perf tools: Fix dso_id inode generation comparison

Synthesized MMAP events have zero ino_generation, so do not compare
them to DSOs with a real ino_generation otherwise we end up with a DSO
without a build id.

Fixes: 0e3149f86b99ddab ("perf dso: Move dso_id from 'struct map' to 'struct dso'")
Signed-off-by: Adrian Hunter <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: [e-mail]
Cc: Namhyung Kim <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
[ Added clarification to the comment from Ian + more detailed explanation from Adrian ]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/dsos.c

···
 	if (a->ino > b->ino) return -1;
 	if (a->ino < b->ino) return 1;
 
-	if (a->ino_generation > b->ino_generation) return -1;
-	if (a->ino_generation < b->ino_generation) return 1;
+	/*
+	 * Synthesized MMAP events have zero ino_generation, avoid comparing
+	 * them with MMAP events with actual ino_generation.
+	 *
+	 * I found it harmful because the mismatch resulted in a new
+	 * dso that did not have a build ID whereas the original dso did have a
+	 * build ID. The build ID was essential because the object was not found
+	 * otherwise. - Adrian
+	 */
+	if (a->ino_generation && b->ino_generation) {
+		if (a->ino_generation > b->ino_generation) return -1;
+		if (a->ino_generation < b->ino_generation) return 1;
+	}
 
 	return 0;
 }