Re: [PATCH] stack and rcu interaction bug in smp_call_function_mask()

From: Nick Piggin
Date: Sun Aug 10 2008 - 02:25:41 EST

On Saturday 09 August 2008 05:37, 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.
>
>
> One way to solve the problem is to have CPU A wait as long as there is a
> rcu read 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?

Nice debugging work.

I'd suggest something like the attached (untested) patch as the simple
fix for now.

I expect the benefits from the less synchronized, multiple-in-flight-data
global queue will still outweigh the costs of dynamic allocations. But
if worst comes to worst then we just go back to a globally synchronous
one-at-a-time implementation, but that would be pretty sad!
Index: linux-2.6/kernel/smp.c
===================================================================
--- linux-2.6.orig/kernel/smp.c
+++ linux-2.6/kernel/smp.c
@@ -260,6 +260,41 @@ void __smp_call_function_single(int cpu,
generic_exec_single(cpu, data);
}

+/* Dummy function */
+static void quiesce_dummy(void *unused)
+{
+}
+
+/*
+ * Ensure stack based data used in call function mask is safe to free.
+ *
+ * This is needed by smp_call_function_mask when using on-stack data, because
+ * a single call function queue is shared by all CPUs, and any CPU may pick up
+ * the data item on the queue at any time before it is deleted. So we need to
+ * ensure that all CPUs have transitioned through a quiescent state after
+ * this call.
+ *
+ * This is a very slow function, implemented by sending synchronous IPIs to
+ * all possible CPUs. For this reason, we have to alloc data rather than use
+ * stack based data even in the case of synchronous calls. The stack based
+ * data is then just used for deadlock/oom fallback which will be very rare.
+ *
+ * If a faster scheme can be made, we could go back to preferring stack based
+ * data -- the data allocation/free is non-zero cost.
+ */
+static void smp_call_function_mask_quiesce_stack(cpumask_t mask)
+{
+ struct call_single_data data;
+ int cpu;
+
+ data.func = quiesce_dummy;
+ data.info = NULL;
+ data.flags = CSD_FLAG_WAIT;
+
+ for_each_cpu_mask(cpu, mask)
+ generic_exec_single(cpu, &data);
+}
+
/**
* smp_call_function_mask(): Run a function on a set of other CPUs.
* @mask: The set of cpus to run on.
@@ -285,6 +320,7 @@ int smp_call_function_mask(cpumask_t mas
cpumask_t allbutself;
unsigned long flags;
int cpu, num_cpus;
+ int slowpath = 0;

/* Can deadlock when called with interrupts disabled */
WARN_ON(irqs_disabled());
@@ -306,15 +342,14 @@ int smp_call_function_mask(cpumask_t mas
return smp_call_function_single(cpu, func, info, wait);
}

- if (!wait) {
- data = kmalloc(sizeof(*data), GFP_ATOMIC);
- if (data)
- data->csd.flags = CSD_FLAG_ALLOC;
- }
- if (!data) {
+ data = kmalloc(sizeof(*data), GFP_ATOMIC);
+ if (data)
+ data->csd.flags = CSD_FLAG_ALLOC;
+ else {
data = &d;
data->csd.flags = CSD_FLAG_WAIT;
wait = 1;
+ slowpath = 1;
}

spin_lock_init(&data->lock);
@@ -331,8 +366,11 @@ int smp_call_function_mask(cpumask_t mas
arch_send_call_function_ipi(mask);

/* optionally wait for the CPUs to complete */
- if (wait)
+ if (wait) {
csd_flag_wait(&data->csd);
+ if (unlikely(slowpath))
+ smp_call_function_mask_quiesce_stack(allbutself);
+ }

return 0;
}