coroutine: Fix setup of sigaltstack coroutines

Use pthread_kill instead of process-wide kill to invoke the signal
handler used for stack switching. This may fix spurious lock-ups with
this backend, easily triggerable by extending the time window between
kill and sigsuspend.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

coroutine-sigaltstack.c

diff --git a/coroutine-sigaltstack.c b/coroutine-sigaltstack.c
index 7ff2d33..b2e316c 100644
--- a/coroutine-sigaltstack.c
+++ b/coroutine-sigaltstack.c
@@ -226,7 +226,7 @@
      * called.
      */
     coTS->tr_called = 0;
-    kill(getpid(), SIGUSR2);
+    pthread_kill(pthread_self(), SIGUSR2);
     sigfillset(&sigs);
     sigdelset(&sigs, SIGUSR2);
     while (!coTS->tr_called) {