If multiple threads are blocked in sigwait() for the same signal that does

not have a user-supplied signal handler, when a signal is delivered, one
thread will receive the signal, and then the code reverts to having no
signal handler for the signal.  This can leave the other sigwait()ing
threads stranded permanently if the signal is later ignored, or can result
in process termination when the process should have delivered the signal to
one of the threads in sigwait().

To fix this problem, maintain a count of sigwait()ers for each signal that
has no default signal handler.  Use the count to correctly install/uninstall
dummy signal handlers.

Reviewed by:	deischen

3 files changed, 37 insertions, 4 deletions

diff --git a/lib/libc_r/uthread/pthread_private.h b/lib/libc_r/uthread/pthread_private.h
index 6b48f23..1ecda3c 100644
--- a/lib/libc_r/uthread/pthread_private.h
+++ b/lib/libc_r/uthread/pthread_private.h
@@ -930,6 +930,13 @@ SCLASS	pthread_cond_t  _gc_cond
 SCLASS struct  sigaction _thread_sigact[NSIG];
 
 /*
+ * Array of counts of dummy handlers for SIG_DFL signals.  This is used to
+ * assure that there is always a dummy signal handler installed while there is a
+ * thread sigwait()ing on the corresponding signal.
+ */
+SCLASS int	_thread_dfl_count[NSIG];
+
+/*
  * Pending signals for this process.
  */
 SCLASS sigset_t	_process_sigpending;
diff --git a/lib/libc_r/uthread/uthread_init.c b/lib/libc_r/uthread/uthread_init.c
index dd5f53f..8e13f90 100644
--- a/lib/libc_r/uthread/uthread_init.c
+++ b/lib/libc_r/uthread/uthread_init.c
@@ -277,6 +277,9 @@ _thread_init(void)
 				 */
 				PANIC("Cannot read signal handler info");
 			}
+
+			/* Initialize the SIG_DFL dummy handler count. */
+			_thread_dfl_count[i] = 0;
 		}
 
 		/*
diff --git a/lib/libc_r/uthread/uthread_sigwait.c b/lib/libc_r/uthread/uthread_sigwait.c
index a509687..b12c028 100644
--- a/lib/libc_r/uthread/uthread_sigwait.c
+++ b/lib/libc_r/uthread/uthread_sigwait.c
@@ -95,6 +95,12 @@ sigwait(const sigset_t * set, int *sig)
 	}
 
 	/*
+	 * Access the _thread_dfl_count array under the protection of signal
+	 * deferral.
+	 */
+	_thread_kern_sig_defer();
+
+	/*
 	 * Enter a loop to find the signals that are SIG_DFL.  For
 	 * these signals we must install a dummy signal handler in
 	 * order for the kernel to pass them in to us.  POSIX says
@@ -107,10 +113,16 @@ sigwait(const sigset_t * set, int *sig)
 	for (i = 1; i < NSIG; i++) {
 		if (sigismember(&waitset, i) &&
 		    (_thread_sigact[i - 1].sa_handler == SIG_DFL)) {
-			if (_thread_sys_sigaction(i,&act,NULL) != 0)
-				ret = -1;
+			_thread_dfl_count[i]++;
+			if (_thread_dfl_count[i] == 1) {
+				if (_thread_sys_sigaction(i,&act,NULL) != 0)
+					ret = -1;
+			}
 		}
 	}
+	/* Done accessing _thread_dfl_count for now. */
+	_thread_kern_sig_undefer();
+
 	if (ret == 0) {
 		/*
 		 * Save the wait signal mask.  The wait signal
@@ -132,15 +144,26 @@ sigwait(const sigset_t * set, int *sig)
 		_thread_run->data.sigwait = NULL;
 	}
 
+	/*
+	 * Access the _thread_dfl_count array under the protection of signal
+	 * deferral.
+	 */
+	_thread_kern_sig_defer();
+
 	/* Restore the sigactions: */
 	act.sa_handler = SIG_DFL;
 	for (i = 1; i < NSIG; i++) {
 		if (sigismember(&waitset, i) &&
 		    (_thread_sigact[i - 1].sa_handler == SIG_DFL)) {
-			if (_thread_sys_sigaction(i,&act,NULL) != 0)
-				ret = -1;
+			_thread_dfl_count[i]--;
+			if (_thread_dfl_count == 0) {
+				if (_thread_sys_sigaction(i,&act,NULL) != 0)
+					ret = -1;
+			}
 		}
 	}
+	/* Done accessing _thread_dfl_count. */
+	_thread_kern_sig_undefer();
 
 	_thread_leave_cancellation_point();