On Fri 30-05-14 18:58:10, Jan Kara wrote: > On Fri 30-05-14 18:19:48, Peter Zijlstra wrote: > > On Fri, May 30, 2014 at 06:16:47PM +0200, Peter Zijlstra wrote: > > > On Fri, May 30, 2014 at 05:50:51PM +0200, Jan Kara wrote: > > > > > [ 7.492350] ====================================================== > > > > > [ 7.492350] [ INFO: possible circular locking dependency detected ] > > > > > [ 7.492350] 3.15.0-rc5-00567-gbafe980 #1 Not tainted > > > > > [ 7.492350] ------------------------------------------------------- > > > > > [ 7.492350] swapper/1 is trying to acquire lock: > > > > > [ 7.492350] (&irq_desc_lock_class){-.-...}, at: [<8107dc8c>] > > > > > __irq_get_desc_lock+0x3c/0x70 > > > > > [ 7.492350] > > > > > [ 7.492350] but task is already holding lock: > > > > > [ 7.492350] (&port_lock_key){......}, at: [<815f5b27>] > > > > > serial8250_startup+0x337/0x720 > > > > > [ 7.492350] > > > > > [ 7.492350] which lock already depends on the new lock. > > > > > [ 7.492350] > > > > > [ 7.492350] > > > > > [ 7.492350] the existing dependency chain (in reverse order) is: > > > > > [ 7.492350] > > > > > -> #2 (&port_lock_key){......}: > > > > > [ 7.492350] [<810750e5>] lock_acquire+0x85/0x190 > > > > > [ 7.492350] [<81baed9d>] _raw_spin_lock_irqsave+0x4d/0x60 > > > > > [ 7.492350] [<8106eb1c>] down_trylock+0xc/0x30 > > > > > [ 7.492350] [<8107b795>] console_trylock+0x15/0xb0 > > > > > [ 7.492350] [<8107be8f>] vprintk_emit+0x14f/0x4d0 > > > > > [ 7.492350] [<81b969b9>] printk+0x38/0x3a > > > > > [ 7.492350] [<82137f78>] print_ICs+0x5b/0x3e7 > > > > > [ 7.492350] [<8212bb41>] do_one_initcall+0x8b/0x128 > > > > > [ 7.492350] [<8212bd7d>] kernel_init_freeable+0x19f/0x236 > > > > > [ 7.492350] [<81b9238b>] kernel_init+0xb/0xd0 > > > > > [ 7.492350] [<81bb0080>] ret_from_kernel_thread+0x20/0x30 > > > > > > > > But this looks really strange. How can we possibly get port_lock_key > > > > in > > > > down_trylock() which calls raw_spin_lock_irqsave() on console_sem->lock? > > > > That looks like some strange lockdep key aliasing issue? Peter do you > > > > have > > > > any idea? > > > > > > No, strange that, I can't say I've ever seen a bogus stracktrace in > > > lockdep reports like this. > > > > > > So this is through: check_prev_add()->save_trace(). And that doesn't > > > reuse entries, at worst it can truncate a trace when we run out of > > > entries, but the above looks complete since it terminates in > > > lock_acquire(), which is the right place to be. > > > > > > But its worse than that, the above trace should link i8259A_lock to > > > port_lock_key, and I can't see where we would have taken i8259A_lock > > > either. > > > > Oh, wait, I missed it, that would be: print_ICs()->print_PIC(), it takes > > that lock there. > Yeah, so as much as the lockdep reported stack trace looks strange I can > now see how a locking problem lockdep reports can happen. We really do call > printk() under i8259A_lock in print_PIC() and so the locking chain lockdep > found is real. Luckily it likely cannot lead to any real problems because > printk only happens during early init. > > In any case it is another example of a problem that was just uncovered by > my change which increased lockdep coverage of printk code. I'll send a fix > to x86 maintainers. Jet, can you please test the attached patch? Thanks!
Honza -- Jan Kara <j...@suse.cz> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/