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, 00: [ 3.654307] WARNING: possible circular locking dependency detected 00: [ 3.654309] 5.4.0-rc1-next-20191004+ #4 Not tainted 00: [ 3.654311] ------------------------------------------------------ 00: [ 3.654313] swapper/0/1 is trying to acquire lock: 00: [ 3.654314] 00000000553f3fb8 (sclp_lock){-.-.}, at: sclp_add_request+0x34 00: /0x308 00: [ 3.654320] 00: [ 3.654322] but task is already holding lock: 00: [ 3.654323] 00000000550c9fc0 (console_owner){....}, at: console_unlock+0x 00: 328/0xa30 00: [ 3.654329] 00: [ 3.654331] which lock already depends on the new lock. 00: [ 3.654332] 00: [ 3.654333] 00: [ 3.654335] the existing dependency chain (in reverse order) is: 00: [ 3.654336] 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 00: [ 3.654374] 00: [ 3.654375] -> #2 (&(&zone->lock)->rlock){....}: 00: [ 3.654382] lock_acquire+0x21a/0x468 00: [ 3.654383] _raw_spin_lock+0x54/0x68 00: [ 3.654385] get_page_from_freelist+0x8b6/0x2d28 00: [ 3.654387] __alloc_pages_nodemask+0x246/0x658 00: [ 3.654389] alloc_slab_page+0x43c/0x858 00: [ 3.654390] allocate_slab+0x90/0x6f0 00: [ 3.654392] new_slab+0x88/0x90 00: [ 3.654393] ___slab_alloc+0x600/0x988 00: [ 3.654395] __slab_alloc+0x5a/0x90 00: [ 3.654396] kmem_cache_alloc+0x340/0x4c0 00: [ 3.654398] fill_pool+0x27a/0x498 00: [ 3.654400] __debug_object_init+0xa8/0x858 00: [ 3.654401] debug_object_activate+0x208/0x370 00: [ 3.654403] __call_rcu+0xb6/0x4a8 00: [ 3.654405] unregister_external_irq+0x13a/0x140 00: [ 3.654406] sclp_init+0x5e0/0x688 00: [ 3.654408] sclp_register+0x2e/0x248 00: [ 3.654409] sclp_rw_init+0x4a/0x70 00: [ 3.654411] sclp_console_init+0x4a/0x1b8 00: [ 3.654413] console_init+0x2c8/0x410 00: [ 3.654414] start_kernel+0x530/0x6a0 00: [ 3.654416] startup_continue+0x70/0xd0 00: [ 3.654417] 00: [ 3.654418] -> #1 (ext_int_hash_lock){....}: 00: [ 3.654424] lock_acquire+0x21a/0x468 00: [ 3.654426] _raw_spin_lock_irqsave+0xcc/0xe8 00: [ 3.654427] register_external_irq+0xb6/0x138 00: [ 3.654429] sclp_init+0x212/0x688 00: [ 3.654430] sclp_register+0x2e/0x248 00: [ 3.654432] sclp_rw_init+0x4a/0x70 00: [ 3.654434] sclp_console_init+0x4a/0x1b8 00: [ 3.654435] console_init+0x2c8/0x410 00: [ 3.654437] start_kernel+0x530/0x6a0 00: [ 3.654438] startup_continue+0x70/0xd0 00: [ 3.654439] 00: [ 3.654440] -> #0 (sclp_lock){-.-.}: 00: [ 3.654446] check_noncircular+0x338/0x3e0 00: [ 3.654448] __lock_acquire+0x1e66/0x2d88 00: [ 3.654450] lock_acquire+0x21a/0x468 00: [ 3.654451] _raw_spin_lock_irqsave+0xcc/0xe8 00: [ 3.654453] sclp_add_request+0x34/0x308 00: [ 3.654455] sclp_conbuf_emit+0x100/0x138 00: [ 3.654456] sclp_console_write+0x96/0x3b8 00: [ 3.654458] console_unlock+0x6dc/0xa30 00: [ 3.654460] vprintk_emit+0x184/0x3c8 00: [ 3.654461] vprintk_default+0x44/0x50 00: [ 3.654463] printk+0xa8/0xc0 00: [ 3.654464] iommu_debugfs_setup+0xf2/0x108 00: [ 3.654466] iommu_init+0x6c/0x78 00: [ 3.654467] do_one_initcall+0x162/0x680 00: [ 3.654469] kernel_init_freeable+0x4e8/0x5a8 00: [ 3.654471] kernel_init+0x2a/0x188 00: [ 3.654472] ret_from_fork+0x30/0x34 00: [ 3.654474] kernel_thread_starter+0x0/0xc 00: [ 3.654475] 00: [ 3.654476] other info that might help us debug this: 00: [ 3.654477] 00: [ 3.654479] Chain exists of: 00: [ 3.654480] sclp_lock --> &(&zone->lock)->rlock --> console_owner 00: [ 3.654488] 00: [ 3.654489] Possible unsafe locking scenario: 00: [ 3.654490] 00: [ 3.654492] CPU0 CPU1 00: [ 3.654493] ---- ---- 00: [ 3.654495] lock(console_owner); 00: [ 3.654498] lock(&(&zone->lock)->rlock); 00: [ 3.654503] lock(console_owner); 00: [ 3.654506] lock(sclp_lock); 00: [ 3.654509] 00: [ 3.654511] *** DEADLOCK *** 00: [ 3.654512] 00: [ 3.654513] 2 locks held by swapper/0/1: 00: [ 3.654514] #0: 00000000550ca200 (console_lock){+.+.}, at: vprintk_emit+ 00: 0x178/0x3c8 00: [ 3.654521] #1: 00000000550c9fc0 (console_owner){....}, at: console_unlo 00: ck+0x328/0xa30 00: [ 3.654529] 00: [ 3.654530] stack backtrace: 00: [ 3.654532] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.4.0-rc1-next-2019 00: 1004+ #4 00: [ 3.654534] Hardware name: IBM 2964 N96 400 (z/VM 6.4.0) 00: [ 3.654535] Call Trace: 00: [ 3.654537] ([<000000005431e218>] show_stack+0x110/0x1b0) 00: [ 3.654539] [<0000000054bdd926>] dump_stack+0x126/0x178 00: [ 3.654541] [<0000000054414b08>] check_noncircular+0x338/0x3e0 00: [ 3.654543] [<000000005441aaf6>] __lock_acquire+0x1e66/0x2d88 00: [ 3.654545] [<0000000054417e12>] lock_acquire+0x21a/0x468 00: [ 3.654547] [<0000000054c18cc4>] _raw_spin_lock_irqsave+0xcc/0xe8 00: [ 3.654549] [<00000000549f2e14>] sclp_add_request+0x34/0x308 00: [ 3.654551] [<00000000549fa7f0>] sclp_conbuf_emit+0x100/0x138 00: [ 3.654553] [<00000000549fa90e>] sclp_console_write+0x96/0x3b8 00: [ 3.654554] [<000000005442b634>] console_unlock+0x6dc/0xa30 00: [ 3.654556] [<000000005442de2c>] vprintk_emit+0x184/0x3c8 00: [ 3.654558] [<000000005442e0b4>] vprintk_default+0x44/0x50 00: [ 3.654560] [<000000005442eb60>] printk+0xa8/0xc0 00: [ 3.654562] [<000000005494ca3a>] iommu_debugfs_setup+0xf2/0x108 00: [ 3.654564] [<000000005557c0ec>] iommu_init+0x6c/0x78 00: [ 3.654566] [<0000000054300fda>] do_one_initcall+0x162/0x680 00: [ 3.654568] [<000000005553b9f0>] kernel_init_freeable+0x4e8/0x5a8 00: [ 3.654569] [<0000000054c04172>] kernel_init+0x2a/0x188 00: [ 3.654571] [<0000000054c19fbc>] ret_from_fork+0x30/0x34 00: [ 3.654573] [<0000000054c19fc0>] kernel_thread_starter+0x0/0xc
