Date: Fri, 22 Feb 2019 12:10:26 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Robert Ayrapetyan <robert.ayrapetyan@gmail.com> Cc: FreeBSD <freebsd-hackers@freebsd.org> Subject: Re: ptrace: SIGTRAP and EXIT race Message-ID: <20190222101026.GX2420@kib.kiev.ua> In-Reply-To: <CAAboi9tT==FFXdqq9XG2v8Lxf8RBuTMx5ns4puZ-hjD5KecFsA@mail.gmail.com> References: <CAAboi9tT==FFXdqq9XG2v8Lxf8RBuTMx5ns4puZ-hjD5KecFsA@mail.gmail.com>
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On Thu, Feb 21, 2019 at 08:43:20PM -0800, Robert Ayrapetyan wrote:
> Hello.
>
> Before creating a bug and providing some test code, would ask a community
> here.
> When tracing a process using ptrace and there are multiple threads in the
> tracing process hitting the same breakpoint, sometimes main thread exits
> (WIFEXITED(status) is TRUE) before last queued TRAP_BKPT signal(s) have
> been delivered to the tracing process. So a final breakpoint hits counter
> is less than it should be.
>
> So in the example below:
>
> #include <iostream>
> #include <thread>
>
> #include <pthread_np.h>
>
> static const int num_threads = 2;
>
> void foo() {
> for (int i = 0; i < 2; ++i) {
> printf("hi: %d (tid: %d)\n", i, pthread_getthreadid_np());
> }
> }
>
> int main() {
> std::thread t[num_threads];
>
> for (int i = 0; i < num_threads; ++i) {
> t[i] = std::thread(foo);
> }
>
> for (int i = 0; i < num_threads; ++i) {
> t[i].join();
> }
>
> return 0;
> }
>
> If we set breakpoint to printf, it should be triggered 4 times (tracing
> process should receive TRAP_BKPT 4 times). However, in ~1 of 5 runs, it
> receives TRAP_BKPT just 2 or 3 times.
>
> Is this expected? Thanks.
I indeed would expect that all four breakpoints triggered before the
main thread exits, assuming that the breakpoints were installed before
the threads are created. Please provide the stand-alone (and preferrably
non-interactive) test to reproduce the issue.
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