linux-kernel - Re: [PATCH v2 20/20] signal: Don't restart fork when signals come in.

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Message-ID: <87pnzc2upf.fsf@xmission.com>
Date:   Tue, 24 Jul 2018 16:24:28 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Wen Yang <wen.yang99@....com.cn>, majiang <ma.jiang@....com.cn>
Subject: Re: [PATCH v2 20/20] signal: Don't restart fork when signals come in.

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Tue, Jul 24, 2018 at 1:40 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>
>> +       if (signal_pending(current)) {
>> +               retval = restart_syscall();
>> +               goto fork_out;
>> +       }
>
> Oh, the previous version had this too, but it wasn't as obvious
> because it was just in a single line:
>
>         return ERR_PTR(restart_syscall());
>
> but it's just crazy.
>
> It should just be
>
>         retval = -ERESTARTNOINTR;
>         if (signal_pending(current))
>                 goto fork_out;
>
> because it's just silly and pointless to change the code to use
> restart_syscall() here.
>
> All restart_syscall() does is
>
>         set_tsk_thread_flag(current, TIF_SIGPENDING);
>         return -ERESTARTNOINTR;
>
> and you just *checked* that TIF_SIGPENDING was already set. So the
> above is completely pointless.
>
> It is not clear why you made that change. The old code had the simpler
> "just return -ERESTARTNOINTR" model.
>
> Did the restart_syscall() thing come in by mistake from some previous
> trials and it just hung around?

I think this is the one place in the kernel where we can restart a
system call and not set TIF_SIGPENDING.

Several years ago I made the mistake in the networking code of returning
-ERESTARTNOINTR and forgetting to set TIF_SIGPENDING.  That wasn't fun.
So I wrote restart_syscall and use it so I don't make that mistake
again.

In this case your suggesting will definitely generate better code so I
am happy to make a V3 with that doesn't use restart_syscall.  A person
working in the guts of fork can reasonably be expected understand and to
have all of the subtleties in cache as they work on that part of fork.

Eric