Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753474AbaGGNka (ORCPT ); Mon, 7 Jul 2014 09:40:30 -0400 Received: from mail-vc0-f175.google.com ([209.85.220.175]:61607 "EHLO mail-vc0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753276AbaGGNk2 (ORCPT ); Mon, 7 Jul 2014 09:40:28 -0400 MIME-Version: 1.0 In-Reply-To: <20140703175231.GF17372@arm.com> References: <1403881067-22690-1-git-send-email-jean.pihet@linaro.org> <1403881067-22690-3-git-send-email-jean.pihet@linaro.org> <20140703175231.GF17372@arm.com> Date: Mon, 7 Jul 2014 15:40:27 +0200 Message-ID: Subject: Re: [PATCH 2/3] ARM: perf: disable the pagefault handler when reading from user space From: Jean Pihet To: Will Deacon Cc: "linux-arm-kernel@lists.infradead.org" , "linaro-kernel@lists.linaro.org" , Sneha Priya , "linux-kernel@vger.kernel.org" , Jiri Olsa , Arnaldo Carvalho de Melo , Steve Capper Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Will, On 3 July 2014 19:52, Will Deacon wrote: > Hi Jean, > > On Fri, Jun 27, 2014 at 03:57:46PM +0100, Jean Pihet wrote: >> As done on other architectures (ARM64, x86, Sparc etc.). >> >> This prevents a deadlock on down_read in do_page_fault when unwinding >> using fp and triggering on kernel tracepoints: > > So is this an issue because you could try setting tracepoints on the > pagefault path? If so, the patch is a little brutal as it would break user > backtracing as soon as we take any old page fault, no? > > Or am I missing something obvious? The problem is a deadlock between the perf events interrupt and copy_from_user, which take the same lock. The commit description has been updated to give all the details about it. Big thanks to Steve on the debugging! A new patch set is on its way. Jean > > Will > >> INFO: task stress:2116 blocked for more than 120 seconds. >> Not tainted 3.15.0-rc4-00364-g3401dfb-dirty #43 >> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. >> stress D c04b41e8 0 2116 2115 0x00000000 >> [] (__schedule) from [] (schedule+0x40/0x90) >> [] (schedule) from [] (__down_read+0xc4/0xfc) >> [] (__down_read) from [] (down_read+0x18/0x1c) >> [] (down_read) from [] (do_page_fault+0xac/0x420) >> [] (do_page_fault) from [] (do_DataAbort+0x44/0xa8) >> [] (do_DataAbort) from [] (__dabt_svc+0x38/0x60) >> Exception stack(0xecbc3af8 to 0xecbc3b40) >> 3ae0: ecbc3b74 b6d72ff4 >> 3b00: ffffffec 00000000 b6d72ff4 ec0fc000 00000000 ec0fc000 0000007e 00000000 >> 3b20: ecbc2000 ecbc3bac 00000014 ecbc3b44 c0019e78 c021ef44 00000013 ffffffff >> [] (__dabt_svc) from [] (__copy_from_user+0xa4/0x3a0) >> >> Signed-off-by: Jean Pihet >> Cc: Will Deacon >> --- >> arch/arm/kernel/perf_event.c | 9 +++++++-- >> 1 file changed, 7 insertions(+), 2 deletions(-) >> >> diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c >> index 6493c4c..f5aeca2 100644 >> --- a/arch/arm/kernel/perf_event.c >> +++ b/arch/arm/kernel/perf_event.c >> @@ -560,11 +560,16 @@ user_backtrace(struct frame_tail __user *tail, >> struct perf_callchain_entry *entry) >> { >> struct frame_tail buftail; >> + unsigned long err; >> >> - /* Also check accessibility of one struct frame_tail beyond */ >> if (!access_ok(VERIFY_READ, tail, sizeof(buftail))) >> return NULL; >> - if (__copy_from_user_inatomic(&buftail, tail, sizeof(buftail))) >> + >> + pagefault_disable(); >> + err = __copy_from_user_inatomic(&buftail, tail, sizeof(buftail)); >> + pagefault_enable(); >> + >> + if (err) >> return NULL; >> >> perf_callchain_store(entry, buftail.lr); >> -- >> 1.8.1.2 >> >> -- 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/