2022-11-12 13:04:14

by Zhen Lei

Subject: [PATCH] mm/vmalloc: allow mem_dump_obj() to be called in interrupt context

The function mem_dump_obj() can sometimes provide valuable debugging
information, but it cannot be called in an interrupt context because
spinlock vmap_area_lock has not been protected against IRQs. If the
current task has held the lock before hard/soft interrupt handler calls
mem_dump_obj(), simply abandoning the dump operation can avoid deadlock.
That is, no deadlock occurs in extreme cases, and dump succeeds in most
cases.

Signed-off-by: Zhen Lei <[email protected]>
---
mm/vmalloc.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index ccaa461998f3c37..cdd36c5a1aa16f8 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -4034,6 +4034,9 @@ bool vmalloc_dump_obj(void *object)
struct vm_struct *vm;
void *objp = (void *)PAGE_ALIGN((unsigned long)object);

+ if (unlikely(spin_is_locked(&vmap_area_lock)))
+ return false;
+
vm = find_vm_area(objp);
if (!vm)
return false;
--
2.25.1

2022-11-15 01:15:34

by Andrew Morton

Subject: Re: [PATCH] mm/vmalloc: allow mem_dump_obj() to be called in interrupt context

On Sat, 12 Nov 2022 20:15:37 +0800 Zhen Lei <[email protected]> wrote:

> The function mem_dump_obj() can sometimes provide valuable debugging
> information, but it cannot be called in an interrupt context because
> spinlock vmap_area_lock has not been protected against IRQs. If the
> current task has held the lock before hard/soft interrupt handler calls
> mem_dump_obj(), simply abandoning the dump operation can avoid deadlock.
> That is, no deadlock occurs in extreme cases, and dump succeeds in most
> cases.
>
> ...
>
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -4034,6 +4034,9 @@ bool vmalloc_dump_obj(void *object)
> struct vm_struct *vm;
> void *objp = (void *)PAGE_ALIGN((unsigned long)object);
>
> + if (unlikely(spin_is_locked(&vmap_area_lock)))
> + return false;
> +
> vm = find_vm_area(objp);
> if (!vm)
> return false;

Yes, but this will worsen the current uses of this function. Consider
the case where task A wants to call vmalloc_dump_obj() but task B holds
vmap_area_lock. No problem, task A will simply spin until task B is
done.

But after this patch, task A's call to vmalloc_dump_obj() will return
without having done anything.