Messages in this thread | | | Date | Wed, 4 Aug 2021 14:31:36 +0200 | From | Petr Mladek <> | Subject | Re: [PATCH printk v1 03/10] kgdb: delay roundup if holding printk cpulock |
| |
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); > + } > }
| |