From: Petr Mladek <pmladek@suse.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: "Daniel Thompson" <daniel.thompson@linaro.org>,
"Sergey Senozhatsky" <senozhatsky@chromium.org>,
"Steven Rostedt" <rostedt@goodmis.org>,
"Thomas Gleixner" <tglx@linutronix.de>,
linux-kernel@vger.kernel.org,
"Michael Ellerman" <mpe@ellerman.id.au>,
"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
"Paul Mackerras" <paulus@samba.org>,
"Ingo Molnar" <mingo@redhat.com>,
"Borislav Petkov" <bp@alien8.de>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
"Jason Wessel" <jason.wessel@windriver.com>,
"Douglas Anderson" <dianders@chromium.org>,
"Srikar Dronamraju" <srikar@linux.vnet.ibm.com>,
"Gautham R. Shenoy" <ego@linux.vnet.ibm.com>,
"Chengyang Fan" <cy.fan@huawei.com>,
"Christophe Leroy" <christophe.leroy@csgroup.eu>,
"Bhaskar Chowdhury" <unixbhaskar@gmail.com>,
"Nicholas Piggin" <npiggin@gmail.com>,
"Cédric Le Goater" <clg@kaod.org>,
"Gustavo A. R. Silva" <gustavoars@kernel.org>,
"Peter Zijlstra" <peterz@infradead.org>,
linuxppc-dev@lists.ozlabs.org,
kgdb-bugreport@lists.sourceforge.net
Subject: Re: [PATCH printk v1 03/10] kgdb: delay roundup if holding printk cpulock
Date: Wed, 4 Aug 2021 14:31:36 +0200 [thread overview]
Message-ID: <YQqIqKjRcNhZSaAZ@alley> (raw)
In-Reply-To: <87tuk635rb.fsf@jogness.linutronix.de>
On Tue 2021-08-03 17:36:32, John Ogness wrote:
> On 2021-08-03, Daniel Thompson <daniel.thompson@linaro.org> wrote:
> > On Tue, Aug 03, 2021 at 03:18:54PM +0206, John Ogness wrote:
> >> kgdb makes use of its own cpulock (@dbg_master_lock, @kgdb_active)
> >> during cpu roundup. This will conflict with the printk cpulock.
> >
> >> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> >> index 3d0c933937b4..1b546e117f10 100644
> >> --- a/kernel/printk/printk.c
> >> +++ b/kernel/printk/printk.c
> >> @@ -214,6 +215,7 @@ int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
> >> #ifdef CONFIG_SMP
> >> static atomic_t printk_cpulock_owner = ATOMIC_INIT(-1);
> >> static atomic_t printk_cpulock_nested = ATOMIC_INIT(0);
> >> +static unsigned int kgdb_cpu = -1;
> >
> > Is this the flag to provoke retriggering? It appears to be a write-only
> > variable (at least in this patch). How is it consumed?
>
> Critical catch! Thank you. I am quite unhappy to see these hunks were
> accidentally dropped when generating this series.
>
> @@ -3673,6 +3675,9 @@ EXPORT_SYMBOL(__printk_cpu_trylock);
> */
> void __printk_cpu_unlock(void)
> {
> + bool trigger_kgdb = false;
> + unsigned int cpu;
> +
> if (atomic_read(&printk_cpulock_nested)) {
> atomic_dec(&printk_cpulock_nested);
> return;
> @@ -3683,6 +3688,12 @@ void __printk_cpu_unlock(void)
> * LMM(__printk_cpu_unlock:A)
> */
>
> + cpu = smp_processor_id();
> + if (kgdb_cpu == cpu) {
> + trigger_kgdb = true;
> + kgdb_cpu = -1;
> + }
Just in case that this approach is used in the end.
This code looks racy. kgdb_roundup_delay() seems to be called in NMI
context. NMI might happen at this point and set kgdb_cpu after
it was checked.
I am afraid that it won't be easy to make this safe using a single
global variable. A solution might be a per-CPU variable set
by kgdb_roundup_delay() when it owns printk_cpu_lock.
__printk_cpu_unlock() would call kgdb_roundup_cpu(cpu) when
the variable is set.
Nit: The name "kgdb_cpu" is too generic. It is not clear what is
so special about this CPU. I would call the per-CPU variable
"kgdb_delayed_roundup" or so.
Best Regards,
Petr
> /*
> * Guarantee loads and stores from this CPU when it was the
> * lock owner are visible to the next lock owner. This pairs
> @@ -3703,6 +3714,21 @@ void __printk_cpu_unlock(void)
> */
> atomic_set_release(&printk_cpulock_owner,
> -1); /* LMM(__printk_cpu_unlock:B) */
> +
> + if (trigger_kgdb) {
> + pr_warn("re-triggering kgdb roundup for CPU#%d\n", cpu);
> + kgdb_roundup_cpu(cpu);
> + }
> }
next prev parent reply other threads:[~2021-08-04 12:31 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-03 13:12 [PATCH printk v1 00/10] printk: introduce atomic consoles and sync mode John Ogness
2021-08-03 13:12 ` [PATCH printk v1 01/10] printk: relocate printk cpulock functions John Ogness
2021-08-04 9:24 ` Petr Mladek
2021-08-03 13:12 ` [PATCH printk v1 02/10] printk: rename printk cpulock API and always disable interrupts John Ogness
2021-08-04 9:52 ` Petr Mladek
2021-08-03 13:12 ` [PATCH printk v1 03/10] kgdb: delay roundup if holding printk cpulock John Ogness
2021-08-03 14:25 ` Daniel Thompson
2021-08-03 15:30 ` John Ogness
2021-08-04 11:31 ` Daniel Thompson
2021-08-04 12:12 ` Petr Mladek
2021-08-04 15:04 ` Daniel Thompson
2021-08-05 3:46 ` John Ogness
2021-08-06 12:06 ` Daniel Thompson
2021-08-04 12:31 ` Petr Mladek [this message]
2021-08-03 13:12 ` [PATCH printk v1 04/10] printk: relocate printk_delay() John Ogness
2021-08-04 13:07 ` Petr Mladek
2021-08-03 13:12 ` [PATCH printk v1 05/10] printk: call boot_delay_msec() in printk_delay() John Ogness
2021-08-04 13:09 ` Petr Mladek
2021-08-31 1:04 ` Sergey Senozhatsky
2021-08-03 13:12 ` [PATCH printk v1 06/10] printk: use seqcount_latch for console_seq John Ogness
2021-08-05 12:16 ` Petr Mladek
2021-08-05 15:26 ` John Ogness
2021-08-06 15:56 ` Petr Mladek
2021-08-31 3:05 ` Sergey Senozhatsky
2021-08-03 13:12 ` [PATCH printk v1 07/10] console: add write_atomic interface John Ogness
2021-08-03 14:02 ` Andy Shevchenko
2021-08-06 10:56 ` John Ogness
2021-08-06 11:18 ` Andy Shevchenko
2021-08-31 2:55 ` Sergey Senozhatsky
2021-08-03 13:12 ` [PATCH printk v1 08/10] printk: introduce kernel sync mode John Ogness
2021-08-05 17:11 ` Petr Mladek
2021-08-05 21:25 ` John Ogness
2021-08-03 13:13 ` [PATCH printk v1 09/10] kdb: if available, only use atomic consoles for output mirroring John Ogness
2021-08-03 13:13 ` [PATCH printk v1 10/10] serial: 8250: implement write_atomic John Ogness
2021-08-03 14:07 ` Andy Shevchenko
2021-08-05 7:47 ` Jiri Slaby
2021-08-05 8:26 ` John Ogness
2021-08-03 13:52 ` [PATCH printk v1 00/10] printk: introduce atomic consoles and sync mode Andy Shevchenko
2021-08-05 15:47 ` Petr Mladek
2021-08-31 0:33 ` Sergey Senozhatsky
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