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Date: Thu, 15 Jan 2015 03:13:25 -0000
From: subashab@...eaurora.org
To: netdev@...r.kernel.org
Subject: [PATCH] net: core: Fix race by protecting process_queues at CPU
hotplug
I am seeing frequent crashes in high throughput conditions in a
multiprocessor system with kernel version 3.10 where cores are getting hot
plugged. I have pinned the network stack to a particular core using
receive packet steering (RPS). At the time of crash, it looks like a
contention of the process_queue between dev_cpu_callback and
process_backlog.
The following is the log at the moment of the crash
75241.598056: <6> Unable to handle kernel NULL pointer dereference at
virtual address 00000004
75241.598064: <6> pgd = c0004000
75241.598072: <2> [00000004] *pgd=00000000
75241.598082: <6> Internal error: Oops: 817 [#1] PREEMPT SMP ARM
75241.598096: <2> Modules linked in: wlan(O) [last unloaded: wlan]
75241.598106: <6> CPU: 0 PID: 3 Comm: ksoftirqd/0 Tainted: G W
O 3.10.49-g0b78c2e-00003-g2eab3e3 #1
75241.598113: <6> task: ee870a80 ti: ee888000 task.ti: ee888000
75241.598133: <2> PC is at process_backlog+0x78/0x140
75241.598142: <2> LR is at __netif_receive_skb_core+0x6fc/0x724
Here is the call stack involved in the crash
-006|__skb_unlink
-006|__skb_dequeue
-006|dev_cpu_callback
-007|notifier_call_chain
-008|__raw_notifier_call_chain
-009|notifier_to_errno
-009|__cpu_notify
-010|cpu_notify_nofail
-011|check_for_tasks
-011|cpu_down
-012|disable_nonboot_cpus()
-013|suspend_enter
-013|suspend_devices_and_enter
-014|enter_state
-014|pm_suspend
-015|state_store
-016|kobj_attr_store
-017|flush_write_buffer
-017|sysfs_write_file
-018|vfs_write
-019|SYSC_write
-019|sys_write
-020|ret_fast_syscall
I have a fix in mind in which the process queues are protected using spin
locks. I do not observe the crash with this patch. Since this patch is in
the hot path for incoming packet processing, I would like some reviews of
this patch. I would also like to know if there are any potential
performance bottlenecks due to this change.
net: core: Protect process_queues at CPU hotplug
When a CPU is hotplugged while processing incoming packets,
dev_cpu_callback() will copy the poll list from the offline
CPU and raise the softIRQ. In the same context, it will also
process process_queue of the offline CPU by de-queueing and
calling netif_rx. Due to this there is a potential for race
condition between process_backlog() and dev_cpu_callback()
accessing the same process_queue resource. This patch
protects this concurrent access by locking.
Signed-off-by: Prasad Sodagudi <psodagud@...eaurora.org>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>
diff --git a/net/core/dev.c b/net/core/dev.c
index df0b522..aa8f503 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3640,6 +3640,7 @@ static void flush_backlog(void *arg)
struct net_device *dev = arg;
struct softnet_data *sd = &__get_cpu_var(softnet_data);
struct sk_buff *skb, *tmp;
+ unsigned long flags;
rps_lock(sd);
skb_queue_walk_safe(&sd->input_pkt_queue, skb, tmp) {
@@ -3651,6 +3652,7 @@ static void flush_backlog(void *arg)
}
rps_unlock(sd);
+ spin_lock_irqsave(&sd->process_queue.lock, flags);
skb_queue_walk_safe(&sd->process_queue, skb, tmp) {
if (skb->dev == dev) {
__skb_unlink(skb, &sd->process_queue);
@@ -3658,6 +3660,7 @@ static void flush_backlog(void *arg)
input_queue_head_incr(sd);
}
}
+ spin_unlock_irqrestore(&sd->process_queue.lock, flags);
}
static int napi_gro_complete(struct sk_buff *skb)
@@ -4021,7 +4024,7 @@ static int process_backlog(struct napi_struct *napi,
int quota)
{
int work = 0;
struct softnet_data *sd = container_of(napi, struct softnet_data,
backlog);
-
+ unsigned long flags;
#ifdef CONFIG_RPS
/* Check if we have pending ipi, its better to send them now,
* not waiting net_rx_action() end.
@@ -4032,18 +4035,19 @@ static int process_backlog(struct napi_struct
*napi, int quota)
}
#endif
napi->weight = weight_p;
- local_irq_disable();
+ spin_lock_irqsave(&sd->process_queue.lock, flags);
while (work < quota) {
struct sk_buff *skb;
unsigned int qlen;
while ((skb = __skb_dequeue(&sd->process_queue))) {
- local_irq_enable();
+ spin_unlock_irqrestore(&sd->process_queue.lock,
flags);
__netif_receive_skb(skb);
- local_irq_disable();
+ spin_lock_irqsave(&sd->process_queue.lock, flags);
input_queue_head_incr(sd);
if (++work >= quota) {
- local_irq_enable();
+
spin_unlock_irqrestore(&sd->process_queue.lock,
+ flags);
return work;
}
}
@@ -4054,6 +4058,7 @@ static int process_backlog(struct napi_struct *napi,
int quota)
skb_queue_splice_tail_init(&sd->input_pkt_queue,
&sd->process_queue);
+
if (qlen < quota - work) {
/*
* Inline a custom version of __napi_complete().
@@ -4069,7 +4074,7 @@ static int process_backlog(struct napi_struct *napi,
int quota)
}
rps_unlock(sd);
}
- local_irq_enable();
+ spin_unlock_irqrestore(&sd->process_queue.lock, flags);
return work;
}
@@ -5991,6 +5996,7 @@ static int dev_cpu_callback(struct notifier_block *nfb,
struct sk_buff *skb;
unsigned int cpu, oldcpu = (unsigned long)ocpu;
struct softnet_data *sd, *oldsd;
+ unsigned long flags;
if (action != CPU_DEAD && action != CPU_DEAD_FROZEN)
return NOTIFY_OK;
@@ -6024,11 +6030,15 @@ static int dev_cpu_callback(struct notifier_block
*nfb,
raise_softirq_irqoff(NET_TX_SOFTIRQ);
local_irq_enable();
+ spin_lock_irqsave(&oldsd->process_queue.lock, flags);
/* Process offline CPU's input_pkt_queue */
while ((skb = __skb_dequeue(&oldsd->process_queue))) {
+ spin_unlock_irqrestore(&oldsd->process_queue.lock, flags);
netif_rx(skb);
+ spin_lock_irqsave(&oldsd->process_queue.lock, flags);
input_queue_head_incr(oldsd);
}
+ spin_unlock_irqrestore(&oldsd->process_queue.lock, flags);
while ((skb = __skb_dequeue(&oldsd->input_pkt_queue))) {
netif_rx(skb);
input_queue_head_incr(oldsd);
Thanks
KS
The Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project
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