pi futex oops in __lock_acquire

[Dave Jones]

I've been able to trigger this for the last week or so.
Unclear whether this is a new bug, or my fuzzer got smarter, but I see the
pi-futex code hasn't changed since the last time it found something..

 > BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
 > IP: [<ffffffff810e185e>] __lock_acquire+0x5e/0x1ba0
 > PGD 8e72c067 PUD 34f07067 PMD 0 
 > Oops: 0000 [#1] PREEMPT SMP 
 > CPU 7 
 > Pid: 27513, comm: trinity-child0 Not tainted 3.7.0-rc2+ #43
 > RIP: 0010:[<ffffffff810e185e>]  [<ffffffff810e185e>] __lock_acquire+0x5e/0x1ba0
 > RSP: 0018:ffff8800803f7b28  EFLAGS: 00010046
 > RAX: 0000000000000086 RBX: 0000000000000000 RCX: 0000000000000000
 > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000018
 > RBP: ffff8800803f7c18 R08: 0000000000000002 R09: 0000000000000000
 > R10: 0000000000000000 R11: 2222222222222222 R12: 0000000000000002
 > R13: ffff880051dd8000 R14: 0000000000000002 R15: 0000000000000018
 > FS:  00007f9fc6ccb740(0000) GS:ffff880148a00000(0000) knlGS:0000000000000000
 > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 > CR2: 0000000000000018 CR3: 000000008e6fb000 CR4: 00000000001407e0
 > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 > DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
 > Process trinity-child0 (pid: 27513, threadinfo ffff8800803f6000, task ffff880051dd8000)
 > Stack:
 >  ffff8800803f7b48 ffffffff816c5c59 ffff8800803f7b48 ffff88014840ebc0
 >  ffff8800803f7b68 ffffffff816c18e3 ffff8800803f7d10 0000000000000001
 >  ffff8800803f7ba8 ffffffff810a1e62 ffff8800803f7d10 0000000000000282
 > Call Trace:
 >  [<ffffffff816c5c59>] ? sub_preempt_count+0x79/0xd0
 >  [<ffffffff816c18e3>] ? _raw_spin_unlock_irqrestore+0x73/0xa0
 >  [<ffffffff810a1e62>] ? hrtimer_try_to_cancel+0x52/0x210
 >  [<ffffffff810eb9e5>] ? debug_rt_mutex_free_waiter+0x15/0x180
 >  [<ffffffff816c0107>] ? rt_mutex_slowlock+0x127/0x1b0
 >  [<ffffffff810b7039>] ? local_clock+0x89/0xa0
 >  [<ffffffff810e3ac2>] lock_acquire+0xa2/0x220
 >  [<ffffffff810e812c>] ? futex_lock_pi.isra.18+0x1cc/0x390
 >  [<ffffffff816c09e0>] _raw_spin_lock+0x40/0x80
 >  [<ffffffff810e812c>] ? futex_lock_pi.isra.18+0x1cc/0x390
 >  [<ffffffff810e812c>] futex_lock_pi.isra.18+0x1cc/0x390
 >  [<ffffffff810a1980>] ? update_rmtp+0x70/0x70
 >  [<ffffffff810e99e4>] do_futex+0x394/0xa50
 >  [<ffffffff8119ec43>] ? might_fault+0x53/0xb0
 >  [<ffffffff810ea12d>] sys_futex+0x8d/0x190
 >  [<ffffffff816ca288>] tracesys+0xe1/0xe6
 > Code: d8 45 0f 45 e0 4c 89 75 f0 4c 89 7d f8 85 c0 0f 84 f8 00 00 00 8b 05 22 fe f3 00 49 89 ff 89 f3 41 89 d2 85 c0 0f 84 02 01 00 00 <49> 8b 07 ba 01 00 00 00 48 3d c0 81 06 82 44 0f 44 e2 83 fb 01 
 > RIP  [<ffffffff810e185e>] __lock_acquire+0x5e/0x1ba0
 >  RSP <ffff8800803f7b28>
 > CR2: 0000000000000018

It looks like we got all the way to lock_acquire with a NULL 'lock' somehow.

Darren, any idea how this could happen ?

	Dave

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