[PATCH v2] mm: Make vmalloc_dump_obj() call in a preemptible context

[Zqiang]

Currently, the mem_dump_obj() is invoked in call_rcu(), the
call_rcu() is maybe invoked in non-preemptive code segment,
for object allocated from vmalloc(), the following scenarios
may occur:

        CPU 0
tasks context
   spin_lock(&vmap_area_lock)
          Interrupt context
              call_rcu()
                mem_dump_obj
                  vmalloc_dump_obj
                    spin_lock(&vmap_area_lock) <--deadlock

and for PREEMPT-RT kernel, the spinlock will convert to sleepable
lock, so the vmap_area_lock spinlock not allowed to get in non-preemptive
code segment. therefore, this commit make the vmalloc_dump_obj() call in
a preemptible context.

Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
---
 v1->v2:
 Add IS_ENABLED(CONFIG_PREEMPT_RT) check.

 mm/util.c    | 4 +++-
 mm/vmalloc.c | 4 ++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/util.c b/mm/util.c
index 12984e76767e..2b0222a728cc 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1128,7 +1128,9 @@ void mem_dump_obj(void *object)
 		return;
 
 	if (virt_addr_valid(object))
-		type = "non-slab/vmalloc memory";
+		type = "non-slab memory";
+	else if (is_vmalloc_addr(object))
+		type = "vmalloc memory";
 	else if (object == NULL)
 		type = "NULL pointer";
 	else if (object == ZERO_SIZE_PTR)
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index ccaa461998f3..018e417b12d6 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -4034,6 +4034,10 @@ bool vmalloc_dump_obj(void *object)
 	struct vm_struct *vm;
 	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
 
+	if (!is_vmalloc_addr(objp) || ((IS_ENABLED(CONFIG_PREEMPT_RT) &&
+				!preemptible()) || in_interrupt()))
+		return false;
+
 	vm = find_vm_area(objp);
 	if (!vm)
 		return false;
-- 
2.25.1