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Ignore SEGV during profiler unwind on Unix #28291

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merged 3 commits into from
Jul 30, 2018
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Ignore SEGV during profiler unwind on Unix #28291

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5730dc0
Ignore SEGV during profiler unwind on Unix.
maleadt Jul 26, 2018
24413c6
Alternative approach based on setjmp/longjmp with ptls->safe_restore.
maleadt Jul 26, 2018
a526b29
Reuse jl_call_in_ctx.
maleadt Jul 27, 2018
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31 changes: 28 additions & 3 deletions src/signals-unix.c
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Original file line number Diff line number Diff line change
Expand Up @@ -225,6 +225,18 @@ static void segv_handler(int sig, siginfo_t *info, void *context)
jl_ptls_t ptls = jl_get_ptls_states();
assert(sig == SIGSEGV || sig == SIGBUS);

// if we're profiling, this segfault is likely caused by the unwinder.
// ignore the signal and jump back to where we came from.
if (running && ptls->safe_restore) {
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Why do you need this? Does it not work with the condition below?

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Just being conservative, only affecting the case where the profiler is running. Do it unconditionally then?

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I mean you shouldn't need any code here in the segfault handler. Have you tested that it doesn't work without this but with the condition a few lines below?

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(Or in another word, it is meant to be doing this unconditionally)

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Ah ok, no it doesn't work, looking closer it triggers a segfault in jl_call_in_ctx (via jl_throw_in_ctx..., jl_stackovf_exception, ...)).
Inferring from the function names, doesn't that behave differently from the plain longjmp I do here, would I need to catch an exception then somehow?

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Hmm, segfault in jl_call_in_ctx? Did you get a NULL context?

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Ah, I guess this thread doesn't have signal_stack allocated. I believe this should fix it (you can merge this with the falllback ifdef below if you want).

diff --git a/src/signals-unix.c b/src/signals-unix.c
index 0fafe121cd..8da89b5fc4 100644
--- a/src/signals-unix.c
+++ b/src/signals-unix.c
@@ -89,6 +89,14 @@ static void jl_call_in_ctx(jl_ptls_t ptls, void (*fptr)(void), int sig, void *_c
     // checks that the syscall is made in the signal handler and that
     // the ucontext address is valid. Hopefully the value of the ucontext
     // will not be part of the validation...
+    if (!ptls->signal_stack) {
+        sigset_t sset;
+        sigemptyset(&sset);
+        sigaddset(&sset, sig);
+        sigprocmask(SIG_UNBLOCK, &sset, NULL);
+        fptr();
+        return;
+    }
     uintptr_t rsp = (uintptr_t)ptls->signal_stack + sig_stack_size;
     assert(rsp % 16 == 0);
 #if defined(_OS_LINUX_) && defined(_CPU_X86_64_)

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Yeah, that works. Thanks!
Why isn't this used for OSX btw? Mimicking profiler_segv_handler which does thread_set_state is what got me here in the first place.

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We don't handle segfault the same way on OSX. I don't really know if the two ways could be used together.

// unblock the signal being handled
sigset_t sset;
sigemptyset(&sset);
sigaddset(&sset, sig);
sigprocmask(SIG_UNBLOCK, &sset, NULL);

jl_longjmp(*ptls->safe_restore, 1);
}

if (jl_addr_is_safepoint((uintptr_t)info->si_addr)) {
#ifdef JULIA_ENABLE_THREADING
jl_set_gc_and_wait();
Expand Down Expand Up @@ -667,9 +679,22 @@ static void *signal_listener(void *arg)
// do backtrace for profiler
if (profile && running) {
if (bt_size_cur < bt_size_max - 1) {
// Get backtrace data
bt_size_cur += rec_backtrace_ctx((uintptr_t*)bt_data_prof + bt_size_cur,
bt_size_max - bt_size_cur - 1, signal_context);
// unwinding can fail, so keep track of the current state
// and restore from the SEGV handler if anything happens.
jl_ptls_t ptls = jl_get_ptls_states();
jl_jmp_buf *old_buf = ptls->safe_restore;
jl_jmp_buf buf;

ptls->safe_restore = &buf;
if (jl_setjmp(buf, 0)) {
jl_safe_printf("WARNING: profiler attempt to access an invalid memory location\n");
} else {
// Get backtrace data
bt_size_cur += rec_backtrace_ctx((uintptr_t*)bt_data_prof + bt_size_cur,
bt_size_max - bt_size_cur - 1, signal_context);
}
ptls->safe_restore = old_buf;

// Mark the end of this block with 0
bt_data_prof[bt_size_cur++] = 0;
}
Expand Down