Date:	Fri, 08 Aug 2008 19:17:36 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
CC:	Jens Axboe <jens.axboe@...cle.com>, Ingo Molnar <mingo@...e.hu>,
	npiggin@...e.de, linux-kernel <linux-kernel@...r.kernel.org>,
	suresh.b.siddha@...el.com
Subject: Re: [PATCH] stack and rcu interaction bug in smp_call_function_mask()

Venki Pallipadi wrote:
> Found a OOPS on a big SMP box during an overnight reboot test with upstream git.
>
> Suresh and I looked at the oops and looks like the root cause is in
> generic_smp_call_function_interrupt() and smp_call_function_mask() with
> wait parameter.
>
> The actual oops looked like
>
> [   11.277260] BUG: unable to handle kernel paging request at ffff8802ffffffff
> [   11.277815] IP: [<ffff8802ffffffff>] 0xffff8802ffffffff
> [   11.278155] PGD 202063 PUD 0
> [   11.278576] Oops: 0010 [1] SMP
> [   11.279006] CPU 5
> [   11.279336] Modules linked in:
> [   11.279752] Pid: 0, comm: swapper Not tainted 2.6.27-rc2-00020-g685d87f #290
> [   11.280039] RIP: 0010:[<ffff8802ffffffff>]  [<ffff8802ffffffff>] 0xffff8802ffffffff
> [   11.280692] RSP: 0018:ffff88027f1f7f70  EFLAGS: 00010086
> [   11.280976] RAX: 00000000ffffffff RBX: 0000000000000000 RCX: 0000000000000000
> [   11.281264] RDX: 0000000000004f4e RSI: 0000000000000001 RDI: 0000000000000000
> [   11.281624] RBP: ffff88027f1f7f98 R08: 0000000000000001 R09: ffffffff802509af
> [   11.281925] R10: ffff8800280c2780 R11: 0000000000000000 R12: ffff88027f097d48
> [   11.282214] R13: ffff88027f097d70 R14: 0000000000000005 R15: ffff88027e571000
> [   11.282502] FS:  0000000000000000(0000) GS:ffff88027f1c3340(0000) knlGS:0000000000000000
> [   11.283096] CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
> [   11.283382] CR2: ffff8802ffffffff CR3: 0000000000201000 CR4: 00000000000006e0
> [   11.283760] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [   11.284048] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> [   11.284337] Process swapper (pid: 0, threadinfo ffff88027f1f2000, task ffff88027f1f0640)
> [   11.284936] Stack:  ffffffff80250963 0000000000000212 0000000000ee8c78 0000000000ee8a66
> [   11.285802]  ffff88027e571550 ffff88027f1f7fa8 ffffffff8021adb5 ffff88027f1f3e40
> [   11.286599]  ffffffff8020bdd6 ffff88027f1f3e40 <EOI>  ffff88027f1f3ef8 0000000000000000
> [   11.287120] Call Trace:
> [   11.287768]  <IRQ>  [<ffffffff80250963>] ? generic_smp_call_function_interrupt+0x61/0x12c
> [   11.288354]  [<ffffffff8021adb5>] smp_call_function_interrupt+0x17/0x27
> [   11.288744]  [<ffffffff8020bdd6>] call_function_interrupt+0x66/0x70
> [   11.289030]  <EOI>  [<ffffffff8024ab3b>] ? clockevents_notify+0x19/0x73
> [   11.289380]  [<ffffffff803b9b75>] ? acpi_idle_enter_simple+0x18b/0x1fa
> [   11.289760]  [<ffffffff803b9b6b>] ? acpi_idle_enter_simple+0x181/0x1fa
> [   11.290051]  [<ffffffff8053aeca>] ? cpuidle_idle_call+0x70/0xa2
> [   11.290338]  [<ffffffff80209f61>] ? cpu_idle+0x5f/0x7d
> [   11.290723]  [<ffffffff8060224a>] ? start_secondary+0x14d/0x152
> [   11.291010]
> [   11.291287]
> [   11.291654] Code:  Bad RIP value.
> [   11.292041] RIP  [<ffff8802ffffffff>] 0xffff8802ffffffff
> [   11.292380]  RSP <ffff88027f1f7f70>
> [   11.292741] CR2: ffff8802ffffffff
> [   11.310951] ---[ end trace 137c54d525305f1c ]---
>
> The problem is with the following sequence of events:
>
> - CPU A calls smp_call_function_mask() for CPU B with wait parameter
> - CPU A sets up the call_function_data on the stack and does an rcu add to
>   call_function_queue
> - CPU A waits until the WAIT flag is cleared
> - CPU B gets the call function interrupt and starts going through the
>   call_function_queue
> - CPU C also gets some other call function interrupt and starts going through
>   the call_function_queue
> - CPU C, which is also going through the call_function_queue, starts referencing
>   CPU A's stack, as that element is still in call_function_queue
> - CPU B finishes the function call that CPU A set up and as there are no other
>   references to it, rcu deletes the call_function_data (which was from CPU A
>   stack)
> - CPU B sees the wait flag and just clears the flag (no call_rcu to free)
> - CPU A which was waiting on the flag continues executing and the stack
>   contents change
>
> - CPU C is still in rcu_read section accessing the CPU A's stack sees
>   inconsistent call_funation_data and can try to execute
>   function with some random pointer, causing stack corruption for A
>   (by clearing the bits in mask field) and oops.
>   

Ah! Thank you!  I had seen this crash a couple of times, but I could 
work out how to reproduce it enough to root cause it.   I'd also only 
ever seen it while running under Xen, so I wasn't sure if it was 
Xen-specific or not.

See Ingo, I wasn't imagining it ;)

> One way to solve the problem is to have CPU A wait as long as there is a rcu
> read happening.

OK, looks like I could get that if a vcpu was preempted while the rcu 
read is happening.

>  But, we cannot use synchronize_rcu() here as we cannot block.
> Another way around is to always allocate call_function_data, instead
> of using CPU A's stack. Below patch does this. But, that will still have to
> handle the kmalloc failure case somehow.
>
> Any other ideas on how to solve this problem?
>   

Nothing leaps to mind at the moment.  Always allocating would not be 
good, as we'd like to use this for tlb flushes.

    J
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