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Date:   Mon, 3 Jun 2019 12:23:54 +0100
From:   Will Deacon <will.deacon@arm.com>
To:     Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Young Xiao <92siuyang@gmail.com>, linux@armlinux.org.uk,
        mark.rutland@arm.com, mingo@redhat.com, bp@alien8.de,
        hpa@zytor.com, x86@kernel.org, kan.liang@linux.intel.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        ravi.bangoria@linux.vnet.ibm.com, mpe@ellerman.id.au,
        acme@redhat.com, eranian@google.com, fweisbec@gmail.com,
        jolsa@redhat.com
Subject: Re: [PATCH] perf: Fix oops when kthread execs user process
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On Fri, May 31, 2019 at 04:37:15PM +0100, Will Deacon wrote:
> Oh, nice! I think this happens because Power doesn't actually initialise
> the regs after a kthread execs() until late in start_thread(). But the plot
> thickens somewhat, since current_pt_regs() is different to
> task_pt_regs(current) on Power (the former cannot return NULL).
> 
> So a really hideous hack on top of Peter's patch might be:
> 
> diff --git a/arch/arm64/kernel/perf_regs.c b/arch/arm64/kernel/perf_regs.c
> index 0bbac612146e..5bde866024b6 100644
> --- a/arch/arm64/kernel/perf_regs.c
> +++ b/arch/arm64/kernel/perf_regs.c
> @@ -57,6 +57,6 @@ void perf_get_regs_user(struct perf_regs *regs_user,
>  			struct pt_regs *regs,
>  			struct pt_regs *regs_user_copy)
>  {
> -	regs_user->regs = task_pt_regs(current);
> +	regs_user->regs = current_pt_regs();
>  	regs_user->abi = perf_reg_abi(current);

^^^ Bah, this was clearly supposed to be a change in the powerpc code, but
you get the idea.

Will