On 2019-05-13 20:52, Jens Axboe wrote:
On 5/13/19 12:20 PM, Roman Penyaev wrote:
This fixes couple of races which lead to infinite wait of park
completion
with the following backtraces:
[20801.303319] Call Trace:
[20801.303321] ? __schedule+0x284/0x650
[20801.303323] schedule+0x33/0xc0
[20801.303324] schedule_timeout+0x1bc/0x210
[20801.303326] ? schedule+0x3d/0xc0
[20801.303327] ? schedule_timeout+0x1bc/0x210
[20801.303329] ? preempt_count_add+0x79/0xb0
[20801.303330] wait_for_completion+0xa5/0x120
[20801.303331] ? wake_up_q+0x70/0x70
[20801.303333] kthread_park+0x48/0x80
[20801.303335] io_finish_async+0x2c/0x70
[20801.303336] io_ring_ctx_wait_and_kill+0x95/0x180
[20801.303338] io_uring_release+0x1c/0x20
[20801.303339] __fput+0xad/0x210
[20801.303341] task_work_run+0x8f/0xb0
[20801.303342] exit_to_usermode_loop+0xa0/0xb0
[20801.303343] do_syscall_64+0xe0/0x100
[20801.303349] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[20801.303380] Call Trace:
[20801.303383] ? __schedule+0x284/0x650
[20801.303384] schedule+0x33/0xc0
[20801.303386] io_sq_thread+0x38a/0x410
[20801.303388] ? __switch_to_asm+0x40/0x70
[20801.303390] ? wait_woken+0x80/0x80
[20801.303392] ? _raw_spin_lock_irqsave+0x17/0x40
[20801.303394] ? io_submit_sqes+0x120/0x120
[20801.303395] kthread+0x112/0x130
[20801.303396] ? kthread_create_on_node+0x60/0x60
[20801.303398] ret_from_fork+0x35/0x40
o kthread_park() waits for park completion, so io_sq_thread() loop
should check kthread_should_park() along with khread_should_stop(),
otherwise if kthread_park() is called before prepare_to_wait()
the following schedule() never returns.
o if the flag ctx->sqo_stop is observed by the io_sq_thread() loop
it is quite possible, that kthread_should_park() check and the
following kthread_parkme() is never called, because kthread_park()
has not been yet called, but few moments later is is called and
waits there for park completion, which never happens, because
kthread has already exited.
It seems that parking here is not needed at all (thread is parked and
then stopped and never unparked), so here in this patch I simply rely
on kthread_should_stop() check and then exit the thread.
The park is a bit of a work-around, without it we get warning spews on
shutdown with SQPOLL set and affinity set to a single CPU.
Also see:
commit 06058632464845abb1af91521122fd04dd3daaec
Author: Jens Axboe <axboe@xxxxxxxxx>
Date: Sat Apr 13 09:26:03 2019 -0600
io_uring: park SQPOLL thread if it's percpu
I see. Do you think it makes sense to fix this is kthread? With
something
as the following:
diff --git a/kernel/kthread.c b/kernel/kthread.c
index be4e8795561a..191f8be5c9e0 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -472,6 +472,10 @@ void kthread_unpark(struct task_struct *k)
{
struct kthread *kthread = to_kthread(k);
+ if (!__kthread_should_park(k))
+ /* Nop if thread was never parked */
+ break;
+
Nevertheless, the other way is to fix io_uring: we quit loop by
only kthread_should_park() check (kthread_park() is synchronous,
so kthread_should_stop() is never observed), and we abandon
->sqo_stop flag, because it is racy. At the end we always parkme(),
because we've exited the loop by park flag.
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 452e35357865..9401ac1bbd46 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -231,7 +231,6 @@ struct io_ring_ctx {
struct task_struct *sqo_thread; /* if using sq thread
polling */
struct mm_struct *sqo_mm;
wait_queue_head_t sqo_wait;
- unsigned sqo_stop;
struct {
/* CQ ring */
@@ -2028,7 +2027,7 @@ static int io_sq_thread(void *data)
set_fs(USER_DS);
timeout = inflight = 0;
- while (!kthread_should_stop() && !ctx->sqo_stop) {
+ while (!kthread_should_park()) {
bool all_fixed, mm_fault = false;
int i;
@@ -2090,7 +2089,7 @@ static int io_sq_thread(void *data)
smp_mb();
if (!io_get_sqring(ctx, &sqes[0])) {
- if (kthread_should_stop()) {
+ if (kthread_should_park()) {
finish_wait(&ctx->sqo_wait,
&wait);
break;
}
@@ -2140,8 +2139,7 @@ static int io_sq_thread(void *data)
mmput(cur_mm);
}
- if (kthread_should_park())
- kthread_parkme();
+ kthread_parkme();
return 0;
}
@@ -2273,8 +2271,11 @@ static int io_sqe_files_unregister(struct
io_ring_ctx *ctx)
static void io_sq_thread_stop(struct io_ring_ctx *ctx)
{
if (ctx->sqo_thread) {
- ctx->sqo_stop = 1;
- mb();
+ /*
+ * The park is a bit of a work-around, without it we get
+ * warning spews on shutdown with SQPOLL set and
affinity
+ * set to a single CPU.
+ */
kthread_park(ctx->sqo_thread);
kthread_stop(ctx->sqo_thread);
ctx->sqo_thread = NULL;
Makes sense?
--
Roman
