sample application: #include <stdlib.h> #include <stdio.h> #include <pthread.h> int count; void *do_request(void *data) { for (int i = 0; i < count; i++) { if (i % 1000 == 0) printf("Alloc for %p count %d\n", data, i); char *buffer = malloc(32 * sizeof(char)); free(buffer); } return NULL; } int main(int argc, char *argv[]) { int i, rc; int threads_count = atoi(argv[1]); count = atoi(argv[2]); pthread_t threads[threads_count]; printf("Creating %d threads\n", threads_count); for (i = 0; i < threads_count; i++) { rc = pthread_create(&threads[i], NULL, do_request, (int*)(i+1)); if (rc) { printf("Error creating thread\n"); exit(1); } } for (i = 0; i < threads_count; i++) if (pthread_join(threads[i], NULL)) { printf("Error joining thread\n"); exit(1); } printf("Done. \n"); return 0; } Best regards, Gyula Farkas