On 5/31/23 20:29, Jürgen Hötzel wrote:
>
> "Richard W.M. Jones" <[email protected]> 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).
Where does that happen? I don't see it.
Either way, wherever it happens: that particular call site should be
modified to call event_callback_wrapper_locked() directly, IMO.
event_callback_wrapper() does nothing other than bracket
event_callback_wrapper_locked() with leave+enter, so if another call
site needs everything that event_callback_wrapper() does *except*
leave+enter (because it already holds the OCaml domain lock), then that
site should just directly call event_callback_wrapper_locked(). For this
purpose, event_callback_wrapper_locked() may have to be made a public
function (but again I don't know where that new call is supposed to come
from).
Laszlo
>
> 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
>
>
>
>
> _______________________________________________
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