Re: [Libguestfs] [PATCH libguestfs 2/2] Only leave/enter blocking_section when OCaml lock is not held

"Richard W.M. Jones" <rjones(a)redhat.com&gt; writes: > On Sat, May 27, 2023 at 03:35:38PM +0200, Jürgen Hötzel wrote: >> Fixes deadlocks on OCaml5 when trying to get the lock that is already >> held: >> >> Fatal error during lock: Resource deadlock avoided >> --- >> ocaml/guestfs-c.c | 8 ++++++-- >> 1 file changed, 6 insertions(+), 2 deletions(-) >> >> diff --git a/ocaml/guestfs-c.c b/ocaml/guestfs-c.c >> index 3888c9456..bcf8e6ab3 100644 >> --- a/ocaml/guestfs-c.c >> +++ b/ocaml/guestfs-c.c >> @@ -395,12 +395,16 @@ event_callback_wrapper (guestfs_h *g, >> /* Ensure we are holding the GC lock before any GC operations are >> * possible. (RHBZ#725824) >> */ >> - caml_leave_blocking_section (); >> + bool in_blocking_section = (caml_state == NULL); >> + >> + if (in_blocking_section) >> + caml_leave_blocking_section (); >> >> event_callback_wrapper_locked (g, data, event, event_handle, flags, >> buf, buf_len, array, array_len); >> >> - caml_enter_blocking_section (); >> + if (in_blocking_section) >> + caml_enter_blocking_section (); >> } > > I don't understand the reason why this patch is needed. the event_callback_wrapper is ALSO called from libguestfs code that already holds the OCaml domain lock. (when blocking = false).

My Patch is just a (confusing) workaround to fix this issue at runtime. An better solution might be using different callbacks at compile time: event_callback_wrapper_locked (when called from a non-blocking function) event_callback_wrapper (when called from a blocking function). This problem also occurs only with OCaml 5 Minimal invalid OCAML/C Code: ============================== stubs.c =============================== #include <caml/alloc.h> #include <caml/custom.h> #include <caml/memory.h> #include <caml/mlvalues.h> void dummy_leave_blocking() { CAMLparam0(); /* we did not call caml_enter_blocking_section_before in C */ caml_leave_blocking_section(); caml_enter_blocking_section(); CAMLreturn0; } ============================== main.ml =============================== external dummy_leave_blocking : unit -> unit = "dummy_leave_blocking" let () = Printf.printf "Hello from OCaml %s\n%!" Sys.ocaml_version; dummy_leave_blocking(); Printf.printf "Bye from OCaml %s\n%!" Sys.ocaml_version; ====================================================================== Using Ocaml 5 results in crash: # dune exec -- bin/main.exe Hello from OCaml 5.0.0 Fatal error: Fatal error during lock: Resource deadlock avoided Aborted (core dumped) VS OCaml 4: # dune exec -- bin/main.exe Hello from OCaml 4.11.1 Bye from OCaml 4.11.1 Jürgen _______________________________________________ Libguestfs mailing list Libguestfs(a)redhat.com https://listman.redhat.com/mailman/listinfo/libguestfs