Reimplement stack capture of running threads on i386 and amd64.

After r355784 the td_oncpu field is no longer synchronized by the thread
lock, so the stack capture interrupt cannot be delievered precisely.
Fix this using a loop which drops the thread lock and restarts if the
wrong thread was sampled from the stack capture interrupt handler.

Change the implementation to use a regular interrupt instead of an NMI.
Now that we drop the thread lock, there is no advantage to the latter.

Simplify the KPIs.  Remove stack_save_td_running() and add a return
value to stack_save_td().  On platforms that do not support stack
capture of running threads, stack_save_td() returns EOPNOTSUPP.  If the
target thread is running in user mode, stack_save_td() returns EBUSY.

Reviewed by:	kib
Reported by:	mjg, pho
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D23355

1 files changed, 2 insertions, 3 deletions

diff --git a/sys/kern/subr_kdb.c b/sys/kern/subr_kdb.c
index 19d88186334b..4e4a55727446 100644
--- a/sys/kern/subr_kdb.c
+++ b/sys/kern/subr_kdb.c
@@ -432,9 +432,8 @@ kdb_backtrace_thread(struct thread *td)
 		struct stack st;
 
 		printf("KDB: stack backtrace of thread %d:\n", td->td_tid);
-		stack_zero(&st);
-		stack_save_td(&st, td);
-		stack_print_ddb(&st);
+		if (stack_save_td(&st, td) == 0)
+			stack_print_ddb(&st);
 	}
 #endif
 }