From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from simark.ca by simark.ca with LMTP id zCkvEOpyN2GcNQAAWB0awg (envelope-from ) for ; Tue, 07 Sep 2021 10:10:50 -0400 Received: by simark.ca (Postfix, from userid 112) id 2FF681EE22; Tue, 7 Sep 2021 10:10:50 -0400 (EDT) X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on simark.ca X-Spam-Level: X-Spam-Status: No, score=-1.7 required=5.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,MAILING_LIST_MULTI,NICE_REPLY_A,RDNS_DYNAMIC, URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.2 Received: from sourceware.org (ip-8-43-85-97.sourceware.org [8.43.85.97]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by simark.ca (Postfix) with ESMTPS id 418291EDF0 for ; Tue, 7 Sep 2021 10:10:48 -0400 (EDT) Received: from server2.sourceware.org (localhost [IPv6:::1]) by sourceware.org (Postfix) with ESMTP id 4777C385702F for ; Tue, 7 Sep 2021 14:10:47 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 4777C385702F DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sourceware.org; s=default; t=1631023847; bh=oUtJbA1UYPSXGg7/kw0jCNkCJBnUtjtz+CFijdJy370=; h=Subject:To:References:Date:In-Reply-To:List-Id:List-Unsubscribe: List-Archive:List-Post:List-Help:List-Subscribe:From:Reply-To:Cc: From; b=sS8PX5uqZNxj+0QdLJbzyGPfmEwwn+8aOfuRmZH79JMe2hKbSMhoFDJE/DpzVseZn 7a27XBmuyv7DSF/Jn9YG5H2CZ0NnU3drG02MSHuGwobaeY/FPFPNSZiD7l5BycNQ0m zqCBOEYCOdIbmxCdBVJFakChp0tsJT8+JijeTHK4= Received: from smtp.polymtl.ca (smtp.polymtl.ca [132.207.4.11]) by sourceware.org (Postfix) with ESMTPS id 239EF3857828 for ; Tue, 7 Sep 2021 14:10:15 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 239EF3857828 Received: from simark.ca (simark.ca [158.69.221.121]) (authenticated bits=0) by smtp.polymtl.ca (8.14.7/8.14.7) with ESMTP id 187EA8BQ030741 (version=TLSv1/SSLv3 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Tue, 7 Sep 2021 10:10:13 -0400 DKIM-Filter: OpenDKIM Filter v2.11.0 smtp.polymtl.ca 187EA8BQ030741 Received: from [10.0.0.11] (192-222-157-6.qc.cable.ebox.net [192.222.157.6]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by simark.ca (Postfix) with ESMTPSA id 22D841EDF0; Tue, 7 Sep 2021 10:10:08 -0400 (EDT) Subject: Re: [PATCH 2/3] gdb: make thread_suspend_state::stop_pc optional To: Andrew Burgess References: <8c0e958f-e5a6-8ed4-7326-99153e7c5faf@polymtl.ca> <20210907132103.GQ2581@embecosm.com> Message-ID: Date: Tue, 7 Sep 2021 10:10:07 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 MIME-Version: 1.0 In-Reply-To: <20210907132103.GQ2581@embecosm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Poly-FromMTA: (simark.ca [158.69.221.121]) at Tue, 7 Sep 2021 14:10:08 +0000 X-BeenThere: gdb-patches@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gdb-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , From: Simon Marchi via Gdb-patches Reply-To: Simon Marchi Cc: gdb-patches@sourceware.org Errors-To: gdb-patches-bounces+public-inbox=simark.ca@sourceware.org Sender: "Gdb-patches" On 2021-09-07 9:21 a.m., Andrew Burgess wrote: > * Simon Marchi [2021-09-01 10:23:32 -0400]: > >> >> >> On 2021-08-30 4:03 p.m., Andrew Burgess wrote: >>> Currently the stop_pc field of thread_suspect_state is a CORE_ADDR and >>> when we want to indicate that there is no stop_pc available we set >>> this field back to a special value. >>> >>> There are actually two special values used, in post_create_inferior >>> the stop_pc is set to 0. This is a little unfortunate, there are >>> plenty of embedded targets where 0 is a valid pc address. The more >>> common special value for stop_pc was set in >>> thread_info::set_executing, where the value (~(CORE_ADDR) 0) was used. >>> >>> This commit changes things so that the stop_pc is instead a >>> gdb::optional. We can now explicitly reset the field to an >>> uninitialised state, we also have (when compiling with _GLIBCXX_DEBUG >>> defined) asserts that we don't read the stop_pc when its in an >>> uninitialised state (see gdbsupport/gdb_optional.h). >> >> Thanks, I think it's a good idea. >> >>> One situation where a thread will not have a stop_pc value is when the >>> thread is stopped as a consequence of GDB being in all stop mode, and >>> some other thread stopped at an interesting event. When GDB brings >>> all the other threads to a stop those other threads will not have a >>> stop_pc set (thus avoiding an unnecessary read of $pc). >>> >>> Previously, when GDB passed through handle_one (in infrun.c) the >>> threads executing flag was set to false and the stop_pc field was left >>> unchanged, i.e. it would (previous) have been left as ~0. >>> >>> Now, handle_one leaves the stop_pc with no value. >>> >>> This caused a problem when we later try to set these threads running >>> again, in proceed() we compare the current pc with the cached >>> stop_pc. If the thread was stopped in via handle_one then the stop_pc >>> would have been left as ~0, and the compare (in proceed) >>> would (likely) fail. Now however, this compare tries to read the >>> stop_pc when it has no value, this would trigger an assert. >>> >>> To resolve this I've added thread_info::stop_pc_p() which returns true >>> if the thread has a cached stop_pc. We should only ever call >>> thread_info::stop_pc() if we know that there is a cached stop_pc. >> >> We could also make stop_pc return gdb::optional. I think it >> would be slightly better, since anybody calling stop_pc would see that >> it returns an optional and be forced to consider that. Otherwise, one >> could call stop_pc and not know that stop_pc_p exists. But otherwise >> it's the same. > > I did consider that initially, but most of the places where > thread_info::stop_pc is called the value is being immediately passed > through to some other function, here's an example pulled randomly from > infrun.c: > > ecs->event_thread->control.stop_bpstat > = bpstat_stop_status (get_current_regcache ()->aspace (), > ecs->event_thread->stop_pc (), > ecs->event_thread, &ecs->ws); > > if we are returned a gdb::optional<> then we might change the code to > do this: > > ecs->event_thread->control.stop_bpstat > = bpstat_stop_status (get_current_regcache ()->aspace (), > *ecs->event_thread->stop_pc (), > ecs->event_thread, &ecs->ws); > > Or maybe, like this: > > auto stop_pc = ecs->event_thread->stop_pc (); > gdb_assert (stop_pc.has_value ()); > ecs->event_thread->control.stop_bpstat > = bpstat_stop_status (get_current_regcache ()->aspace (), > *stop_pc, > ecs->event_thread, &ecs->ws); > > In the first case, it doesn't feel like we've gained much over my > patch, where thead_info::stop_pc() accesses the value for us. > Further, once we've normalised the pattern of accessing the stop_pc as > `*ecs->event_thread->stop_pc ()`, I worry people still wouldn't > actually consider whether the stop_pc value was valid or not, they'd > just duplicate the existing code. > > The second case seems excessively verbose, so much so, that you might > even be tempted to write a wrapper, say thread_info::stop_pc_value(), > which kind lands us back on my original patch... > > Initially, I'd relied on the asserts within gdb::optional to ensure > that we didn't access the stop_pc when it had no value, but these > asserts are only present when compiling with _GLIBCXX_DEBUG defined - > I do this, but it's certainly not going to be standard in a release > build of GDB. So, I wonder if this would be a good change: > > /* Return this thread's stop PC. This should only be called when it is > known that stop_pc has a value. If this function is being used in a > situation where a thread may not have had a stop_pc assigned, then > stop_pc_p() can be used to check if the stop_pc is defined. */ > > CORE_ADDR stop_pc () const > { > gdb_assert (m_suspend.stop_pc.has_value ()); > return *m_suspend.stop_pc; > } That one looks good to me, stating preconditions in the documentation and enforcing them using assertions. That's similar to how I designed other things, like dynamic_prop. Simon