Re: [PATCH 0/2] Don't show PF_IO_WORKER in /proc//task/

Jens Axboe
2021-03-25T21:45:23Z

On 3/25/21 2:40 PM, Jens Axboe wrote:
> On 3/25/21 2:12 PM, Linus Torvalds wrote:
>> On Thu, Mar 25, 2021 at 12:42 PM Linus Torvalds
>> <torvalds@linux-foundation.org> wrote:
>>>
>>> On Thu, Mar 25, 2021 at 12:38 PM Linus Torvalds
>>> <torvalds@linux-foundation.org> wrote:
>>>>
>>>> I don't know what the gdb logic is, but maybe there's some other
>>>> option that makes gdb not react to them?
>>>
>>> .. maybe we could have a different name for them under the task/
>>> subdirectory, for example (not just the pid)? Although that probably
>>> messes up 'ps' too..
>>
>> Actually, maybe the right model is to simply make all the io threads
>> take signals, and get rid of all the special cases.
>>
>> Sure, the signals will never be delivered to user space, but if we
>>
>> - just made the thread loop do "get_signal()" when there are pending signals
>>
>> - allowed ptrace_attach on them
>>
>> they'd look pretty much like regular threads that just never do the
>> user-space part of signal handling.
>>
>> The whole "signals are very special for IO threads" thing has caused
>> so many problems, that maybe the solution is simply to _not_ make them
>> special?
>
> Just to wrap up the previous one, yes it broke all sorts of things to
> make the 'tid' directory different. They just end up being hidden anyway
> through that, for both ps and top.
>
> Yes, I do think that maybe it's better to just embrace maybe just
> embrace the signals, and have everything just work by default. It's
> better than continually trying to make the threads special. I'll see
> if there are some demons lurking down that path.

In the spirit of "let's just try it", I ran with the below patch. With
that, I can gdb attach just fine to a test case that creates an io_uring
and a regular thread with pthread_create(). The regular thread uses
the ring, so you end up with two iou-mgr threads. Attach:

[root@archlinux ~]# gdb -p 360
[snip gdb noise]
Attaching to process 360
[New LWP 361]
[New LWP 362]
[New LWP 363]

warning: Selected architecture i386:x86-64 is not compatible with reported target architecture i386

warning: Architecture rejected target-supplied description
Error while reading shared library symbols for /usr/lib/libpthread.so.0:
Cannot find user-level thread for LWP 363: generic error
0x00007f7aa526e125 in clock_nanosleep@GLIBC_2.2.5 () from /usr/lib/libc.so.6
(gdb) info threads
Id Target Id Frame
* 1 LWP 360 "io_uring" 0x00007f7aa526e125 in clock_nanosleep@GLIBC_2.2.5 ()
from /usr/lib/libc.so.6
2 LWP 361 "iou-mgr-360" 0x0000000000000000 in ?? ()
3 LWP 362 "io_uring" 0x00007f7aa52a0a9d in syscall () from /usr/lib/libc.so.6
4 LWP 363 "iou-mgr-362" 0x0000000000000000 in ?? ()
(gdb) thread 2
[Switching to thread 2 (LWP 361)]
#0 0x0000000000000000 in ?? ()
(gdb) bt
#0 0x0000000000000000 in ?? ()
Backtrace stopped: Cannot access memory at address 0x0
(gdb) cont
Continuing.
^C
Thread 1 "io_uring" received signal SIGINT, Interrupt.
[Switching to LWP 360]
0x00007f7aa526e125 in clock_nanosleep@GLIBC_2.2.5 () from /usr/lib/libc.so.6
(gdb) q
A debugging session is active.

Inferior 1 [process 360] will be detached.

Quit anyway? (y or n) y
Detaching from program: /root/git/fio/t/io_uring, process 360
[Inferior 1 (process 360) detached]

The iou-mgr-x threads are stopped just fine, gdb obviously can't get any
real info out of them. But it works... Regular test cases work fine too,
just a sanity check. Didn't expect them not to.

Only thing that I dislike a bit, but I guess that's just a Linuxism, is
that if can now kill an io_uring owning task by sending a signal to one
of its IO thread workers.

diff --git a/fs/io-wq.c b/fs/io-wq.c
index b7c1fa932cb3..2dbdc552f3ba 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -505,8 +505,14 @@ static int io_wqe_worker(void *data)
ret = schedule_timeout(WORKER_IDLE_TIMEOUT);
if (try_to_freeze() || ret)
continue;
- if (fatal_signal_pending(current))
- break;
+ if (signal_pending(current)) {
+ struct ksignal ksig;
+
+ if (fatal_signal_pending(current))
+ break;
+ get_signal(&ksig);
+ continue;
+ }
/* timed out, exit unless we're the fixed worker */
if (test_bit(IO_WQ_BIT_EXIT, &wq->state) ||
!(worker->flags & IO_WORKER_F_FIXED))
@@ -715,8 +721,15 @@ static int io_wq_manager(void *data)
io_wq_check_workers(wq);
schedule_timeout(HZ);
try_to_freeze();
- if (fatal_signal_pending(current))
- set_bit(IO_WQ_BIT_EXIT, &wq->state);
+ if (signal_pending(current)) {
+ struct ksignal ksig;
+
+ if (fatal_signal_pending(current))
+ set_bit(IO_WQ_BIT_EXIT, &wq->state);
+ else
+ get_signal(&ksig);
+ continue;
+ }
} while (!test_bit(IO_WQ_BIT_EXIT, &wq->state));

io_wq_check_workers(wq);
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 54ea561db4a5..3a9d021db328 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -6765,8 +6765,14 @@ static int io_sq_thread(void *data)
timeout = jiffies + sqd->sq_thread_idle;
continue;
}
- if (fatal_signal_pending(current))
- break;
+ if (signal_pending(current)) {
+ struct ksignal ksig;
+
+ if (fatal_signal_pending(current))
+ break;
+ get_signal(&ksig);
+ continue;
+ }
sqt_spin = false;
cap_entries = !list_is_singular(&sqd->ctx_list);
list_for_each_entry(ctx, &sqd->ctx_list, sqd_list) {
diff --git a/kernel/fork.c b/kernel/fork.c
index d3171e8e88e5..3b45d0f04044 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2436,6 +2436,7 @@ struct task_struct *create_io_thread(int (*fn)(void *), void *arg, int node)
if (!IS_ERR(tsk)) {
sigfillset(&tsk->blocked);
sigdelsetmask(&tsk->blocked, sigmask(SIGKILL));
+ sigdelsetmask(&tsk->blocked, sigmask(SIGSTOP));
}
return tsk;
}
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 821cf1723814..61db50f7ca86 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -375,7 +375,7 @@ static int ptrace_attach(struct task_struct *task, long request,
audit_ptrace(task);

retval = -EPERM;
- if (unlikely(task->flags & (PF_KTHREAD | PF_IO_WORKER)))
+ if (unlikely(task->flags & PF_KTHREAD))
goto out;
if (same_thread_group(task, current))
goto out;
diff --git a/kernel/signal.c b/kernel/signal.c
index f2a1b898da29..a5700557eb50 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -91,7 +91,7 @@ static bool sig_task_ignored(struct task_struct *t, int sig, bool force)
return true;

/* Only allow kernel generated signals to this kthread */
- if (unlikely((t->flags & (PF_KTHREAD | PF_IO_WORKER)) &&
+ if (unlikely((t->flags & PF_KTHREAD) &&
(handler == SIG_KTHREAD_KERNEL) && !force))
return true;

@@ -288,8 +288,7 @@ bool task_set_jobctl_pending(struct task_struct *task, unsigned long mask)
JOBCTL_STOP_SIGMASK | JOBCTL_TRAPPING));
BUG_ON((mask & JOBCTL_TRAPPING) && !(mask & JOBCTL_PENDING_MASK));

- if (unlikely(fatal_signal_pending(task) ||
- (task->flags & (PF_EXITING | PF_IO_WORKER))))
+ if (unlikely(fatal_signal_pending(task) || task->flags & PF_EXITING))
return false;

if (mask & JOBCTL_STOP_SIGMASK)
@@ -834,9 +833,6 @@ static int check_kill_permission(int sig, struct kernel_siginfo *info,

if (!valid_signal(sig))
return -EINVAL;
- /* PF_IO_WORKER threads don't take any signals */
- if (t->flags & PF_IO_WORKER)
- return -ESRCH;

if (!si_fromuser(info))
return 0;

--
Jens Axboe