On Wed, 12 Dec 2018 17:28:14 -0500 Waiman Long <[email protected]> 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. Patch looks sensible, but I have a nit about the variable names. > --- a/lib/debugobjects.c > +++ b/lib/debugobjects.c > @@ -375,6 +375,8 @@ static void debug_object_is_on_stack(void *addr, int > onstack) > struct debug_bucket *db; > struct debug_obj *obj; > unsigned long flags; > + bool debug_printobj = false; "debug_printobject" would be better, but this code already intermingles "obj" and "object". > + bool debug_chkstack = false; Not so good. Is it debug_chkstack or debug_checkstk or ... This file uses "check" consistently so let's not depart from that? Linux style is to avoid these tricky little abbreviations and to use full words. ie, debug_checkstack, please. Better would be debug_check_stack. Or simply check_stack: the "debug" doesn't add anything useful.
