perf/x86/intel/pt: Fix sampling synchronization

commit d92792a upstream.

pt_event_snapshot_aux() uses pt->handle_nmi to determine if tracing
needs to be stopped, however tracing can still be going because
pt->handle_nmi is set to zero before tracing is stopped in pt_event_stop,
whereas pt_event_snapshot_aux() requires that tracing must be stopped in
order to copy a sample of trace from the buffer.

Instead call pt_config_stop() always, which anyway checks config for
RTIT_CTL_TRACEEN and does nothing if it is already clear.

Note pt_event_snapshot_aux() can continue to use pt->handle_nmi to
determine if the trace needs to be restarted afterwards.

Fixes: 25e8920 ("perf/x86/intel/pt: Add sampling support")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20240715160712.127117-2-adrian.hunter@intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: ahunter6

arch/x86/events/intel/pt.c

@@ -1602,6 +1602,7 @@ static void pt_event_stop(struct perf_event *event, int mode)
 	 * see comment in intel_pt_interrupt().
 	 */
 	WRITE_ONCE(pt->handle_nmi, 0);
+	barrier();
 
 	pt_config_stop(event);
 
@@ -1653,11 +1654,10 @@ static long pt_event_snapshot_aux(struct perf_event *event,
 		return 0;
 
 	/*
-	 * Here, handle_nmi tells us if the tracing is on
+	 * There is no PT interrupt in this mode, so stop the trace and it will
+	 * remain stopped while the buffer is copied.
 	 */
-	if (READ_ONCE(pt->handle_nmi))
-		pt_config_stop(event);
-
+	pt_config_stop(event);
 	pt_read_offset(buf);
 	pt_update_head(pt);
 
@@ -1669,11 +1669,10 @@ static long pt_event_snapshot_aux(struct perf_event *event,
 	ret = perf_output_copy_aux(&pt->handle, handle, from, to);
 
 	/*
-	 * If the tracing was on when we turned up, restart it.
-	 * Compiler barrier not needed as we couldn't have been
-	 * preempted by anything that touches pt->handle_nmi.
+	 * Here, handle_nmi tells us if the tracing was on.
+	 * If the tracing was on, restart it.
 	 */
-	if (pt->handle_nmi)
+	if (READ_ONCE(pt->handle_nmi))
 		pt_config_start(event);
 
 	return ret;