On 7/8/24 11:42, Oleg Nesterov wrote:
On 07/07, Pavel Begunkov wrote:
io_uring can asynchronously add a task_work while the task is getting
freezed. TIF_NOTIFY_SIGNAL will prevent the task from sleeping in
do_freezer_trap(), and since the get_signal()'s relock loop doesn't
retry task_work, the task will spin there not being able to sleep
until the freezing is cancelled / the task is killed / etc.
Cc: stable@xxxxxxxxxxxxxxx
Link: https://github.com/systemd/systemd/issues/33626
Fixes: 3146cba99aa28 ("io-wq: make worker creation resilient against signals")
I don't think we should blame io_uring even if so far it is the only user
of TWA_SIGNAL.
And it's not entirely correct even for backporting purposes,
I'll pin it to when freezing was introduced then.
Perhaps we should change do_freezer_trap() somehow, not sure... It assumes
that TIF_SIGPENDING is the only reason to not sleep in TASK_INTERRUPTIBLE,
today this is not true.
Let's CC Peter Zijlstra and Tejun in case they might have
some input on that.
Link to this patch for convenience:
https://lore.kernel.org/all/1d935e9d87fd8672ef3e8a9a0db340d355ea08b4.1720368770.git.asml.silence@xxxxxxxxx/
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2694,6 +2694,10 @@ bool get_signal(struct ksignal *ksig)
try_to_freeze();
relock:
+ clear_notify_signal();
+ if (unlikely(task_work_pending(current)))
+ task_work_run();
+
spin_lock_irq(&sighand->siglock);
Well, but can't we kill the same code at the start of get_signal() then?
Of course, in this case get_signal() should check signal_pending(), not
task_sigpending().
Should be fine, but I didn't want to change the
try_to_freeze() -> __refrigerator() path, which also reschedules.
Or perhaps something like the patch below makes more sense? I dunno...
It needs a far backporting, I'd really prefer to keep it
lean and without more side effects if possible, unless
there is a strong opinion on that.
Oleg.
diff --git a/kernel/signal.c b/kernel/signal.c
index 1f9dd41c04be..e2ae85293fbb 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2676,6 +2676,7 @@ bool get_signal(struct ksignal *ksig)
struct signal_struct *signal = current->signal;
int signr;
+start:
clear_notify_signal();
if (unlikely(task_work_pending(current)))
task_work_run();
@@ -2760,10 +2761,11 @@ bool get_signal(struct ksignal *ksig)
if (current->jobctl & JOBCTL_TRAP_MASK) {
do_jobctl_trap();
spin_unlock_irq(&sighand->siglock);
+ goto relock;
} else if (current->jobctl & JOBCTL_TRAP_FREEZE)
do_freezer_trap();
-
- goto relock;
+ goto start;
+ }
}
/*
--
Pavel Begunkov
