Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752476AbbH0Cnt (ORCPT ); Wed, 26 Aug 2015 22:43:49 -0400 Received: from mail-ob0-f180.google.com ([209.85.214.180]:36798 "EHLO mail-ob0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751016AbbH0Cns (ORCPT ); Wed, 26 Aug 2015 22:43:48 -0400 MIME-Version: 1.0 In-Reply-To: References: <1440556841-17219-1-git-send-email-chunyan.zhang@spreadtrum.com> <20150826111205.GD30466@arm.com> Date: Thu, 27 Aug 2015 10:43:47 +0800 Message-ID: Subject: Re: [PATCH] arm64: fix bug for reloading FPSIMD state after execve on cpu 0. From: Jianhua Liu To: Ard Biesheuvel Cc: Will Deacon , Chunyan Zhang , Catalin Marinas , "linux-arm-kernel@lists.infradead.org" , "orson.zhai@spreadtrum.com" , "xiongshan.an@spreadtrum.com" , "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2963 Lines: 83 On Wed, Aug 26, 2015 at 7:32 PM, Ard Biesheuvel wrote: > On 26 August 2015 at 13:12, 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. >> > > I think the problem is real, but it would be better to set the > fpsimd_state::cpu field to an invalid value like we do in > fpsimd_flush_task_state() > > diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c > index 44d6f7545505..c56956a16d3f 100644 > --- a/arch/arm64/kernel/fpsimd.c > +++ b/arch/arm64/kernel/fpsimd.c > @@ -158,6 +158,7 @@ void fpsimd_thread_switch(struct task_struct *next) > void fpsimd_flush_thread(void) > { > memset(¤t->thread.fpsimd_state, 0, sizeof(struct fpsimd_state)); > + fpsimd_flush_task_state(current); > set_thread_flag(TIF_FOREIGN_FPSTATE); > } > > (note the memset erroneously initializes that field to CPU 0) > > This more accurately reflects the state of the process after forking, > i.e., that its FPSIMD state has never been loaded into any CPU. I agree with you. Can you send a patch to maintainer as soon as possible to solve this issue? Janet > > -- > Ard. > >> 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/