Re: [Libguestfs] [PATCH libnbd] ocaml: Release runtime lock around call to nbd_close

OCaml 5 is stricter than earlier versions about correct locking. We must release the OCaml runtime lock when calling nbd_close since it may do some long-running operations and we want to allow concurrent threads to run. However specifically if there are callbacks (eg. a debug callback) then we would end up trying to re-acquire the lock in the callback, resulting in a crash:

(gdb) bt #0 __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44 #1 0x00007f47ffc8f773 in __pthread_kill_internal (signo=6, threadid=<optimized out>) at pthread_kill.c:78 #2 0x00007f47ffc3e71e in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26 #3 0x00007f47ffc2687f in __GI_abort () at abort.c:79 #4 0x0000560c9eb62779 in caml_fatal_error () #5 0x0000560c9eb63238 in caml_plat_fatal_error () #6 0x0000560c9eb4ea77 in caml_acquire_domain_lock () #7 0x0000560c9eb65cdc in caml_leave_blocking_section () #8 0x0000560c9eaf8a87 in debug_wrapper (user_data=0x560ca0af2670, context=0x7f47fff8ca60 "nbd_close", msg=0x560ca0af28b0 "closing handle") at ../nbd-c.c:187 #9 0x00007f47fff7072f in nbd_internal_debug (h=h@entry=0x560ca0b57db0, context=0x7f47fff8ca60 "nbd_close", context@entry=0x0, fs=fs@entry=0x7f47fff8ca6a "closing handle") at /home/rjones/d/libnbd/lib/debug.c:90 #10 0x00007f47fff73f23 in nbd_close (h=0x560ca0b57db0) at /home/rjones/d/libnbd/lib/handle.c:127 #11 0x0000560c9eae8dbe in nbd_internal_ocaml_handle_finalize ( hv=<optimized out>) at ../handle.c:39 #12 nbd_internal_ocaml_nbd_close (hv=<optimized out>) at ../handle.c:62 #13 <signal handler called> #14 0x0000560c9eae84dc in camlTest_140_explicit_close__entry () at NBD.ml:148 #15 0x0000560c9eae5c5b in caml_program () #16 <signal handler called> #17 0x0000560c9eb6cb77 in caml_startup_common () #18 0x0000560c9eb6cbef in caml_main () #19 0x0000560c9eae5910 in main () --- ocaml/handle.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/ocaml/handle.c b/ocaml/handle.c index fb45b30102..b3e5a0fc33 100644 --- a/ocaml/handle.c +++ b/ocaml/handle.c @@ -26,6 +26,7 @@ #include <caml/fail.h> #include <caml/memory.h> #include <caml/mlvalues.h> +#include <caml/threads.h> #include <libnbd.h> @@ -36,7 +37,9 @@ nbd_internal_ocaml_handle_finalize (value hv) { struct nbd_handle *h = NBD_val (hv); + caml_enter_blocking_section (); nbd_close (h); + caml_leave_blocking_section (); } value