On Tue 08-10-19 09:43:57, Petr Mladek wrote: > On Mon 2019-10-07 16:49:37, Michal Hocko wrote: > > [Cc s390 maintainers - the lockdep is http://lkml.kernel.org/r/1570228005-24979-1-git-send-email-cai@xxxxxx > > Petr has explained it is a false positive > > http://lkml.kernel.org/r/20191007143002.l37bt2lzqtnqjqxu@xxxxxxxxxxxxxxx] > > On Mon 07-10-19 16:30:02, Petr Mladek wrote: > > [...] > > > I believe that it cannot really happen because: > > > > > > static int __init > > > sclp_console_init(void) > > > { > > > [...] > > > rc = sclp_rw_init(); > > > [...] > > > register_console(&sclp_console); > > > return 0; > > > } > > > > > > sclp_rw_init() is called before register_console(). And > > > console_unlock() will never call sclp_console_write() before > > > the console is registered. > > > > > > AFAIK, lockdep only compares existing chain of locks. It does > > > not know about console registration that would make some > > > code paths mutually exclusive. > > > > > > I believe that it is a false positive. I do not know how to > > > avoid this lockdep report. I hope that it will disappear > > > by deferring all printk() calls rather soon. > > > > Thanks a lot for looking into this Petr. I have also checked the code > > and I really fail to see why the allocation has to be done under the > > lock in the first place. sclp_read_sccb and sclp_init_sccb are global > > variables but I strongly suspect that they need a synchronization during > > early init, callbacks are registered only later IIUC: > > Good idea. It would work when the init function is called only once. > But see below. > > > diff --git a/drivers/s390/char/sclp.c b/drivers/s390/char/sclp.c > > index d2ab3f07c008..4b1c033e3255 100644 > > --- a/drivers/s390/char/sclp.c > > +++ b/drivers/s390/char/sclp.c > > @@ -1169,13 +1169,13 @@ sclp_init(void) > > unsigned long flags; > > int rc = 0; > > > > + sclp_read_sccb = (void *) __get_free_page(GFP_ATOMIC | GFP_DMA); > > + sclp_init_sccb = (void *) __get_free_page(GFP_ATOMIC | GFP_DMA); > > spin_lock_irqsave(&sclp_lock, flags); > > /* Check for previous or running initialization */ > > if (sclp_init_state != sclp_init_state_uninitialized) > > goto fail_unlock; > > It seems that sclp_init() could be called several times in parallel. > I see it called from sclp_register() and sclp_initcall(). Interesting. Something for s390 people to answer I guess. Anyway, this should be quite trivial to workaround by a cmpxch or alike. -- Michal Hocko SUSE Labs
