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Date: Thu, 27 Aug 2015 10:43:47 +0800 From: Jianhua Liu <jianhua.ljh@...il.com> To: Ard Biesheuvel <ard.biesheuvel@...aro.org> Cc: Will Deacon <will.deacon@....com>, Chunyan Zhang <chunyan.zhang@...eadtrum.com>, Catalin Marinas <Catalin.Marinas@....com>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, "orson.zhai@...eadtrum.com" <orson.zhai@...eadtrum.com>, "xiongshan.an@...eadtrum.com" <xiongshan.an@...eadtrum.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] arm64: fix bug for reloading FPSIMD state after execve on cpu 0. On Wed, Aug 26, 2015 at 7:32 PM, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote: > On 26 August 2015 at 13:12, Will Deacon <will.deacon@....com> wrote: >> Hello, >> >> [adding Ard] >> >> On Wed, Aug 26, 2015 at 03:40:41AM +0100, Chunyan Zhang wrote: >>> From: Janet Liu <janet.liu@...eadtrum.com> >>> >>> 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 <janet.liu@...eadtrum.com> >>> Signed-off-by: Chunyan Zhang <chunyan.zhang@...eadtrum.com> >>> --- >>> 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@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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