[PATCH/RFC] perf buildid: Cache kernel DSO created when reading buildid header table

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Namhyung,

	Can you take a look and perhaps give me your Acked-by?

- Arnaldo

From: Arnaldo Carvalho de Melo <acme@redhat.com>

No need to traverse it all again using the filename extension to
disambiguate kernel from kernel modules, just cache it when
reading the buildid table.

This also fixes a refcount bug in the error path, i.e. in the error path
for the __machine__create_kernel_maps() call we do a dso__put(), which
is ok and expected if that dso came from machine__findnew_dso(), but
wasn't when we found it by traversing the machine dso list without
grabbing a dso refcount (dso__get()) before dropping the list lock.

Fixes: b837a8bdc489 ("perf tools: Fix build-id matching on vmlinux")
Link: http://lkml.kernel.org/n/tip-48fh3cf6pvs4zs2fj4nhc4b5@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 tools/perf/util/header.c  |  7 +++++++
 tools/perf/util/machine.c | 34 +++-------------------------------
 tools/perf/util/machine.h |  1 +
 3 files changed, 11 insertions(+), 31 deletions(-)

diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
index 43838003c1a1..6bc92ae1e99a 100644
--- a/tools/perf/util/header.c
+++ b/tools/perf/util/header.c
@@ -1260,6 +1260,13 @@ static int __event_process_build_id(struct build_id_event *bev,
 
 		if (!is_kernel_module(filename, cpumode))
 			dso->kernel = dso_type;
+		/*
+		 * We don't need to grab a reference as long as
+		 * the kernel dso is in the machine dso list.
+		 */
+		if (cpumode == PERF_RECORD_MISC_KERNEL ||
+		    cpumode == PERF_RECORD_MISC_GUEST_KERNEL)
+			machine->kernel_dso = dso;
 
 		build_id__sprintf(dso->build_id, sizeof(dso->build_id),
 				  sbuild_id);
diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
index d07a38678e14..f6e689b2c83c 100644
--- a/tools/perf/util/machine.c
+++ b/tools/perf/util/machine.c
@@ -62,6 +62,7 @@ int machine__init(struct machine *machine, const char *root_dir, pid_t pid)
 	}
 
 	machine->current_tid = NULL;
+	machine->kernel_dso = NULL;
 
 	return 0;
 }
@@ -1180,39 +1181,10 @@ static int machine__process_kernel_mmap_event(struct machine *machine,
 		 * Should be there already, from the build-id table in
 		 * the header.
 		 */
-		struct dso *kernel = NULL;
-		struct dso *dso;
+		struct dso *kernel;
 
 		pthread_rwlock_rdlock(&machine->dsos.lock);
-
-		list_for_each_entry(dso, &machine->dsos.head, node) {
-
-			/*
-			 * The cpumode passed to is_kernel_module is not the
-			 * cpumode of *this* event. If we insist on passing
-			 * correct cpumode to is_kernel_module, we should
-			 * record the cpumode when we adding this dso to the
-			 * linked list.
-			 *
-			 * However we don't really need passing correct
-			 * cpumode.  We know the correct cpumode must be kernel
-			 * mode (if not, we should not link it onto kernel_dsos
-			 * list).
-			 *
-			 * Therefore, we pass PERF_RECORD_MISC_CPUMODE_UNKNOWN.
-			 * is_kernel_module() treats it as a kernel cpumode.
-			 */
-
-			if (!dso->kernel ||
-			    is_kernel_module(dso->long_name,
-					     PERF_RECORD_MISC_CPUMODE_UNKNOWN))
-				continue;
-
-
-			kernel = dso;
-			break;
-		}
-
+		kernel = dso__get(machine->kernel_dso);
 		pthread_rwlock_unlock(&machine->dsos.lock);
 
 		if (kernel == NULL)
diff --git a/tools/perf/util/machine.h b/tools/perf/util/machine.h
index 9dfc4281f940..d7b044a1046f 100644
--- a/tools/perf/util/machine.h
+++ b/tools/perf/util/machine.h
@@ -38,6 +38,7 @@ struct machine {
 	struct dsos	  dsos;
 	struct map_groups kmaps;
 	struct map	  *vmlinux_maps[MAP__NR_TYPES];
+	struct dso	  *kernel_dso;
 	u64		  kernel_start;
 	symbol_filter_t	  symbol_filter;
 	pid_t		  *current_tid;
-- 
2.1.0