linux-kernel - Re: [PATCH 2/7] io_uring: handle signals for IO threads like a normal thread

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Message-ID: <m1k0ptv9kj.fsf@fess.ebiederm.org>
Date:   Fri, 26 Mar 2021 17:23:08 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Jens Axboe <axboe@...nel.dk>
Cc:     io-uring@...r.kernel.org, torvalds@...ux-foundation.org,
        metze@...ba.org, oleg@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/7] io_uring: handle signals for IO threads like a normal thread

Jens Axboe <axboe@...nel.dk> writes:

> On 3/26/21 2:29 PM, Eric W. Biederman wrote:
>> Jens Axboe <axboe@...nel.dk> writes:
>> 
>>> We go through various hoops to disallow signals for the IO threads, but
>>> there's really no reason why we cannot just allow them. The IO threads
>>> never return to userspace like a normal thread, and hence don't go through
>>> normal signal processing. Instead, just check for a pending signal as part
>>> of the work loop, and call get_signal() to handle it for us if anything
>>> is pending.
>>>
>>> With that, we can support receiving signals, including special ones like
>>> SIGSTOP.
>>>
>>> Signed-off-by: Jens Axboe <axboe@...nel.dk>
>>> ---
>>>  fs/io-wq.c    | 24 +++++++++++++++++-------
>>>  fs/io_uring.c | 12 ++++++++----
>>>  2 files changed, 25 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/fs/io-wq.c b/fs/io-wq.c
>>> index b7c1fa932cb3..3e2f059a1737 100644
>>> --- a/fs/io-wq.c
>>> +++ b/fs/io-wq.c
>>> @@ -16,7 +16,6 @@
>>>  #include <linux/rculist_nulls.h>
>>>  #include <linux/cpu.h>
>>>  #include <linux/tracehook.h>
>>> -#include <linux/freezer.h>
>>>  
>>>  #include "../kernel/sched/sched.h"
>>>  #include "io-wq.h"
>>> @@ -503,10 +502,16 @@ static int io_wqe_worker(void *data)
>>>  		if (io_flush_signals())
>>>  			continue;
>>>  		ret = schedule_timeout(WORKER_IDLE_TIMEOUT);
>>> -		if (try_to_freeze() || ret)
>>> +		if (signal_pending(current)) {
>>> +			struct ksignal ksig;
>>> +
>>> +			if (fatal_signal_pending(current))
>>> +				break;
>>> +			if (get_signal(&ksig))
>>> +				continue;
>>                         ^^^^^^^^^^^^^^^^^^^^^^
>> 
>> That is wrong.  You are promising to deliver a signal to signal
>> handler and them simply discarding it.  Perhaps:
>> 
>> 			if (!get_signal(&ksig))
>>                         	continue;
>> 			WARN_ON(!sig_kernel_stop(ksig->sig));
>>                         break;
>
> Thanks, updated.

Gah.  Kill the WARN_ON.

I was thinking "WARN_ON(!sig_kernel_fatal(ksig->sig));"
The function sig_kernel_fatal does not exist.

Fatal is the state that is left when a signal is neither
ignored nor a stop signal, and does not have a handler.

The rest of the logic still works.

Eric