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[86.9.19.6]) by smtp.gmail.com with ESMTPSA id y186sm10500048wmb.41.2019.10.10.08.07.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 10 Oct 2019 08:07:36 -0700 (PDT) Date: Thu, 10 Oct 2019 16:07:35 +0100 From: Daniel Thompson To: Doug Anderson Cc: Catalin Marinas , Will Deacon , Jason Wessel , kgdb-bugreport@lists.sourceforge.net, Christophe Leroy , LKML Subject: Re: [PATCH v3 3/4] kdb: Fix "btc " crash if the CPU didn't round up Message-ID: <20191010150735.dhrj3pbjgmjrdpwr@holly.lan> References: <20190925200220.157670-1-dianders@chromium.org> <20190925125811.v3.3.Id33c06cbd1516b49820faccd80da01c7c4bf15c7@changeid> <20191007135459.lj3qc2tqzcv3xcia@holly.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 07, 2019 at 04:34:55PM -0700, Doug Anderson wrote: > Hi, > > On Mon, Oct 7, 2019 at 6:55 AM Daniel Thompson > wrote: > > > > On Wed, Sep 25, 2019 at 01:02:19PM -0700, Douglas Anderson wrote: > > > > > > I noticed that when I did "btc " and the CPU I passed in hadn't > > > rounded up that I'd crash. I was going to copy the same fix from > > > commit 162bc7f5afd7 ("kdb: Don't back trace on a cpu that didn't round > > > up") into the "not all the CPUs" case, but decided it'd be better to > > > clean things up a little bit. > > > > > > This consolidates the two code paths. It is _slightly_ wasteful in in > > > that the checks for "cpu" being too small or being offline isn't > > > really needed when we're iterating over all online CPUs, but that > > > really shouldn't hurt. Better to have the same code path. > > > > > > While at it, eliminate at least one slightly ugly (and totally > > > needless) recursive use of kdb_parse(). > > > > > > Signed-off-by: Douglas Anderson > > > --- > > > > > > Changes in v3: > > > - Patch ("kdb: Fix "btc " crash if the CPU...") new for v3. > > > > > > Changes in v2: None > > > > > > kernel/debug/kdb/kdb_bt.c | 61 ++++++++++++++++++++++----------------- > > > 1 file changed, 34 insertions(+), 27 deletions(-) > > > > > > diff --git a/kernel/debug/kdb/kdb_bt.c b/kernel/debug/kdb/kdb_bt.c > > > index 120fc686c919..d9af139f9a31 100644 > > > --- a/kernel/debug/kdb/kdb_bt.c > > > +++ b/kernel/debug/kdb/kdb_bt.c > > > @@ -101,6 +101,27 @@ kdb_bt1(struct task_struct *p, unsigned long mask, bool btaprompt) > > > return 0; > > > } > > > > > > +static void > > > +kdb_bt_cpu(unsigned long cpu) > > > +{ > > > + struct task_struct *kdb_tsk; > > > + > > > + if (cpu >= num_possible_cpus() || !cpu_online(cpu)) { > > > + kdb_printf("WARNING: no process for cpu %ld\n", cpu); > > > + return; > > > + } > > > + > > > + /* If a CPU failed to round up we could be here */ > > > + kdb_tsk = KDB_TSK(cpu); > > > + if (!kdb_tsk) { > > > + kdb_printf("WARNING: no task for cpu %ld\n", cpu); > > > + return; > > > + } > > > + > > > + kdb_set_current_task(kdb_tsk); > > > + kdb_bt1(kdb_tsk, ~0UL, false); > > > +} > > > + > > > int > > > kdb_bt(int argc, const char **argv) > > > { > > > @@ -161,7 +182,6 @@ kdb_bt(int argc, const char **argv) > > > } else if (strcmp(argv[0], "btc") == 0) { > > > unsigned long cpu = ~0; > > > struct task_struct *save_current_task = kdb_current_task; > > > - char buf[80]; > > > if (argc > 1) > > > return KDB_ARGCOUNT; > > > if (argc == 1) { > > > @@ -169,35 +189,22 @@ kdb_bt(int argc, const char **argv) > > > if (diag) > > > return diag; > > > } > > > - /* Recursive use of kdb_parse, do not use argv after > > > - * this point */ > > > - argv = NULL; > > > if (cpu != ~0) { > > > - if (cpu >= num_possible_cpus() || !cpu_online(cpu)) { > > > - kdb_printf("no process for cpu %ld\n", cpu); > > > - return 0; > > > - } > > > - sprintf(buf, "btt 0x%px\n", KDB_TSK(cpu)); > > > - kdb_parse(buf); > > > - return 0; > > > - } > > > - kdb_printf("btc: cpu status: "); > > > - kdb_parse("cpu\n"); > > > - for_each_online_cpu(cpu) { > > > - void *kdb_tsk = KDB_TSK(cpu); > > > - > > > - /* If a CPU failed to round up we could be here */ > > > - if (!kdb_tsk) { > > > - kdb_printf("WARNING: no task for cpu %ld\n", > > > - cpu); > > > - continue; > > > + kdb_bt_cpu(cpu); > > > + } else { > > > + /* > > > + * Recursive use of kdb_parse, do not use argv after > > > + * this point. > > > + */ > > > + argv = NULL; > > > + kdb_printf("btc: cpu status: "); > > > + kdb_parse("cpu\n"); > > > + for_each_online_cpu(cpu) { > > > + kdb_bt_cpu(cpu); > > > + touch_nmi_watchdog(); > > > } > > > - > > > - sprintf(buf, "btt 0x%px\n", kdb_tsk); > > > - kdb_parse(buf); > > > - touch_nmi_watchdog(); > > > + kdb_set_current_task(save_current_task); > > > } > > > - kdb_set_current_task(save_current_task); > > > > Why does this move out into only one of the conditional branches? > > Don't both of the above paths modify the current task? > > The old code has a "return 0 in the case that "cpu != ~0", so this > basically matches the prior behavior in restoring the current task for > a "btc" but not leaving the current task changed in the case of "btc > ". Thus my patch doesn't actually change the existing behavior, > but I guess that it does make the control flow simpler so it's easier > to understand what the behavior is. ;-) Point taken. Horrific though it may be ;-) . > Reading through other control flows of the various backtrace commands, > it looks like it is intentional to leave the current task changed when > you explicitly do an action on that task (or a CPU). > > Actually, though, it wasn't clear to me that it ever made sense for > any of these commands to implicitly leave the current task changed. > If you agree, I can send a follow-up patch to change this behavior. Personally I don't like implicit changes of state but I might need a bit more thinking to agree (or disagree ;-) ). Daniel.