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

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Message-ID: <20190207190007.jz4rz6e6qxwazxm7@pburton-laptop>
Date:   Thu, 7 Feb 2019 19:00:09 +0000
From:   Paul Burton <paul.burton@...s.com>
To:     Davidlohr Bueso <dave@...olabs.net>
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()

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.

Thanks,
    Paul