Support for dumping current PrettyStackTrace on SIGINFO (Ctrl-T)

Support SIGINFO (and SIGUSR1 for POSIX purposes) to tell what
long-running jobs are doing, as inspired by BSD tools (including on
macOS), by dumping the current PrettyStackTrace.

This adds a new kind of signal handler for non-fatal "info" signals,
similar to the "interrupt" handler that already exists for SIGINT
(Ctrl-C). It then uses that handler to update a "generation count"
managed by the PrettyStackTrace infrastructure, which is then checked
whenever a PrettyStackTraceEntry is pushed or popped on each
thread. If the generation has changed---i.e. if the user has pressed
Ctrl-T---the stack trace is dumped, though unfortunately it can't
include the deepest entry because that one is currently being
constructed/destructed.

https://reviews.llvm.org/D63750

llvm-svn: 365911

1 files changed, 62 insertions, 3 deletions

diff --git a/llvm/lib/Support/PrettyStackTrace.cpp b/llvm/lib/Support/PrettyStackTrace.cpp
index 497084ecda9..e566440f49a 100644
--- a/llvm/lib/Support/PrettyStackTrace.cpp
+++ b/llvm/lib/Support/PrettyStackTrace.cpp
@@ -16,6 +16,7 @@
 #include "llvm/ADT/SmallString.h"
 #include "llvm/Config/config.h"
 #include "llvm/Support/Compiler.h"
+#include "llvm/Support/SaveAndRestore.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/Watchdog.h"
 #include "llvm/Support/raw_ostream.h"
@@ -41,6 +42,22 @@ using namespace llvm;
 // thread-local variable.
 static LLVM_THREAD_LOCAL PrettyStackTraceEntry *PrettyStackTraceHead = nullptr;
 
+// The use of 'volatile' here is to ensure that any particular thread always
+// reloads the value of the counter. The 'std::atomic' allows us to specify that
+// this variable is accessed in an unsychronized way (it's not actually
+// synchronizing). This does technically mean that the value may not appear to
+// be the same across threads running simultaneously on different CPUs, but in
+// practice the worst that will happen is that we won't print a stack trace when
+// we could have.
+//
+// This is initialized to 1 because 0 is used as a sentinel for "not enabled on
+// the current thread". If the user happens to overflow an 'unsigned' with
+// SIGINFO requests, it's possible that some threads will stop responding to it,
+// but the program won't crash.
+static volatile std::atomic<unsigned> GlobalSigInfoGenerationCounter =
+    ATOMIC_VAR_INIT(1);
+static LLVM_THREAD_LOCAL unsigned ThreadLocalSigInfoGenerationCounter = 0;
+
 namespace llvm {
 PrettyStackTraceEntry *ReverseStackTrace(PrettyStackTraceEntry *Head) {
   PrettyStackTraceEntry *Prev = nullptr;
@@ -56,8 +73,9 @@ static void PrintStack(raw_ostream &OS) {
   // to fail if we crashed due to stack overflow), we do an up-front pass to
   // reverse the stack, then print it, then reverse it again.
   unsigned ID = 0;
-  PrettyStackTraceEntry *ReversedStack =
-      llvm::ReverseStackTrace(PrettyStackTraceHead);
+  SaveAndRestore<PrettyStackTraceEntry *> SavedStack{PrettyStackTraceHead,
+                                                     nullptr};
+  PrettyStackTraceEntry *ReversedStack = ReverseStackTrace(SavedStack.get());
   for (const PrettyStackTraceEntry *Entry = ReversedStack; Entry;
        Entry = Entry->getNextEntry()) {
     OS << ID++ << ".\t";
@@ -67,7 +85,10 @@ static void PrintStack(raw_ostream &OS) {
   llvm::ReverseStackTrace(ReversedStack);
 }
 
-/// PrintCurStackTrace - Print the current stack trace to the specified stream.
+/// Print the current stack trace to the specified stream.
+///
+/// Marked NOINLINE so it can be called from debuggers.
+LLVM_ATTRIBUTE_NOINLINE
 static void PrintCurStackTrace(raw_ostream &OS) {
   // Don't print an empty trace.
   if (!PrettyStackTraceHead) return;
@@ -127,10 +148,24 @@ static void CrashHandler(void *) {
 #endif
 }
 
+static void printForSigInfoIfNeeded() {
+  unsigned CurrentSigInfoGeneration =
+      GlobalSigInfoGenerationCounter.load(std::memory_order_relaxed);
+  if (ThreadLocalSigInfoGenerationCounter == 0 ||
+      ThreadLocalSigInfoGenerationCounter == CurrentSigInfoGeneration) {
+    return;
+  }
+
+  PrintCurStackTrace(errs());
+  ThreadLocalSigInfoGenerationCounter = CurrentSigInfoGeneration;
+}
+
 #endif // ENABLE_BACKTRACES
 
 PrettyStackTraceEntry::PrettyStackTraceEntry() {
 #if ENABLE_BACKTRACES
+  // Handle SIGINFO first, because we haven't finished constructing yet.
+  printForSigInfoIfNeeded();
   // Link ourselves.
   NextEntry = PrettyStackTraceHead;
   PrettyStackTraceHead = this;
@@ -142,6 +177,8 @@ PrettyStackTraceEntry::~PrettyStackTraceEntry() {
   assert(PrettyStackTraceHead == this &&
          "Pretty stack trace entry destruction is out of order");
   PrettyStackTraceHead = NextEntry;
+  // Handle SIGINFO first, because we already started destructing.
+  printForSigInfoIfNeeded();
 #endif
 }
 
@@ -188,6 +225,28 @@ void llvm::EnablePrettyStackTrace() {
 #endif
 }
 
+void llvm::EnablePrettyStackTraceOnSigInfoForThisThread(bool ShouldEnable) {
+#if ENABLE_BACKTRACES
+  if (!ShouldEnable) {
+    ThreadLocalSigInfoGenerationCounter = 0;
+    return;
+  }
+
+  // The first time this is called, we register the SIGINFO handler.
+  static bool HandlerRegistered = []{
+    sys::SetInfoSignalFunction([]{
+      GlobalSigInfoGenerationCounter.fetch_add(1, std::memory_order_relaxed);
+    });
+    return false;
+  }();
+  (void)HandlerRegistered;
+
+  // Next, enable it for the current thread.
+  ThreadLocalSigInfoGenerationCounter =
+      GlobalSigInfoGenerationCounter.load(std::memory_order_relaxed);
+#endif
+}
+
 const void *llvm::SavePrettyStackState() {
 #if ENABLE_BACKTRACES
   return PrettyStackTraceHead;