From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 88371 invoked by alias); 11 Mar 2016 11:05:45 -0000 Mailing-List: contact gdb-patches-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-patches-owner@sourceware.org Received: (qmail 88357 invoked by uid 89); 11 Mar 2016 11:05:45 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-1.9 required=5.0 tests=BAYES_00,RP_MATCHES_RCVD,SPF_HELO_PASS autolearn=ham version=3.3.2 spammy=sigstepexp, UD:sigstep.exp, sigstep.exp, sk:control X-HELO: mx1.redhat.com Received: from mx1.redhat.com (HELO mx1.redhat.com) (209.132.183.28) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with (AES256-GCM-SHA384 encrypted) ESMTPS; Fri, 11 Mar 2016 11:05:43 +0000 Received: from int-mx10.intmail.prod.int.phx2.redhat.com (int-mx10.intmail.prod.int.phx2.redhat.com [10.5.11.23]) by mx1.redhat.com (Postfix) with ESMTPS id B6800B5956; Fri, 11 Mar 2016 11:05:42 +0000 (UTC) Received: from [127.0.0.1] (ovpn01.gateway.prod.ext.phx2.redhat.com [10.5.9.1]) by int-mx10.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id u2BB5fhl000836; Fri, 11 Mar 2016 06:05:41 -0500 Subject: Re: [PATCH 3/8] Deliver signal in hardware single step To: Yao Qi , gdb-patches@sourceware.org References: <1457088276-1170-1-git-send-email-yao.qi@linaro.org> <1457088276-1170-4-git-send-email-yao.qi@linaro.org> From: Pedro Alves Message-ID: <56E2A685.2080602@redhat.com> Date: Fri, 11 Mar 2016 11:05:00 -0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 MIME-Version: 1.0 In-Reply-To: <1457088276-1170-4-git-send-email-yao.qi@linaro.org> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-SW-Source: 2016-03/txt/msg00178.txt.bz2 On 03/04/2016 10:44 AM, Yao Qi wrote: > GDBserver doesn't deliver signal when stepping over a breakpoint even > hardware single step is used. When GDBserver started to step over > (thread creation) breakpoint for mutlit-threaded debugging in 2002 [1], > GDBserver behaves this way. > > This behaviour gets trouble on conditional breakpoints on branch to > self instruction like this, > > 0x00000000004005b6 <+29>: jmp 0x4005b6 > > and I set breakpoint > > $(gdb) break branch-to-self.c:43 if counter > 3 > > and the variable counter will be set to 5 in SIGALRM signal handler. > Since GDBserver keeps stepping over breakpoint, the SIGALRM can never > be dequeued and delivered to the inferior, so the program can't stop. > The test can be found in gdb.base/branch-to-self.exp. > > I can understand why does GDBserver queue signal for software single > step, but I can't figure out a reason we should queue signal for > hardware single step. With this patch applied, GDBserver forward the > signal to inferior and the program can stop correctly. > > [1] PATCH: Multithreaded debugging for gdbserver > https://sourceware.org/ml/gdb-patches/2002-06/msg00157.html > Because the signal handler might recurse and call the same code that had the breakpoint (or some other removed breakpoint), and thus we'd miss a breakpoint hit in the signal handler. GDB / infrun.c handles it here: if (ecs->event_thread->prev_pc == stop_pc && ecs->event_thread->control.trap_expected && ecs->event_thread->control.step_resume_breakpoint == NULL) { int was_in_line; /* We were just starting a new sequence, attempting to single-step off of a breakpoint and expecting a SIGTRAP. Instead this signal arrives. This signal will take us out of the stepping range so GDB needs to remember to, when the signal handler returns, resume stepping off that breakpoint. */ /* To simplify things, "continue" is forced to use the same code paths as single-step - set a breakpoint at the signal return address and then, once hit, step off that breakpoint. */ IIRC, some of sigstep.exp, signull.exp, signest.exp exercise this. Note that this also lets all threads run while the signal handler runs. Thanks, Pedro Alves