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. >
