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Date:   Mon, 13 May 2019 14:17:04 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>
Cc:     linux-kernel@vger.kernel.org, tj@kernel.org, guro@fb.com,
        kernel-team@fb.com
Subject: Re: [REGRESSION] ptrace broken from "cgroup: cgroup v2 freezer"
 (76f969e)
Message-ID: <20190513121703.GA24724@redhat.com>
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On 05/12, Alex Xu (Hello71) wrote:
>
> Hi,
>
> I was trying to use strace recently and found that it exhibited some
> strange behavior. I produced this minimal test case:
>
> #include <unistd.h>
>
> int main() {
>     write(1, "a", 1);
>     return 0;
> }
>
> which, when run using "gcc test.c && strace ./a.out" produces this
> strace output:
>
> [ pre-main omitted ]
> write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
> write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
> write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
> write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
> write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
> write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
> [ repeats forever ]

Yes, cgroup_enter_frozen() alone is wrong, we have already discussed this
a bit... see https://lore.kernel.org/lkml/20190508152536.GA17058@redhat.com/

Probably we add leave_frozen(true) after freezable_schedule() for now, then
think try to make something better...

But I am not sure I 100% understand whats going on in this case, could you
try the patch below? (Just in case, of course it is wrong).

Oleg.

--- x/kernel/signal.c
+++ x/kernel/signal.c
@@ -149,8 +149,7 @@
 {
 	if ((t->jobctl & (JOBCTL_PENDING_MASK | JOBCTL_TRAP_FREEZE)) ||
 	    PENDING(&t->pending, &t->blocked) ||
-	    PENDING(&t->signal->shared_pending, &t->blocked) ||
-	    cgroup_task_frozen(t)) {
+	    PENDING(&t->signal->shared_pending, &t->blocked) {
 		set_tsk_thread_flag(t, TIF_SIGPENDING);
 		return true;
 	}