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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 1 Jan 2020 21:51:02 +0100
Message-ID: <CAK8P3a2a1aY9G+Nh9fy+NU=YA_m1dxm-4SCHgydVO5kcydh77g@mail.gmail.com>
Subject: Re: [PATCH] MIPS: Don't declare __current_thread_info globally
To:     Paul Burton <paulburton@kernel.org>
Cc:     "open list:BROADCOM NVRAM DRIVER" <linux-mips@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Christian Brauner <christian.brauner@canonical.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        "# 3.4.x" <stable@vger.kernel.org>
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On Wed, Jan 1, 2020 at 6:57 PM Paul Burton <paulburton@kernel.org> wrote:
> diff --git a/arch/mips/include/asm/thread_info.h b/arch/mips/include/asm/thread_info.h
> index 4993db40482c..aceefc3f9a1a 100644
> --- a/arch/mips/include/asm/thread_info.h
> +++ b/arch/mips/include/asm/thread_info.h
> @@ -50,10 +50,10 @@ struct thread_info {
>  }
>
>  /* How to get the thread information struct from C.  */
> -register struct thread_info *__current_thread_info __asm__("$28");
> -
>  static inline struct thread_info *current_thread_info(void)
>  {
> +       register struct thread_info *__current_thread_info __asm__("$28");
> +
>         return __current_thread_info;
>  }

This looks like a nice fix, but are you sure it doesn't allow the compiler to
reuse $28 for another purpose in the kernel under register pressure,
which would break current_thread_info()?

I see in the MIPS ABI document that $28 is preserved across function
calls, but I don't see any indication that a function is not allowed
to modify it and later restore the original content.

        Arnd