Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756817AbbHZLjY (ORCPT ); Wed, 26 Aug 2015 07:39:24 -0400 Received: from foss.arm.com ([217.140.101.70]:44589 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752228AbbHZLjW (ORCPT ); Wed, 26 Aug 2015 07:39:22 -0400 Date: Wed, 26 Aug 2015 12:39:22 +0100 From: Will Deacon To: Ard Biesheuvel Cc: Chunyan Zhang , Catalin Marinas , "linux-arm-kernel@lists.infradead.org" , "jianhua.ljh@gmail.com" , "orson.zhai@spreadtrum.com" , "xiongshan.an@spreadtrum.com" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] arm64: fix bug for reloading FPSIMD state after execve on cpu 0. Message-ID: <20150826113922.GE30466@arm.com> References: <1440556841-17219-1-git-send-email-chunyan.zhang@spreadtrum.com> <20150826111205.GD30466@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2252 Lines: 50 On Wed, Aug 26, 2015 at 12:32:03PM +0100, Ard Biesheuvel wrote: > On 26 August 2015 at 13:12, Will Deacon wrote: > > 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) Aha, I see. So the problem is actually that we get a view on our fpsimd state before the exec, rather than a view on some kernel state. > This more accurately reflects the state of the process after forking, > i.e., that its FPSIMD state has never been loaded into any CPU. Yup, that's much clearer. Will -- 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/