[PATCH libgpiod] gpiosim: fix data race that corrupts heap

[Erik Schilling <erik.schilling@xxxxxxxxxx>]

Hit this while seeing some heap corruptions when running cargo test on
the Rust bindings.

Took a bit to track down since I first used address sanitizers, but with
the thread sanitizer it becomes obvious immediately (output simplified):

    ==================
    WARNING: ThreadSanitizer: data race (pid=288119)
      Write of size 8 at 0x0000018f1e78 by thread T6:
        #0 id_free /libgpiod/tests/gpiosim/gpiosim.c:141:17
        #1 dev_release /libgpiod/tests/gpiosim/gpiosim.c:600:2
        #2 refcount_dec /libgpiod/tests/gpiosim/gpiosim.c:176:3
        #3 gpiosim_dev_unref /libgpiod/tests/gpiosim/gpiosim.c:671:2
        #4 bank_release /libgpiod/tests/gpiosim/gpiosim.c:873:2
        #5 refcount_dec /libgpiod/tests/gpiosim/gpiosim.c:176:3
        #6 gpiosim_bank_unref /libgpiod/tests/gpiosim/gpiosim.c:941:2
        [...]

      Previous write of size 8 at 0x0000018f1e78 by thread T1:
        #0 id_free /libgpiod/tests/gpiosim/gpiosim.c:141:17
        #1 bank_release /libgpiod/tests/gpiosim/gpiosim.c:878:2
        #2 refcount_dec /libgpiod/tests/gpiosim/gpiosim.c:176:3
        #3 gpiosim_bank_unref /libgpiod/tests/gpiosim/gpiosim.c:941:2
        [...]

      Location is global 'id_del_ctx' of size 16 at 0x0000018f1e70

      Thread T6 'chip::verify::f' (tid=288126, running) created by main thread at:
        #7 test::run_tests::hd53a07a011bd771f /.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test/src/lib.rs:407:21
        [...]

      Thread T1 'chip::open::gpi' (tid=288121, finished) created by main thread at:
        #7 test::run_tests::hd53a07a011bd771f /.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test/src/lib.rs:407:21
        [...]

    SUMMARY: ThreadSanitizer: data race /libgpiod/tests/gpiosim/gpiosim.c:141:17 in id_free
    ==================

This eventually can either lead to leaks or double free's that corrupt
the heap and lead to crashes.

The issue got introduced when a previously local variable that did not
require protection was turned into a global variable.

Fixes: 5e111df2fca56d57193a1825e45e78dd8b76c0f1
Signed-off-by: Erik Schilling <erik.schilling@xxxxxxxxxx>
---
To: Linux-GPIO <linux-gpio@xxxxxxxxxxxxxxx>
Cc: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
---
 tests/gpiosim/gpiosim.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tests/gpiosim/gpiosim.c b/tests/gpiosim/gpiosim.c
index b49a61a..fca6b7f 100644
--- a/tests/gpiosim/gpiosim.c
+++ b/tests/gpiosim/gpiosim.c
@@ -137,11 +137,11 @@ static int id_alloc(void)
 
 static void id_free(int id)
 {
+	pthread_mutex_lock(&id_lock);
+
 	id_del_ctx.id = id;
 	id_del_ctx.idp = NULL;
 
-	pthread_mutex_lock(&id_lock);
-
 	twalk(id_root, id_del);
 	if (id_del_ctx.idp) {
 		tdelete(id_del_ctx.idp, &id_root, id_compare);

---
base-commit: d04639ddd11ed7d02c630e693bf07d97f53e17d3
change-id: 20230626-datarace-e62e9bcfa3ee

Best regards,
-- 
Erik Schilling <erik.schilling@xxxxxxxxxx>