generic-ipi: fix stack and rcu interaction bug in smp_call_function_mask()

* Venki Pallipadi <[e-mail]> 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.

Nice debugging work.

I'd suggest something like the attached (boot tested) 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!

Signed-off-by: Ingo Molnar <[e-mail]>

kernel/smp.c

···
 	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.
···
 	cpumask_t allbutself;
 	unsigned long flags;
 	int cpu, num_cpus;
+	int slowpath = 0;
 
 	/* Can deadlock when called with interrupts disabled */
 	WARN_ON(irqs_disabled());
···
 		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;
+		if (wait)
+			data->csd.flags |= CSD_FLAG_WAIT;
+	} else {
 		data = &d;
 		data->csd.flags = CSD_FLAG_WAIT;
 		wait = 1;
+		slowpath = 1;
 	}
 
 	spin_lock_init(&data->lock);
···
 	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;
 }