System information (version)

• OpenCV => 4.5.1
• Operating System / Platform => Linux
• Compiler => clang

Detailed description

Our thread sanitizer finds a race condition:

WARNING: ThreadSanitizer: data race (pid=3836)
Read of size 1 at 0x7b240000e1e8 by main thread:
#0 cv::ThreadPool::run(cv::Range const&, cv::ParallelLoopBody const&, double) third_party/OpenCV/v4_5_1/modules/core/src/parallel_impl.cpp:574:32
#1 cv::parallel_for_pthreads(cv::Range const&, cv::ParallelLoopBody const&, double) third_party/OpenCV/v4_5_1/modules/core/src/parallel_impl.cpp:731:28
#2 parallel_for_impl third_party/OpenCV/v4_5_1/modules/core/src/parallel.cpp:584:9
#3 cv::parallel_for_(cv::Range const&, cv::ParallelLoopBody const&, double) third_party/OpenCV/v4_5_1/modules/core/src/parallel.cpp:512:13
....

Previous write of size 1 at 0x7b240000e1e8 by thread T14 (mutexes: write M16796):
#0 cv::WorkerThread::thread_body() third_party/OpenCV/v4_5_1/modules/core/src/parallel_impl.cpp:396:22
#1 cv::WorkerThread::thread_loop_wrapper(void*) third_party/OpenCV/v4_5_1/modules/core/src/parallel_impl.cpp:278:41

It seems IsActive is set in the thread with no mutex at

while the threadpool can read by the main thread at

CV_USE_GLOBAL_WORKERS_COND_VAR is not defined.
Defining it solves the issue. Do we want to have code that is not thread-safe by default ?

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues,
  forum.opencv.org, Stack Overflow, etc and have not found solution
• I updated to latest OpenCV version and the issue is still there