hi,
On 06/06/2020 18:12, Xiaoguang Wang wrote:
While testing io_uring in our internal kernel, note it's not upstream
kernel, we see below panic:
[ 872.498723] x29: ffff00002d553cf0 x28: 0000000000000000
[ 872.508973] x27: ffff807ef691a0e0 x26: 0000000000000000
[ 872.519116] x25: 0000000000000000 x24: ffff0000090a7980
[ 872.529184] x23: ffff000009272060 x22: 0000000100022b11
[ 872.539144] x21: 0000000046aa5668 x20: ffff80bee8562b18
[ 872.549000] x19: ffff80bee8562080 x18: 0000000000000000
[ 872.558876] x17: 0000000000000000 x16: 0000000000000000
[ 872.568976] x15: 0000000000000000 x14: 0000000000000000
[ 872.578762] x13: 0000000000000000 x12: 0000000000000000
[ 872.588474] x11: 0000000000000000 x10: 0000000000000c40
[ 872.598324] x9 : ffff000008100c00 x8 : 000000007ffff000
[ 872.608014] x7 : ffff80bee8562080 x6 : ffff80beea862d30
[ 872.617709] x5 : 0000000000000000 x4 : ffff80beea862d48
[ 872.627399] x3 : ffff80bee8562b18 x2 : 0000000000000000
[ 872.637044] x1 : ffff0000090a7000 x0 : 0000000000208040
[ 872.646575] Call trace:
[ 872.653139] task_numa_work+0x4c/0x310
[ 872.660916] task_work_run+0xb0/0xe0
[ 872.668400] io_sq_thread+0x164/0x388
[ 872.675829] kthread+0x108/0x138
The reason is that once io_sq_thread has a valid mm, schedule subsystem
may call task_tick_numa() adding a task_numa_work() callback, which will
visit mm, then above panic will happen.
To fix this bug, only call task_work_run() before dropping mm.
So, the problem is that poll/async paths re-issue requests with
__io_queue_sqe(), which doesn't care about current->mm, and which
can be NULL for io_sq_thread(). Right?
No, above panic is not triggered by poll/async paths.
See below code path:
==> task_tick_fair()
====> task_tick_numa()
======> task_work_add, work is task_numa_work, which will visit mm.
In sqpoll mode, there maybe are sqes that need mm, then above codes
maybe executed by schedule subsystem. In io_sq_thread, we drop mm before
task_work_run, if there is a task_numa_work, panic occurs.
Regards,
Xiaoguang Wang
Signed-off-by: Xiaoguang Wang <xiaoguang.wang@xxxxxxxxxxxxxxxxx>
---
fs/io_uring.c | 15 ++++++++-------
1 file changed, 8 insertions(+), 7 deletions(-)
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 6391a00ff8b7..32381984b2a6 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -6134,6 +6134,13 @@ static int io_sq_thread(void *data)
* to enter the kernel to reap and flush events.
*/
if (!to_submit || ret == -EBUSY) {
+ /*
+ * Current task context may already have valid mm, that
+ * means some works that visit mm may have been queued,
+ * so we must execute the works before dropping mm.
+ */
+ if (current->task_works)
+ task_work_run();
Even though you're not dropping mm, the thread might not have it in the first
place. see how it's done in io_init_req(). How about setting mm either lazily
in io_poll_task_func()/io_async_task_func(), or before task_work_run() in
io_sq_thread().
/*
* Drop cur_mm before scheduling, we can't hold it for
* long periods (or over schedule()). Do this before
@@ -6152,8 +6159,6 @@ static int io_sq_thread(void *data)
if (!list_empty(&ctx->poll_list) ||
(!time_after(jiffies, timeout) && ret != -EBUSY &&
!percpu_ref_is_dying(&ctx->refs))) {
- if (current->task_works)
- task_work_run();
cond_resched();
continue;
}
@@ -6185,11 +6190,7 @@ static int io_sq_thread(void *data)
finish_wait(&ctx->sqo_wait, &wait);
break;
}
- if (current->task_works) {
- task_work_run();
- finish_wait(&ctx->sqo_wait, &wait);
- continue;
- }
+
if (signal_pending(current))
flush_signals(current);
schedule();
