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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 16 May 2023 13:38:51 -0700
Message-ID: <CAHk-=wg6TWSvb8Lk5uEu=QKQRFj7vewMsqLhuah4_7r2S=dXWA@mail.gmail.com>
Subject: Re: [GIT PULL] x86/shstk for 6.4
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        "keescook@chromium.org" <keescook@chromium.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>
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On Mon, May 15, 2023 at 3:40=E2=80=AFPM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> So I *think* fork() could do something like this:
>
>   struct fork_cookie; // dummy type purely for type checking
>   static struct fork_cookie *is_singe_threaded(void)
>   {
>         struct mm_struct *mm =3D current->mm;
>         mutex_lock(&mm->fork_lock);

Actually, let's not even bother with a new fork_lock.

I for some reason thought that fork only took the mmap_lock for
reading (because it kind of does just read the old VM data), but when
I actually look at dup_mmap() I notice that what it does is actually
just

        if (mmap_write_lock_killable(oldmm)) ..

and so I think we can just use that as the lock.

So then all we'd need is to use mmap_read_lock(mm) in kthread_use_mm()
around the mmgrab.

I don't think we even need it in kthread_unuse_mm(), because
*decrementing* the mm counters isn't even something we need to worry
about.

How does that sound?

                Linus