bug#59055: [PATCH] Fix possible deadlock.

bug#59055: [PATCH] Fix possible deadlock.

[Olivier Dion via Bug reports for GUILE, GNU's Ubiquitous Extension Language]

If we got interrupted while waiting on our condition variable, we unlock
the kernel mutex momentarily while executing asynchronous operations
before putting us back into the waiting queue.

However, we have to retry acquiring the mutex before getting back into
the queue, otherwise it's possible that we wait indefinitely since
nobody could be the owner for a while.

* libguile/threads.c (lock_mutex): Try acquring the mutex after signal
interruption.
---
 libguile/threads.c | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/libguile/threads.c b/libguile/threads.c
index 280d306bf..0f5cf2ed5 100644
--- a/libguile/threads.c
+++ b/libguile/threads.c
@@ -1022,14 +1022,7 @@ lock_mutex (enum scm_mutex_kind kind, struct scm_mutex *m,
 
         if (err == 0)
           {
-            if (scm_is_eq (m->owner, SCM_BOOL_F))
-              {
-                m->owner = current_thread->handle;
-                scm_i_pthread_mutex_unlock (&m->lock);
-                return SCM_BOOL_T;
-              }
-            else
-              continue;
+            goto maybe_acquire;
           }
         else if (err == ETIMEDOUT)
           {
@@ -1041,7 +1034,7 @@ lock_mutex (enum scm_mutex_kind kind, struct scm_mutex *m,
             scm_i_pthread_mutex_unlock (&m->lock);
             scm_async_tick ();
             scm_i_scm_pthread_mutex_lock (&m->lock);
-            continue;
+            goto maybe_acquire;
           }
         else
           {
@@ -1050,6 +1043,14 @@ lock_mutex (enum scm_mutex_kind kind, struct scm_mutex *m,
             errno = err;
             SCM_SYSERROR;
           }
+
+      maybe_acquire:
+        if (scm_is_eq (m->owner, SCM_BOOL_F))
+          {
+            m->owner = current_thread->handle;
+            scm_i_pthread_mutex_unlock (&m->lock);
+            return SCM_BOOL_T;
+          }
       }
 }
 #undef FUNC_NAME
-- 
2.38.0

bug#59055: [PATCH] Fix possible deadlock.

[Ludovic Courtès]

Hi,

Olivier Dion <olivier.dion@polymtl.ca> skribis:

> If we got interrupted while waiting on our condition variable, we unlock
> the kernel mutex momentarily while executing asynchronous operations
> before putting us back into the waiting queue.
>
> However, we have to retry acquiring the mutex before getting back into
> the queue, otherwise it's possible that we wait indefinitely since
> nobody could be the owner for a while.
>
> * libguile/threads.c (lock_mutex): Try acquring the mutex after signal
> interruption.

Looks reasonable to me; applied.

Did you try to come up with a reproducer?  That would be awesome but I
guess it’s hard because you need to trigger EINTR at the right point.

Thanks,
Ludo’.

bug#59055: [PATCH] Fix possible deadlock.

[Olivier Dion via Bug reports for GUILE, GNU's Ubiquitous Extension Language]

On Sun, 20 Nov 2022, Ludovic Courtès <ludo@gnu.org> wrote:

> Did you try to come up with a reproducer?  That would be awesome but I
> guess it’s hard because you need to trigger EINTR at the right point.

With a stress test in guile-parallel.  Very hard to reproduce indeed.
You can also reproduce it with `ice-9 futures` I think.

Here's the stress test that I've been using:
--8<---------------cut here---------------start------------->8---
(use-modules
  ((ice-9 futures) #:prefix ice-9:)
  (srfi srfi-1)
  (srfi srfi-26))

(define (run-stress-test N future touch)
  (for-each
   touch
   (unfold
    (cut = N <>)
    (lambda (_)
      (future
       (const #t)))
    1+
    0)))

(run-stress-test 10000000 ice-9:make-future ice-9:touch)
--8<---------------cut here---------------end--------------->8---

-- 
Olivier Dion
oldiob.dev