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[86.9.19.6]) by smtp.gmail.com with ESMTPSA id w5sm11200334wrc.82.2019.08.30.07.52.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 30 Aug 2019 07:52:39 -0700 (PDT) Date: Fri, 30 Aug 2019 15:52:37 +0100 From: Daniel Thompson To: Douglas Anderson Cc: Catalin Marinas , Will Deacon , Jason Wessel , kgdb-bugreport@lists.sourceforge.net, Christophe Leroy , linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] kdb: Fix stack crawling on 'running' CPUs that aren't the master Message-ID: <20190830145237.aoysubwetqe3eggj@holly.lan> References: <20190731183732.178134-1-dianders@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190731183732.178134-1-dianders@chromium.org> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 31, 2019 at 11:37:32AM -0700, Douglas Anderson wrote: > In kdb when you do 'btc' (back trace on CPU) it doesn't necessarily > give you the right info. Specifically on many architectures > (including arm64, where I tested) you can't dump the stack of a > "running" process that isn't the process running on the current CPU. > This can be seen by this: > > echo SOFTLOCKUP > /sys/kernel/debug/provoke-crash/DIRECT > # wait 2 seconds > g > > Here's what I see now on rk3399-gru-kevin. I see the stack crawl for > the CPU that handled the sysrq but everything else just shows me stuck > in __switch_to() which is bogus: > > ====== > > [0]kdb> btc > btc: cpu status: Currently on cpu 0 > Available cpus: 0, 1-3(I), 4, 5(I) > Stack traceback for pid 0 > 0xffffff801101a9c0 0 0 1 0 R 0xffffff801101b3b0 *swapper/0 > Call trace: > dump_backtrace+0x0/0x138 > ... > kgdb_compiled_brk_fn+0x34/0x44 > ... > sysrq_handle_dbg+0x34/0x5c > Stack traceback for pid 0 > 0xffffffc0f175a040 0 0 1 1 I 0xffffffc0f175aa30 swapper/1 > Call trace: > __switch_to+0x1e4/0x240 > 0xffffffc0f65616c0 > Stack traceback for pid 0 > 0xffffffc0f175d040 0 0 1 2 I 0xffffffc0f175da30 swapper/2 > Call trace: > __switch_to+0x1e4/0x240 > 0xffffffc0f65806c0 > Stack traceback for pid 0 > 0xffffffc0f175b040 0 0 1 3 I 0xffffffc0f175ba30 swapper/3 > Call trace: > __switch_to+0x1e4/0x240 > 0xffffffc0f659f6c0 > Stack traceback for pid 1474 > 0xffffffc0dde8b040 1474 727 1 4 R 0xffffffc0dde8ba30 bash > Call trace: > __switch_to+0x1e4/0x240 > __schedule+0x464/0x618 > 0xffffffc0dde8b040 > Stack traceback for pid 0 > 0xffffffc0f17b0040 0 0 1 5 I 0xffffffc0f17b0a30 swapper/5 > Call trace: > __switch_to+0x1e4/0x240 > 0xffffffc0f65dd6c0 > > === > > The problem is that 'btc' eventually boils down to > show_stack(task_struct, NULL); > > ...and show_stack() doesn't work for "running" CPUs because their > registers haven't been stashed. > > On x86 things might work better (I haven't tested) because kdb has a > special case for x86 in kdb_show_stack() where it passes the stack > pointer to show_stack(). This wouldn't work on arm64 where the stack > crawling function seems needs the "fp" and "pc", not the "sp" which is > presumably why arm64's show_stack() function totally ignores the "sp" > parameter. > > NOTE: we _can_ get a good stack dump for all the cpus if we manually > switch each one to the kdb master and do a back trace. AKA: > cpu 4 > bt > ...will give the expected trace. That's because now arm64's > dump_backtrace will now see that "tsk == current" and go through a > different path. > > In this patch I fix the problems by catching a request to stack crawl > a task that's running on a CPU and then I ask that CPU to do the stack > crawl. > > NOTE: this will (presumably) change what stack crawls are printed for > x86 machines. Now kdb functions will show up in the stack crawl. > Presumably this is OK but if it's not we can go back and add a special > case for x86 again. > > Signed-off-by: Douglas Anderson I think this approach can be made work but there are problems as things exist today, see below. > --- > > Changes in v2: > - Totally new approach; now arch agnostic. > > kernel/debug/debug_core.c | 5 +++++ > kernel/debug/debug_core.h | 1 + > kernel/debug/kdb/kdb_bt.c | 44 ++++++++++++++++++++++++++++++--------- > 3 files changed, 40 insertions(+), 10 deletions(-) > > diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c > index 5cc608de6883..a89c72714fe6 100644 > --- a/kernel/debug/debug_core.c > +++ b/kernel/debug/debug_core.c > @@ -92,6 +92,8 @@ static int kgdb_use_con; > bool dbg_is_early = true; > /* Next cpu to become the master debug core */ > int dbg_switch_cpu; > +/* cpu number of slave we request a stack crawl of */ > +int dbg_slave_dumpstack_cpu = -1; > > /* Use kdb or gdbserver mode */ > int dbg_kdb_mode = 1; > @@ -580,6 +582,9 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs, > atomic_xchg(&kgdb_active, cpu); > break; > } > + } else if (dbg_slave_dumpstack_cpu == cpu) { Couldn't this be encoded in the exception state? > + dump_stack(); > + dbg_slave_dumpstack_cpu = -1; > } else if (kgdb_info[cpu].exception_state & DCPU_IS_SLAVE) { > if (!raw_spin_is_locked(&dbg_slave_lock)) > goto return_normal; > diff --git a/kernel/debug/debug_core.h b/kernel/debug/debug_core.h > index b4a7c326d546..dca74d5caef2 100644 > --- a/kernel/debug/debug_core.h > +++ b/kernel/debug/debug_core.h > @@ -62,6 +62,7 @@ extern int dbg_io_get_char(void); > /* Switch from one cpu to another */ > #define DBG_SWITCH_CPU_EVENT -123456 > extern int dbg_switch_cpu; > +extern int dbg_slave_dumpstack_cpu; > > /* gdbstub interface functions */ > extern int gdb_serial_stub(struct kgdb_state *ks); > diff --git a/kernel/debug/kdb/kdb_bt.c b/kernel/debug/kdb/kdb_bt.c > index 7e2379aa0a1e..10095ae05826 100644 > --- a/kernel/debug/kdb/kdb_bt.c > +++ b/kernel/debug/kdb/kdb_bt.c > @@ -10,6 +10,7 @@ > */ > > #include > +#include > #include > #include > #include > @@ -22,20 +23,43 @@ > static void kdb_show_stack(struct task_struct *p, void *addr) > { > int old_lvl = console_loglevel; > + int time_left; > + int cpu; > + > console_loglevel = CONSOLE_LOGLEVEL_MOTORMOUTH; > kdb_trap_printk++; > - kdb_set_current_task(p); > - if (addr) { > - show_stack((struct task_struct *)p, addr); > - } else if (kdb_current_regs) { > -#ifdef CONFIG_X86 > - show_stack(p, &kdb_current_regs->sp); > -#else > - show_stack(p, NULL); > -#endif > + > + if (!addr && kdb_task_has_cpu(p)) { > + cpu = kdb_process_cpu(p); > + > + if (cpu == raw_smp_processor_id()) { > + dump_stack(); > + goto exit; This goto is not for error recovery but looks like it is. Why can't we use normal control flow here (extracting the remote stack dump logic into a seperate function if the right margin is getting too close)? In fact to be honest a function call would be useful anyway since I'd rather have all the resulting horror in a single file (debug_core.c). > + } > + > + /* > + * In general architectures don't support dumping the stack > + * of a "running" process that's not the current one so if > + * we want to dump the stack of a running process that's not > + * the master then we'll set a global letting the slave > + * (which should be looping) know to dump its own stack. > + */ > + dbg_slave_dumpstack_cpu = cpu; > + for (time_left = MSEC_PER_SEC; time_left; time_left--) { > + udelay(1000); > + if (dbg_slave_dumpstack_cpu == -1) > + break; > + } This timeout does not interact correctly with the pager (the timer does not get reset when we sit in the pager loop waiting for user to tell us to continue). > + if (dbg_slave_dumpstack_cpu != -1) { > + kdb_printf("ERROR: Timeout dumping CPU %d stack\n", > + cpu); > + dbg_slave_dumpstack_cpu = -1; > + } > } else { > - show_stack(p, NULL); > + show_stack(p, addr); > } > + > +exit: > console_loglevel = old_lvl; > kdb_trap_printk--; > } > -- > 2.22.0.770.g0f2c4a37fd-goog