Restored the Linux port mutex retryv6.4.1_cert

This was reverted because a NetX Secure regression test segfaulted with it
in place. The cause was the test: it hands the certificate verify path a
stack-local certificate whose signature it never assigns, and this change
moves the stack layout that decides whether the unassigned slot reads null
or a readable leftover. That test has been fixed, so the retry comes back.

Measured with the test unfixed and this change in place, on a layout built
without the stack protector: eight of eight runs segfault, the fault read
from the faulting process as a null signature dereferenced by the ECDSA
verify. With the test fixed, nineteen build configurations run it eight
times each with no fault.

Without this change, a thread signalled into the port's suspend handler
never returns to the contended-mutex loop, the next unlock's wake-up is
lost, the scheduler is left parked, and the timer interrupt never runs --
which stalls the tick and hangs about 0.39% of instances of a few TCP
tests for their full timeout.

2 files changed, 57 insertions, 1 deletions

diff --git a/ports/linux/gnu/inc/tx_port.h b/ports/linux/gnu/inc/tx_port.h
index dc39b5f9..70a9fb9d 100644
--- a/ports/linux/gnu/inc/tx_port.h
+++ b/ports/linux/gnu/inc/tx_port.h
@@ -525,7 +525,7 @@ VOID   _tx_thread_interrupt_restore(UINT previous_posture);
 #define TX_RESTORE                          _tx_linux_debug_entry_insert("RESTORE", __FILE__, __LINE__); \
                                             _tx_thread_interrupt_restore(tx_saved_posture);
 #endif /* TX_LINUX_DEBUG_ENABLE */
-#define tx_linux_mutex_lock(p)              pthread_mutex_lock(&p)
+#define tx_linux_mutex_lock(p)              _tx_linux_mutex_lock_retry(&p)
 #define tx_linux_mutex_unlock(p)            pthread_mutex_unlock(&p)
 #define tx_linux_mutex_recursive_unlock(p)  {\
                                                 int _recursive_count = (int)tx_linux_mutex_recursive_count;\
@@ -566,6 +566,17 @@ extern  CHAR                    _tx_version_id[];
 /* Define externals for the Linux port of ThreadX.  */
 
 extern pthread_mutex_t                          _tx_linux_mutex;
+
+/* Define how long a thread waits on the Linux mutex before retrying. A thread
+   parked on the mutex can be suspended by the port's signal handler and so never
+   act on the wake-up the next unlock sends it, which leaves the wake-up lost and
+   every other waiter parked on a mutex that is free.  */
+
+#ifndef TX_LINUX_MUTEX_RETRY_NSEC
+#define TX_LINUX_MUTEX_RETRY_NSEC       1000000
+#endif
+
+void    _tx_linux_mutex_lock_retry(pthread_mutex_t *mutex);
 extern sem_t                                    _tx_linux_semaphore;
 extern sem_t                                    _tx_linux_semaphore_no_idle;
 extern ULONG                                    _tx_linux_global_int_disabled_flag;
diff --git a/ports/linux/gnu/src/tx_thread_schedule.c b/ports/linux/gnu/src/tx_thread_schedule.c
index d65104a1..c66aa891 100644
--- a/ports/linux/gnu/src/tx_thread_schedule.c
+++ b/ports/linux/gnu/src/tx_thread_schedule.c
@@ -8,6 +8,8 @@
  * SPDX-License-Identifier: MIT
  **************************************************************************/
 
+// Portions of this file were generated with AI assistance.
+
 
 /**************************************************************************/
 /**************************************************************************/
@@ -206,6 +208,49 @@ struct timespec ts;
     }
 }
 
+/* Define the ThreadX Linux mutex lock function.  The wait is timed and retried
+   rather than left to pthread_mutex_lock, because a thread can be signalled into
+   the port's suspend handler while it is parked on this mutex.  That handler does
+   not return until the thread is resumed, so the wake-up the next unlock sends is
+   delivered to a thread that never retries and is lost.  Any other thread parked
+   on the mutex then waits on a mutex that is free.  Retrying on a timeout costs
+   nothing when the mutex is handed over normally, and turns that lost wake-up into
+   a delay of at most the retry period.  */
+
+void  _tx_linux_mutex_lock_retry(pthread_mutex_t *mutex)
+{
+
+INT             linux_status;
+struct timespec ts;
+
+
+    do
+    {
+
+        /* Set the deadline for this attempt.  */
+        clock_gettime(CLOCK_REALTIME, &ts);
+        ts.tv_nsec =  ts.tv_nsec + TX_LINUX_MUTEX_RETRY_NSEC;
+        if (ts.tv_nsec >= 1000000000)
+        {
+
+            ts.tv_nsec =  ts.tv_nsec - 1000000000;
+            ts.tv_sec++;
+        }
+
+        linux_status =  pthread_mutex_timedlock(mutex, &ts);
+
+        /* Anything but the deadline expiring is a real failure to obtain the
+           mutex, so stop retrying.  */
+        if ((linux_status != 0) && (linux_status != ETIMEDOUT))
+        {
+
+            break;
+        }
+
+    } while (linux_status != 0);
+}
+
+
 void _tx_thread_delete_port_completion(TX_THREAD *thread_ptr, UINT tx_saved_posture)
 {
 INT             linux_status;