[PATCH v4 07/17] perf vdso: Fix unlocked __dsos__add() and race in __machine__addnew_vdso()

[Ian Rogers]

__machine__addnew_vdso() called __dsos__add(&machine->dsos, dso) without
holding machine->dsos.lock. Acquire machine->dsos.lock in write mode,
double-check __dsos__find() to avoid inserting duplicate VDSO entries
when racing with another thread, and set the long name before calling
__dsos__add() so it does not attempt to re-acquire machine->dsos.lock.

Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
---
 tools/perf/util/dsos.c |  7 ++++++-
 tools/perf/util/dsos.h |  1 +
 tools/perf/util/vdso.c | 17 +++++++++++++++--
 3 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/tools/perf/util/dsos.c b/tools/perf/util/dsos.c
index a2cd37bd1660..40267d3a725d 100644
--- a/tools/perf/util/dsos.c
+++ b/tools/perf/util/dsos.c
@@ -284,12 +284,17 @@ static struct dso *__dsos__find_id(struct dsos *dsos, const char *name, const st
 	return res;
 }
 
+struct dso *__dsos__find(struct dsos *dsos, const char *name, bool cmp_short, bool write_locked)
+{
+	return __dsos__find_id(dsos, name, &dso_id_empty, cmp_short, write_locked);
+}
+
 struct dso *dsos__find(struct dsos *dsos, const char *name, bool cmp_short)
 {
 	struct dso *res;
 
 	down_read(&dsos->lock);
-	res = __dsos__find_id(dsos, name, &dso_id_empty, cmp_short, /*write_locked=*/false);
+	res = __dsos__find(dsos, name, cmp_short, /*write_locked=*/false);
 	up_read(&dsos->lock);
 	return res;
 }
diff --git a/tools/perf/util/dsos.h b/tools/perf/util/dsos.h
index a26774950866..857a79fd8805 100644
--- a/tools/perf/util/dsos.h
+++ b/tools/perf/util/dsos.h
@@ -30,6 +30,7 @@ void dsos__exit(struct dsos *dsos);
 
 int __dsos__add(struct dsos *dsos, struct dso *dso);
 int dsos__add(struct dsos *dsos, struct dso *dso);
+struct dso *__dsos__find(struct dsos *dsos, const char *name, bool cmp_short, bool write_locked);
 struct dso *dsos__find(struct dsos *dsos, const char *name, bool cmp_short);
 
 struct dso *dsos__findnew_id(struct dsos *dsos, const char *name, const struct dso_id *id);
diff --git a/tools/perf/util/vdso.c b/tools/perf/util/vdso.c
index c12f5d8c4bf6..dca4f683be9b 100644
--- a/tools/perf/util/vdso.c
+++ b/tools/perf/util/vdso.c
@@ -127,13 +127,26 @@ void machine__exit_vdso(struct machine *machine)
 static struct dso *__machine__addnew_vdso(struct machine *machine, const char *short_name,
 					  const char *long_name)
 {
-	struct dso *dso;
+	struct dso *dso, *existing;
+
+	down_write(&machine->dsos.lock);
+	existing = __dsos__find(&machine->dsos, short_name, true, /*write_locked=*/true);
+	if (existing) {
+		up_write(&machine->dsos.lock);
+		return existing;
+	}
 
 	dso = dso__new(short_name);
 	if (dso != NULL) {
-		__dsos__add(&machine->dsos, dso);
+		/*
+		 * Set the long name before adding to machine->dsos so that
+		 * __dsos__add() inserts it in sorted order without needing to
+		 * re-take machine->dsos.lock.
+		 */
 		dso__set_long_name(dso, long_name, false);
+		__dsos__add(&machine->dsos, dso);
 	}
+	up_write(&machine->dsos.lock);
 
 	return dso;
 }
-- 
2.56.0.212.g129ede20d9-goog