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[109.158.45.47]) by smtp.gmail.com with ESMTPSA id h18sm1808490wrb.33.2021.09.07.06.21.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 07 Sep 2021 06:21:05 -0700 (PDT) Date: Tue, 7 Sep 2021 14:21:03 +0100 From: Andrew Burgess To: Simon Marchi Subject: Re: [PATCH 2/3] gdb: make thread_suspend_state::stop_pc optional Message-ID: <20210907132103.GQ2581@embecosm.com> References: <8c0e958f-e5a6-8ed4-7326-99153e7c5faf@polymtl.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8c0e958f-e5a6-8ed4-7326-99153e7c5faf@polymtl.ca> X-Operating-System: Linux/5.8.18-100.fc31.x86_64 (x86_64) X-Uptime: 14:07:38 up 21 days, 2:03, X-Editor: GNU Emacs [ http://www.gnu.org/software/emacs ] 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: , Cc: gdb-patches@sourceware.org Errors-To: gdb-patches-bounces+public-inbox=simark.ca@sourceware.org Sender: "Gdb-patches" * 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; } I've (a) extended the comment to mention stop_pc_p(), and added an assert that the stop_pc has a value. Given that (currently) all builds of GDB do check assertions, this should hopefully make it much more likely that if someone does access stop_pc when they shouldn't then GDB will rapidly point this out to them. What are your thoughts? Thanks, Andrew