Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756777AbbHZMCe (ORCPT ); Wed, 26 Aug 2015 08:02:34 -0400 Received: from mail-ob0-f171.google.com ([209.85.214.171]:33635 "EHLO mail-ob0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755582AbbHZMCc convert rfc822-to-8bit (ORCPT ); Wed, 26 Aug 2015 08:02:32 -0400 MIME-Version: 1.0 In-Reply-To: <20150826111205.GD30466@arm.com> References: <1440556841-17219-1-git-send-email-chunyan.zhang@spreadtrum.com> <20150826111205.GD30466@arm.com> Date: Wed, 26 Aug 2015 20:02:32 +0800 Message-ID: Subject: Re: [PATCH] arm64: fix bug for reloading FPSIMD state after execve on cpu 0. From: Jianhua Liu To: Will Deacon Cc: Chunyan Zhang , Catalin Marinas , "linux-arm-kernel@lists.infradead.org" , "orson.zhai@spreadtrum.com" , "xiongshan.an@spreadtrum.com" , "linux-kernel@vger.kernel.org" , ard.biesheuvel@linaro.org Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2789 Lines: 76 NO, kernel thread will not use fpsimd. kernel threads get scheduled in, per_cpu(fpsimd_last_state) will not be updated. The following is the condion that bug comes. 1. Process A runa on CPU 0, and does execve syscall. 2. Before execve syscall return to userspace, A gets scheduled out. 3. some kernel threads get scheduled in on CPU 0, per_cpu(fpsimd_last_state) will not be updated, still points to A's fpsimd_state 4. Process A get scheduled in on CPU 0, 1)process A's fpsimd_state.cpu contains 0, 2)cpu 0's fpsimd_last_state per-cpu variable points to process A 's fpsimd_state, These two are the conditions that TIF_FOREIGN_FPSTATE get cleared。 The kernel will not reload the fpsimd context during A returns back to user space. But A's fpsimd context is cleared to zero after execve syscall and must to be reload. I am not seeing an exact issue in practice. Janet On Wed, Aug 26, 2015 at 7:12 PM, Will Deacon wrote: > Hello, > > [adding Ard] > > On Wed, Aug 26, 2015 at 03:40:41AM +0100, Chunyan Zhang wrote: >> From: Janet Liu >> >> If process A is running on CPU 0 and do execve syscall and after sched_exec, >> dest_cpu is 0, fpsimd_state.cpu is 0. If at the time Process A get scheduled >> out and after some kernel threads running on CPU 0, process A is back in CPU 0, >> A's fpsimd_state.cpu is current cpu id "0", and per_cpu(fpsimd_last_state) >> points A's fpsimd_state, TIF_FOREIGN_FPSTATE will be clear, kernel will not >> reload the context during it return to userspace. so set the cpu's >> fpsimd_last_state to NULL to avoid this. > > AFAICT, this is only a problem if one of the kernel threads uses the fpsimd > registers, right? However, kernel_neon_begin_partial clobbers > fpsimd_last_state, so I'm struggling to see the problem. > > Are you seeing an issue in practice? > > Will > >> Signed-off-by: Janet Liu >> Signed-off-by: Chunyan Zhang >> --- >> arch/arm64/kernel/fpsimd.c | 1 + >> 1 file changed, 1 insertion(+) >> >> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c >> index 44d6f75..ec58d94 100644 >> --- a/arch/arm64/kernel/fpsimd.c >> +++ b/arch/arm64/kernel/fpsimd.c >> @@ -159,6 +159,7 @@ void fpsimd_flush_thread(void) >> { >> memset(¤t->thread.fpsimd_state, 0, sizeof(struct fpsimd_state)); >> set_thread_flag(TIF_FOREIGN_FPSTATE); >> + this_cpu_write(fpsimd_last_state, NULL); >> } >> >> /* >> -- >> 1.7.9.5 >> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/