On Tue, 2019-10-08 at 20:35 +0200, Michal Hocko wrote: > On Tue 08-10-19 12:08:37, Qian Cai wrote: > > On Tue, 2019-10-08 at 14:56 +0200, Christian Borntraeger wrote: > > > Adding Peter Oberparleiter. > > > Peter, can you have a look? > > > > > > On 08.10.19 10:27, Michal Hocko wrote: > > > > 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. > > > > > > > > The above fix is simply insufficient, > > Isn't this yet another init time lockdep false possitive? Again, this is not 100% false positive for sure yet. > > > 00: [ 3.654337] -> #3 (console_owner){....}: > > 00: [ 3.654343] lock_acquire+0x21a/0x468 > > 00: [ 3.654345] console_unlock+0x3a6/0xa30 > > 00: [ 3.654346] vprintk_emit+0x184/0x3c8 > > 00: [ 3.654348] vprintk_default+0x44/0x50 > > 00: [ 3.654349] printk+0xa8/0xc0 > > 00: [ 3.654351] get_random_u64+0x40/0x108 > > 00: [ 3.654360] add_to_free_area_random+0x188/0x1c0 > > 00: [ 3.654364] free_one_page+0x72/0x128 > > 00: [ 3.654366] __free_pages_ok+0x51c/0xca0 > > 00: [ 3.654368] memblock_free_all+0x30a/0x3b0 > > 00: [ 3.654370] mem_init+0x84/0x200 > > 00: [ 3.654371] start_kernel+0x384/0x6a0 > > 00: [ 3.654373] startup_continue+0x70/0xd0 > > This one is actually a nice example why trying to get printk out of the > zone->lock is simply not viable. This one is likely a printk to warn > that the random pool is not fully intiailized. Just because the > allocator tries to randomize the initial free memory pool. You are not > going to remove that printk, right? Well, Sergey had a patch to convert that one to printk_deferred(), but even with his patch, it will still trigger the lockdep splat here because the lock dependency between zone->lock --> console_owner is still there from memory offline. > > I fully agree that this class of lockdep splats are annoying especially > when they make the lockdep unusable but please discuss this with lockdep > maintainers and try to find some solution rather than go and try to > workaround the problem all over the place. If there are places that > would result in a cleaner code then go for it but please do not make the > code worse just because of a non existent problem flagged by a false > positive. It makes me wonder what make you think it is a false positive for sure.
