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From: Oleg Nesterov <>
To: Jens Axboe <>
Cc: io-uring <>,
	"" <>,
	Peter Zijlstra <>,
	Thomas Gleixner <>
Subject: Re: [PATCH RFC v2] kernel: decouple TASK_WORK TWA_SIGNAL handling from signals
Date: Thu, 1 Oct 2020 18:27:20 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>


I'll read this version tomorrow, but:

On 10/01, Jens Axboe wrote:
>  static inline int signal_pending(struct task_struct *p)
>  {
> -	return unlikely(test_tsk_thread_flag(p,TIF_SIGPENDING));
> +#ifdef TIF_TASKWORK
> +	/*
> +	 * TIF_TASKWORK isn't really a signal, but it requires the same
> +	 * behavior of restarting the system call to force a kernel/user
> +	 * transition.
> +	 */
> +	return unlikely(test_tsk_thread_flag(p, TIF_SIGPENDING) ||
> +			test_tsk_thread_flag(p, TIF_TASKWORK));
> +#else
> +	return unlikely(test_tsk_thread_flag(p, TIF_SIGPENDING));
> +#endif

This change alone is already very wrong.

signal_pending(task) == T means that this task will do get_signal() as
soon as it can, and this basically means you can't "divorce" SIGPENDING

Simple example. Suppose we have a single-threaded task T.

Someone does task_work_add(T, TWA_SIGNAL). This makes signal_pending()==T
and this is what we need.

Now suppose that another task sends a signal to T before T calls
task_work_run() and clears TIF_TASKWORK. In this case SIGPENDING won't
be set because signal_pending() is already set (see wants_signal), and
this means that T won't notice this signal.


  reply	other threads:[~2020-10-01 16:27 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-01 15:03 Jens Axboe
2020-10-01 16:27 ` Oleg Nesterov [this message]
2020-10-01 17:27   ` Jens Axboe
     [not found]   ` <>
2020-10-02 13:44     ` Jens Axboe

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