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Subject: Re: [PATCH 4/6] kernel: add support for TIF_NOTIFY_SIGNAL
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     linux-kernel@vger.kernel.org, io-uring@vger.kernel.org,
        peterz@infradead.org, tglx@linutronix.de
References: <20201005150438.6628-1-axboe@kernel.dk>
 <20201005150438.6628-5-axboe@kernel.dk> <20201008135325.GG9995@redhat.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <b691ff60-8847-e48f-956b-41f8f5c1275b@kernel.dk>
Date:   Thu, 8 Oct 2020 08:07:55 -0600
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On 10/8/20 7:53 AM, Oleg Nesterov wrote:
> On 10/05, Jens Axboe wrote:
>>
>>  static inline int signal_pending(struct task_struct *p)
>>  {
>> +#ifdef TIF_NOTIFY_SIGNAL
>> +	/*
>> +	 * TIF_NOTIFY_SIGNAL isn't really a signal, but it requires the same
>> +	 * behavior in terms of ensuring that we break out of wait loops
>> +	 * so that notify signal callbacks can be processed.
>> +	 */
>> +	if (unlikely(test_tsk_thread_flag(p, TIF_NOTIFY_SIGNAL)))
>> +		return 1;
>> +#endif
>>  	return task_sigpending(p);
>>  }
> 
> perhaps we can add test_tsk_thread_mask() later...

Yeah would be nice, and I bet there are a lot of cases in the kernel
that test multiple bits like that.

>>  static inline void restore_saved_sigmask_unless(bool interrupted)
>>  {
>> -	if (interrupted)
>> +	if (interrupted) {
>> +#ifdef TIF_NOTIFY_SIGNAL
>> +		WARN_ON(!test_thread_flag(TIF_SIGPENDING) &&
>> +			!test_thread_flag(TIF_NOTIFY_SIGNAL));
>> +#else
>>  		WARN_ON(!test_thread_flag(TIF_SIGPENDING));
>> -	else
>> +#endif
>> +	} else {
>>  		restore_saved_sigmask();
>> +	}
> 
> I'd suggest to simply do
> 
> 	-	WARN_ON(!test_thread_flag(TIF_SIGPENDING));
> 	+	WARN_ON(!signal_pending(current);

Ah yes, that's much better. I'll make the edit.

>> --- a/kernel/entry/kvm.c
>> +++ b/kernel/entry/kvm.c
>> @@ -8,6 +8,9 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work)
>>  	do {
>>  		int ret;
>>  
>> +		if (ti_work & _TIF_NOTIFY_SIGNAL)
>> +			tracehook_notify_signal();
> 
> Can't really comment this change, but to me it would be more safe to
> simply return -EINTR.
> 
> Or perhaps even better, treat _TIF_NOTIFY_SIGNAL and _TIF_SIGPENDING
> equally:
> 
> 	-	if (ti_work & _TIF_SIGPENDING) {
> 	+	if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL)) {
> 			kvm_handle_signal_exit(vcpu);
> 			return -EINTR;

Not sure I follow your logic here. Why treat it any different than
NOTIFY_RESUME from this perspective?

-- 
Jens Axboe