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From: Michael S. Tsirkin on 16 Mar 2010 13:20
In 2.6.34-rc1, removing vhost_net module causes an oops in sync_mm_rss
(called from do_exit) when workqueue is destroyed. This does not happen on
net-next, or with vhost on top of to 2.6.33.

The issue seems to be introduced by
34e55232e59f7b19050267a05ff1226e5cd122a5: that commit added function
sync_mm_rss that is passed task->mm, and dereferences it without
checking. If task is a kernel thread, mm might be NULL.
I think this might also happen e.g. with aio.

This patch fixes the oops by calling sync_mm_rss when task->mm
is set to NULL. I also added BUG_ON to detect any other cases
where counters get incremented while mm is NULL.

The oops I observed looks like this:

BUG: unable to handle kernel NULL pointer dereference at 00000000000002a8
IP: [<ffffffff810b436d>] sync_mm_rss+0x33/0x6f
PGD 0
Oops: 0002 [#1] SMP
last sysfs file: /sys/devices/system/cpu/cpu7/cache/index2/shared_cpu_map
CPU 2
Modules linked in: vhost_net(-) tun bridge stp sunrpc ipv6 cpufreq_ondemand acpi_cpufreq freq_table kvm_intel kvm i5000_edac edac_core rtc_cmos bnx2 button i2c_i801 i2c_core rtc_core e1000e sg joydev ide_cd_mod serio_raw pcspkr rtc_lib cdrom virtio_net virtio_blk virtio_pci virtio_ring virtio af_packet e1000 shpchp aacraid uhci_hcd ohci_hcd ehci_hcd [last unloaded: microcode]

Pid: 2046, comm: vhost Not tainted 2.6.34-rc1-vhost #25 System Planar/IBM System x3550 -[7978B3G]-
RIP: 0010:[<ffffffff810b436d>] [<ffffffff810b436d>] sync_mm_rss+0x33/0x6f
RSP: 0018:ffff8802379b7e60 EFLAGS: 00010202
RAX: 0000000000000008 RBX: ffff88023f2390c0 RCX: 0000000000000000
RDX: ffff88023f2396b0 RSI: 0000000000000000 RDI: ffff88023f2390c0
RBP: ffff8802379b7e60 R08: 0000000000000000 R09: 0000000000000000
R10: ffff88023aecfbc0 R11: 0000000000013240 R12: 0000000000000000
R13: ffffffff81051a6c R14: ffffe8ffffc0f540 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff880001e80000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 00000000000002a8 CR3: 000000023af23000 CR4: 00000000000406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process vhost (pid: 2046, threadinfo ffff8802379b6000, task ffff88023f2390c0)
Stack:
ffff8802379b7ee0 ffffffff81040687 ffffe8ffffc0f558 ffffffffa00a3e2d
<0> 0000000000000000 ffff88023f2390c0 ffffffff81055817 ffff8802379b7e98
<0> ffff8802379b7e98 0000000100000286 ffff8802379b7ee0 ffff88023ad47d78
Call Trace:
[<ffffffff81040687>] do_exit+0x147/0x6c4
[<ffffffffa00a3e2d>] ? handle_rx_net+0x0/0x17 [vhost_net]
[<ffffffff81055817>] ? autoremove_wake_function+0x0/0x39
[<ffffffff81051a6c>] ? worker_thread+0x0/0x229
[<ffffffff810553c9>] kthreadd+0x0/0xf2
[<ffffffff810038d4>] kernel_thread_helper+0x4/0x10
[<ffffffff81055342>] ? kthread+0x0/0x87
[<ffffffff810038d0>] ? kernel_thread_helper+0x0/0x10
Code: 00 8b 87 6c 02 00 00 85 c0 74 14 48 98 f0 48 01 86 a0 02 00 00 c7 87 6c 02 00 00 00 00 00 00 8b 87 70 02 00 00 85 c0 74 14 48 98 <f0> 48 01 86 a8 02 00 00 c7 87 70 02 00 00 00 00 00 00 8b 87 74
RIP [<ffffffff810b436d>] sync_mm_rss+0x33/0x6f
RSP <ffff8802379b7e60>
CR2: 00000000000002a8
---[ end trace 41603ba922beddd2 ]---
Fixing recursive fault but reboot is needed!

(note: handle_rx_net is a work item using workqueue in question).
sync_mm_rss+0x33/0x6f gave me a hint. I also tried reverting
34e55232e59f7b19050267a05ff1226e5cd122a5 and the oops goes away.

The module in question calls use_mm and later unuse_mm from a kernel
thread. It is when this kernel thread is destroyed that the crash
happens.

Signed-off-by: Michael S. Tsirkin <mst(a)redhat.com>
---
mm/memory.c | 1 +
mm/mmu_context.c | 1 +
2 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index d1153e3..27022b3 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -130,6 +130,7 @@ void __sync_task_rss_stat(struct task_struct *task, struct mm_struct *mm)

for (i = 0; i < NR_MM_COUNTERS; i++) {
if (task->rss_stat.count[i]) {
+ BUG_ON(!mm);
add_mm_counter(mm, i, task->rss_stat.count[i]);
task->rss_stat.count[i] = 0;
}
diff --git a/mm/mmu_context.c b/mm/mmu_context.c
index 0777654..9e82e93 100644
--- a/mm/mmu_context.c
+++ b/mm/mmu_context.c
@@ -53,6 +53,7 @@ void unuse_mm(struct mm_struct *mm)
struct task_struct *tsk = current;

task_lock(tsk);
+ sync_mm_rss(tsk, mm);
tsk->mm = NULL;
/* active_mm is still 'mm' */
enter_lazy_tlb(mm, tsk);
--
1.7.0.18.g0d53a5
--
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Prev: SCSI: lpfc, fix lock imbalances
Next: [PATCH] gigaset: correct range checking off by one error