Add memory barrier when waiting on futex

[Ma, Xindong]

We encountered following panic several times:
[   74.671982] BUG: unable to handle kernel NULL pointer dereference at 00000008
[   74.672101] IP: [<c129bb27>] wake_futex+0x47/0x80
[   74.672185] *pdpt = 0000000010108001 *pde = 0000000000000000 
[   74.672278] Oops: 0002 [#1] PREEMPT SMP 
[   74.672403] Modules linked in: atomisp_css2400b0_v2 atomisp_css2400_v2 dfrgx bcm_bt_lpm videobuf_vmalloc videobuf_core hdmi_audio tngdisp bcm4335 kct_daemon(O) cfg80211
[   74.672815] CPU: 0 PID: 1477 Comm: zygote Tainted: G        W  O 3.10.1-259934-g0bfb86e #1
[   74.672855] Hardware name: Intel Corporation Merrifield/SALT BAY, BIOS 404 2013.10.09:15.29.48
[   74.672894] task: d4c97220 ti: cfaa8000 task.ti: cfaa8000
[   74.672933] EIP: 0060:[<c129bb27>] EFLAGS: 00210246 CPU: 0
[   74.672975] EIP is at wake_futex+0x47/0x80
[   74.673012] EAX: 00000000 EBX: 00000000 ECX: 00000000 EDX: 00000000
[   74.673049] ESI: def4de5c EDI: ffffffff EBP: cfaa9eb4 ESP: cfaa9ea0
[   74.673086]  DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
[   74.673123] CR0: 8005003b CR2: 00000008 CR3: 10109000 CR4: 001007f0
[   74.673160] DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
[   74.673196] DR6: ffff0ff0 DR7: 00000400
[   74.673229] Stack:
[   74.673260]  00000000 00000001 00000000 def4de5c c225eb50 cfaa9ee4 c129bc29 00000000
[   74.673536]  00000000 7fffffff c225eb30 b4f38000 ec1a4b40 00000f90 7fffffff 00000001
[   74.673814]  b4f38f90 cfaa9f58 c129da0b ffffffff ffffffff cfaa9f10 c195d835 00000001
[   74.674092] Call Trace:
[   74.674144]  [<c129bc29>] futex_wake+0xc9/0x110
[   74.674195]  [<c129da0b>] do_futex+0xeb/0x950
[   74.674246]  [<c195d835>] ? sub_preempt_count+0x55/0xe0
[   74.674293]  [<c1275aee>] ? wake_up_new_task+0xee/0x190
[   74.674341]  [<c195a31b>] ? _raw_spin_unlock_irqrestore+0x3b/0x70
[   74.674388]  [<c1275aee>] ? wake_up_new_task+0xee/0x190
[   74.674436]  [<c1241afc>] ? do_fork+0xec/0x350
[   74.674484]  [<c129e30b>] SyS_futex+0x9b/0x140
[   74.674533]  [<c1312298>] ? SyS_mprotect+0x188/0x1e0
[   74.674582]  [<c195a718>] syscall_call+0x7/0xb

On smp systems, setting current task to q->task in queue_me() may
not visible immediately to another cpu, some times this will
cause panic in wake_futex(). Adding memory barrier to avoid this.

Signed-off-by: Leon Ma <xindong.ma@xxxxxxxxx>
Signed-off-by: xiaobing tu <xiaobing.tu@xxxxxxxxx>
---
 kernel/futex.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 80ba086..792cd41 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1529,6 +1529,7 @@ static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
 	plist_node_init(&q->list, prio);
 	plist_add(&q->list, &hb->chain);
 	q->task = current;
+	smp_mb();
 	spin_unlock(&hb->lock);
 }
 
-- 
1.7.4.1


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