Messages in this thread | | | Subject | Re: [PATCH] debugobjects: Move printk out of db lock critical sections | From | Dmitry Safonov <> | Date | Thu, 13 Dec 2018 04:35:04 +0000 |
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Hi Waiman,
On 12/12/18 10:28 PM, Waiman Long wrote: > The db->lock is a raw spinlock and so the lock hold time is supposed > to be short. This will not be the case when printk() is being involved > in some of the critical sections. In order to avoid the long hold time, > in case some messages need to be printed, the debug_object_is_on_stack() > and debug_print_object() calls are now moved out of those critical > sections. > > Holding the db->lock while calling printk() may lead to deadlock if > printk() somehow requires the allocation/freeing of debug object that > happens to be in the same hash bucket or a circular lock dependency > warning from lockdep as reported in https://lkml.org/lkml/2018/12/11/143. > > [ 87.209665] WARNING: possible circular locking dependency detected > [ 87.210547] 4.20.0-rc4-00057-gc96cf92 #1 Tainted: G W > [ 87.211449] ------------------------------------------------------ > [ 87.212405] getty/519 is trying to acquire lock: > [ 87.213074] (____ptrval____) (&obj_hash[i].lock){-.-.}, at: debug_check_no_obj_freed+0xb4/0x302 > [ 87.214343] > [ 87.214343] but task is already holding lock: > [ 87.215174] (____ptrval____) (&port_lock_key){-.-.}, at: uart_shutdown+0x3a3/0x4e2 > [ 87.216260] > [ 87.216260] which lock already depends on the new lock. > > This patch was also found to be able to fix a boot hanging problem > when the initramfs image was switched on after a debugobjects splat > from the EFI code. > > Signed-off-by: Waiman Long <longman@redhat.com> > ---
I've tried to review it and found minor issues like missed debug_object_is_on_stack() for initializing already active object.
But than I come to opinion that it's just generally unsafe: debug_obj life-time is protected by bucket's spin_lock. Check the conditions when free_object() is being called.
So, I resulted by finishing my stack-copy version which I've started yesterday on test robot report when I didn't know about your patch.
I've sent it now concurrently.
Thanks, Dima
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