Hi Sebastian,

thanks for looking into this.

We could reproduce the issue with QEMU.
At runtime you need to set mdev as the kernel's hotplug client (/proc/sys/kernel/hotplug) and give it a dummy /etc/mdev.conf like (.* 1:1 777). Then just do a loop and insmod/rmmod crc4.ko and crc7.ko.

Swapping the mm assignment did not work -> exception after 1900 iterations
Your second suggestion with check.patch (attached to this email for completeness, only protecting the exec_mmap function) did not work eighter -> exception after 2600 iterations

Your third suggestion (a modification to the original revert) enclosed in this e-mail does seem to work. Still no problems after 30000 iterations.

By the way, as noticed in your kernel config, we would be quite interested in a gcc 9 compiler for our platform. Is there a mainline/maintained version or fork for this or another possibility to get it?
 
Regards,
Thomas

-----Original Message-----
From: Sebastian Andrzej Siewior [mailto:bigeasy@linutronix.de] 
Sent: Monday, July 06, 2020 6:50 PM
To: Mark Marshall <markmarshall14@gmail.com>
Cc: linux-rt-users <linux-rt-users@vger.kernel.org>; Mark Marshall <mark.marshall@omicronenergy.com>; Thomas Graziadei <thomas.graziadei@omicronenergy.com>; Thomas Gleixner <tglx@linutronix.de>; linux-kernel@vger.kernel.org; rostedt@goodmis.org
Subject: Re: Kernel crash due to memory corruption with v5.4.26-rt17 and PowerPC e500

On 2020-05-29 18:37:22 [+0200], To Mark Marshall wrote:
> On 2020-05-29 18:15:18 [+0200], To Mark Marshall wrote:
> > In order to get it back into the RT queue I need to understand why 
> > it is required. What exactly is it fixing. Let me stare at for a 
> > little…
> 
> it used to be local_irq_disable() which then became preempt_disable()
> local_irq_disable() due to ARM's limitation.

Any luck on your side?

I *think* if you swap the mm assignment in exec_mmap() then it should be gone. Basically:
|         tsk->active_mm = mm;
|         tsk->mm = mm;

However I think to apply something like this:

diff --git a/fs/exec.c b/fs/exec.c
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1035,11 +1035,15 @@ static int exec_mmap(struct mm_struct *mm)
 		}
 	}
 	task_lock(tsk);
+
+	task_lock_mm();
 	active_mm = tsk->active_mm;
 	membarrier_exec_mmap(mm);
 	tsk->mm = mm;
 	tsk->active_mm = mm;
 	activate_mm(active_mm, mm);
+	task_unlock_mm();
+
 	tsk->mm->vmacache_seqnum = 0;
 	vmacache_flush(tsk);
 	task_unlock(tsk);
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -176,4 +176,31 @@ static inline void task_unlock(struct task_struct *p)
 	spin_unlock(&p->alloc_lock);
 }
 
+#ifdef CONFIG_PREEMPT_RT
+/*
+ * Protects ->mm and ->active_mm.
+ * Avoids scheduling so switch_mm() or enter_lazy_tlb() will not read 
+the
+ * members while they are updated.
+ */
+static inline void task_lock_mm(void)
+{
+	preempt_disable();
+}
+
+static inline void task_unlock_mm(void) {
+	preempt_enable();
+}
+
+#else
+
+static inline void task_lock_mm(void)
+{
+}
+
+static inline void task_unlock_mm(void) { } #endif
+
 #endif /* _LINUX_SCHED_TASK_H */
diff --git a/mm/mmu_context.c b/mm/mmu_context.c
--- a/mm/mmu_context.c
+++ b/mm/mmu_context.c
@@ -25,6 +25,7 @@ void use_mm(struct mm_struct *mm)
 	struct task_struct *tsk = current;
 
 	task_lock(tsk);
+	task_lock_mm();
 	active_mm = tsk->active_mm;
 	if (active_mm != mm) {
 		mmgrab(mm);
@@ -32,6 +33,7 @@ void use_mm(struct mm_struct *mm)
 	}
 	tsk->mm = mm;
 	switch_mm(active_mm, mm, tsk);
+	task_unlock_mm();
 	task_unlock(tsk);
 #ifdef finish_arch_post_lock_switch
 	finish_arch_post_lock_switch();
@@ -55,10 +57,12 @@ void unuse_mm(struct mm_struct *mm)
 	struct task_struct *tsk = current;
 
 	task_lock(tsk);
+	task_lock_mm();
 	sync_mm_rss(mm);
 	tsk->mm = NULL;
 	/* active_mm is still 'mm' */
 	enter_lazy_tlb(mm, tsk);
+	task_unlock_mm();
 	task_unlock(tsk);
 }
 EXPORT_SYMBOL_GPL(unuse_mm);
--
2.27.0

> > > Best regards,
> > > Mark

Sebastian