On Tue 2021-08-03 17:36:32, John Ogness wrote:
quoted hunk ↗ jump to hunk
On 2021-08-03, Daniel Thompson [off-list ref] wrote:
quoted
On Tue, Aug 03, 2021 at 03:18:54PM +0206, John Ogness wrote:
quoted
kgdb makes use of its own cpulock (@dbg_master_lock, @kgdb_active)
during cpu roundup. This will conflict with the printk cpulock.
quoted
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
quoted hunk ↗ jump to hunk
/*
* 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);
+ }
}