There is a window where threads are removed from the process list and where

the thread destructor is invoked. Catch that window by waiting for all
task_struct allocations to be returned before freeing the UMA zone in the
LinuxKPI. Else UMA may fail to release the zone due to concurrent access
and panic:

panic() - Bad link element prev->next != elm
zone_release()
bucket_drain()
bucket_free()
zone_dtor()
zone_free_item()
uma_zdestroy()
linux_current_uninit()

This failure can be triggered by loading and unloading the LinuxKPI module
in a loop:

while true
do
kldload linuxkpi
kldunload linuxkpi
done

Discussed with:	kib@
MFC after:	1 week
Sponsored by:	Mellanox Technologies // NVIDIA Networking

sys/compat/linuxkpi/common/src/linux_current.c

@@ -45,6 +45,7 @@ extern u_int first_msi_irq, num_msi_irqs;
 
 static eventhandler_tag linuxkpi_thread_dtor_tag;
 
+static atomic_t linux_current_allocs;
 static uma_zone_t linux_current_zone;
 static uma_zone_t linux_mm_zone;
 
@@ -146,6 +147,10 @@ linux_alloc_current(struct thread *td, int flags)
 	/* free mm_struct pointer, if any */
 	uma_zfree(linux_mm_zone, mm);
 
+	/* keep track of number of allocations */
+	if (atomic_add_return(1, &linux_current_allocs) == INT_MAX)
+		panic("linux_alloc_current: Refcount too high!");
+
 	return (0);
 }
 
@@ -173,6 +178,10 @@ linux_free_current(struct task_struct *ts)
 {
 	mmput(ts->mm);
 	uma_zfree(linux_current_zone, ts);
+
+	/* keep track of number of allocations */
+	if (atomic_sub_return(1, &linux_current_allocs) < 0)
+		panic("linux_free_current: Negative refcount!");
 }
 
 static void
@@ -271,10 +280,6 @@ SYSCTL_INT(_compat_linuxkpi, OID_AUTO, task_struct_reserve,
 static void
 linux_current_init(void *arg __unused)
 {
-	lkpi_alloc_current = linux_alloc_current;
-	linuxkpi_thread_dtor_tag = EVENTHANDLER_REGISTER(thread_dtor,
-	    linuxkpi_thread_dtor, NULL, EVENTHANDLER_PRI_ANY);
-
 	TUNABLE_INT_FETCH("compat.linuxkpi.task_struct_reserve",
 	    &lkpi_task_resrv);
 	if (lkpi_task_resrv == 0) {
@@ -298,6 +303,12 @@ linux_current_init(void *arg __unused)
 	    UMA_ALIGN_PTR, 0);
 	uma_zone_reserve(linux_mm_zone, lkpi_task_resrv);
 	uma_prealloc(linux_mm_zone, lkpi_task_resrv);
+
+	atomic_thread_fence_seq_cst();
+
+	lkpi_alloc_current = linux_alloc_current;
+	linuxkpi_thread_dtor_tag = EVENTHANDLER_REGISTER(thread_dtor,
+	    linuxkpi_thread_dtor, NULL, EVENTHANDLER_PRI_ANY);
 }
 SYSINIT(linux_current, SI_SUB_EVENTHANDLER, SI_ORDER_SECOND,
     linux_current_init, NULL);
@@ -309,6 +320,10 @@ linux_current_uninit(void *arg __unused)
 	struct task_struct *ts;
 	struct thread *td;
 
+	lkpi_alloc_current = linux_alloc_current_noop;
+
+	atomic_thread_fence_seq_cst();
+
 	sx_slock(&allproc_lock);
 	FOREACH_PROC_IN_SYSTEM(p) {
 		PROC_LOCK(p);
@@ -321,8 +336,18 @@ linux_current_uninit(void *arg __unused)
 		PROC_UNLOCK(p);
 	}
 	sx_sunlock(&allproc_lock);
+
+	/*
+	 * There is a window where threads are removed from the
+	 * process list and where the thread destructor is invoked.
+	 * Catch that window by waiting for all task_struct
+	 * allocations to be returned before freeing the UMA zone.
+	 */
+	while (atomic_read(&linux_current_allocs) != 0)
+		pause("W", 1);
+
 	EVENTHANDLER_DEREGISTER(thread_dtor, linuxkpi_thread_dtor_tag);
-	lkpi_alloc_current = linux_alloc_current_noop;
+	
 	uma_zdestroy(linux_current_zone);
 	uma_zdestroy(linux_mm_zone);
 }