If getSiginfo in libgo does not know how to determine the PC, it will call runtime_callers. That can happen in a thread that was started by non-Go code, in which case the TLS variable g will not be set, in which case runtime_lock will crash.
Avoid the problem by using atomic operations for the lock. This is OK since creating a backtrace state is fast and never blocks. The test case is TestCgoExternalThreadSIGPROF in the runtime package on a system that getSiginfo doesn't handle specially. Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu and powerpc64le-unknown-linux-gnu. Committed to mainline. Ian
Index: gcc/go/gofrontend/MERGE =================================================================== --- gcc/go/gofrontend/MERGE (revision 250436) +++ gcc/go/gofrontend/MERGE (working copy) @@ -1,4 +1,4 @@ -a9f1aeced86691de891fbf2a8c97e848faf1962e +b712bacd939466e66972337744983e180849c535 The first line of this file holds the git revision number of the last merge done from the gofrontend repository. Index: libgo/runtime/go-caller.c =================================================================== --- libgo/runtime/go-caller.c (revision 250406) +++ libgo/runtime/go-caller.c (working copy) @@ -74,7 +74,7 @@ static void *back_state; /* A lock to control creating back_state. */ -static Lock back_state_lock; +static uint32 back_state_lock; /* The program arguments. */ @@ -85,7 +85,15 @@ extern Slice runtime_get_args(void); struct backtrace_state * __go_get_backtrace_state () { - runtime_lock (&back_state_lock); + uint32 set; + + /* We may not have a g here, so we can't use runtime_lock. */ + set = 0; + while (!__atomic_compare_exchange_n (&back_state_lock, &set, 1, false, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED)) + { + runtime_osyield (); + set = 0; + } if (back_state == NULL) { Slice args; @@ -113,7 +121,7 @@ __go_get_backtrace_state () back_state = backtrace_create_state (filename, 1, error_callback, NULL); } - runtime_unlock (&back_state_lock); + __atomic_store_n (&back_state_lock, 0, __ATOMIC_RELEASE); return back_state; }