linux-kernel - Re: [PATCH 2/2] MIPS/c-r4k: do no use mmap_sem for gup

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Date:   Thu, 7 Feb 2019 11:25:55 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Paul Burton <paul.burton@...s.com>
Cc:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 2/2] MIPS/c-r4k: do no use mmap_sem for gup_fast()

On Thu, 07 Feb 2019, Paul Burton wrote:

>Hi Davidlohr,
>
>On Wed, Feb 06, 2019 at 09:37:40PM -0800, Davidlohr Bueso wrote:
>> It is well known that because the mm can internally
>> call the regular gup_unlocked if the lockless approach
>> fails and take the sem there, the caller must not hold
>> the mmap_sem already.
>>
>> Fixes: e523f289fe4d (MIPS: c-r4k: Fix sigtramp SMP call to use kmap)
>> Cc: Ralf Baechle <ralf@...ux-mips.org>
>> Cc: Paul Burton <paul.burton@...s.com>
>> Cc: James Hogan <jhogan@...nel.org>
>> Cc: linux-mips@...r.kernel.org
>> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
>
>Thanks - this looks good, but:
>
> 1) The problem it fixes was introduced in v4.8.
>
> 2) Commit adcc81f148d7 ("MIPS: math-emu: Write-protect delay slot
>    emulation pages") actually left flush_cache_sigtramp unused, and has
>    been backported to stable kernels also as far as v4.8.
>
>Therefore this will just fix code that never gets called, and I'll go
>delete the whole thing instead.

Even better.

Thanks,
Davidlohr