netdev - Re: [PATCH v2] 9p/trans_fd: perform read/write with TIF

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Date:   Fri, 2 Sep 2022 07:25:30 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Christian Schoenebeck <linux_oss@...debyte.com>,
        Eric Van Hensbergen <ericvh@...il.com>,
        Latchesar Ionkov <lucho@...kov.net>,
        Dominique Martinet <asmadeus@...ewreck.org>
Cc:     syzbot <syzbot+8b41a1365f1106fd0f33@...kaller.appspotmail.com>,
        v9fs-developer@...ts.sourceforge.net,
        syzkaller-bugs@...glegroups.com, netdev@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v2] 9p/trans_fd: perform read/write with TIF_SIGPENDING
 set

On 2022/09/02 0:23, Christian Schoenebeck wrote:
> So the intention in this alternative approach is to allow user space apps  
> still being able to perform blocking I/O, while at the same time making the 
> kernel thread interruptible to fix this hung task issue, correct?

Making the kernel thread "non-blocking" (rather than "interruptible") in order
not to be blocked on I/O on pipes.

Since kernel threads by default do not receive signals, being "interruptible"
or "killable" does not help (except for silencing khungtaskd warning). Being
"non-blocking" like I/O on sockets helps.

>> --- a/net/9p/trans_fd.c
>> +++ b/net/9p/trans_fd.c
>> @@ -256,11 +256,13 @@ static int p9_fd_read(struct p9_client *client, void
>> *v, int len) if (!ts)
>>  		return -EREMOTEIO;
>>
>> -	if (!(ts->rd->f_flags & O_NONBLOCK))
>> -		p9_debug(P9_DEBUG_ERROR, "blocking read ...\n");
>> -
>>  	pos = ts->rd->f_pos;
>> +	/* Force non-blocking read() even without O_NONBLOCK. */
>> +	set_thread_flag(TIF_SIGPENDING);
>>  	ret = kernel_read(ts->rd, v, len, &pos);
>> +	spin_lock_irq(&current->sighand->siglock);
>> +	recalc_sigpending();
>> +	spin_unlock_irq(&current->sighand->siglock);
> 
> Is the recalc_sigpending() block here actually needed? The TIF_SIGPENDING flag 
> is already cleared by net/9p/client.c, no?

This is actually needed.

The thread which processes this function is a kernel workqueue thread which
is supposed to process other functions (which might call "interruptible"
functions even if signals are not received by default).

The thread which currently clearing the TIF_SIGPENDING flag is a user process
(which are calling "killable" functions from syscall context but effectively
"uninterruptible" due to clearing the TIF_SIGPENDING flag and retrying).
By the way, clearing the TIF_SIGPENDING flag before retrying "killable" functions
(like p9_client_rpc() does) is very bad and needs to be avoided...