Re: [PATCH] mm: Fix possible deadlock in console_trylock_spinning

[Petr Mladek <pmladek@suse.com>]

On Fri 2025-08-01 16:52:28, Andrew Morton wrote:
> On Wed, 30 Jul 2025 17:49:14 +0800 Gu Bowen <gubowen5@huawei.com> wrote:
> 
> > kmemleak_scan_thread() invokes scan_block() which may invoke a nomal
> > printk() to print warning message. This can cause a deadlock in the
> > scenario reported below:
> > 
> >        CPU0                    CPU1
> >        ----                    ----
> >   lock(kmemleak_lock);
> >                                lock(&port->lock);
> >                                lock(kmemleak_lock);
> >   lock(console_owner);
> > 
> > To solve this problem, switch to printk_safe mode before printing warning
> > message, this will redirect all printk()-s to a special per-CPU buffer,
> > which will be flushed later from a safe context (irq work), and this
> > deadlock problem can be avoided.
> > 
> > Our syztester report the following lockdep error:
> > 
> > ======================================================
> > WARNING: possible circular locking dependency detected
> > 5.10.0-22221-gca646a51dd00 #16 Not tainted
> > ------------------------------------------------------
> > 
> > ...
> >
> > Chain exists of:
> >   console_owner --> &port->lock --> kmemleak_lock
> > 
> > Cc: stable@vger.kernel.org # 5.10
> > Signed-off-by: Gu Bowen <gubowen5@huawei.com>
> > ---
> >  mm/kmemleak.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> > index 4801751cb6b6..d322897a1de1 100644
> > --- a/mm/kmemleak.c
> > +++ b/mm/kmemleak.c
> > @@ -390,9 +390,11 @@ static struct kmemleak_object *lookup_object(unsigned long ptr, int alias)
> >  		else if (object->pointer == ptr || alias)
> >  			return object;
> >  		else {
> > +			__printk_safe_enter();
> >  			kmemleak_warn("Found object by alias at 0x%08lx\n",
> >  				      ptr);
> >  			dump_object_info(object);
> > +			__printk_safe_exit();
> >  			break;
> >  		}
> >  	}
> 
> umm,
> 
> --- a/mm/kmemleak.c~a
> +++ a/mm/kmemleak.c
> @@ -103,6 +103,8 @@
>  #include <linux/kmemleak.h>
>  #include <linux/memory_hotplug.h>
>  
> +#include "../kernel/printk/internal.h"		/* __printk_safe_enter */
> +
>  /*
>   * Kmemleak configuration and common defines.
>   */
> 
> 
> I'm not sure we're allowed to do that.  Is there an official way?

The official way is to use printk_deferred_enter()/exit().

Note that the API is using a per-CPU variable. It must be called
with CPU migration disabled. The comment suggests disabling IRQs.
But it should be enough to disable preemption.

Best Regards,
Petr