Re: [prink] BUG: spinlock lockup suspected on CPU#0, swapper/1

[Jan Kara <jack@suse.cz>]

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 <jack@suse.cz>
SUSE Labs, CR