perf comm: Fix comm_str__put() for reference count checking

Searching for the entry in the array needs to avoid the intermediate
pointer with reference count checking.

Refactor the array removal to binary search for the entry.

Change the array to hold an entry with a reference count (so the
intermediate pointer can work) and remove from the array when the
reference count on a comm_str falls to 1.

Fixes: 13ca628 ("perf comm: Add reference count checking to 'struct comm_str'")
Signed-off-by: Ian Rogers <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Leo Yan <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: captain5050

tools/perf/util/comm.c

@@ -19,6 +19,8 @@ static struct comm_strs {
 	int capacity;
 } _comm_strs;
 
+static void comm_strs__remove_if_last(struct comm_str *cs);
+
 static void comm_strs__init(void)
 {
 	init_rwsem(&_comm_strs.lock);
@@ -58,22 +60,15 @@ static struct comm_str *comm_str__get(struct comm_str *cs)
 
 static void comm_str__put(struct comm_str *cs)
 {
-	if (cs && refcount_dec_and_test(comm_str__refcnt(cs))) {
-		struct comm_strs *comm_strs = comm_strs__get();
-		int i;
+	if (!cs)
+		return;
 
-		down_write(&comm_strs->lock);
-		for (i = 0; i < comm_strs->num_strs; i++) {
-			if (comm_strs->strs[i] == cs)
-				break;
-		}
-		for (; i < comm_strs->num_strs - 1; i++)
-			comm_strs->strs[i] = comm_strs->strs[i + 1];
-
-		comm_strs->num_strs--;
-		up_write(&comm_strs->lock);
+	if (refcount_dec_and_test(comm_str__refcnt(cs))) {
 		RC_CHK_FREE(cs);
 	} else {
+		if (refcount_read(comm_str__refcnt(cs)) == 1)
+			comm_strs__remove_if_last(cs);
+
 		RC_CHK_PUT(cs);
 	}
 }
@@ -107,6 +102,28 @@ static int comm_str__search(const void *_key, const void *_member)
 	return strcmp(key, comm_str__str(member));
 }
 
+static void comm_strs__remove_if_last(struct comm_str *cs)
+{
+	struct comm_strs *comm_strs = comm_strs__get();
+
+	down_write(&comm_strs->lock);
+	/*
+	 * Are there only references from the array, if so remove the array
+	 * reference under the write lock so that we don't race with findnew.
+	 */
+	if (refcount_read(comm_str__refcnt(cs)) == 1) {
+		struct comm_str **entry;
+
+		entry = bsearch(comm_str__str(cs), comm_strs->strs, comm_strs->num_strs,
+				sizeof(struct comm_str *), comm_str__search);
+		comm_str__put(*entry);
+		for (int i = entry - comm_strs->strs; i < comm_strs->num_strs - 1; i++)
+			comm_strs->strs[i] = comm_strs->strs[i + 1];
+		comm_strs->num_strs--;
+	}
+	up_write(&comm_strs->lock);
+}
+
 static struct comm_str *__comm_strs__find(struct comm_strs *comm_strs, const char *str)
 {
 	struct comm_str **result;
@@ -158,7 +175,7 @@ static struct comm_str *comm_strs__findnew(const char *str)
 		}
 	}
 	up_write(&comm_strs->lock);
-	return result;
+	return comm_str__get(result);
 }
 
 struct comm *comm__new(const char *str, u64 timestamp, bool exec)