Re: [PATCH printk v1 03/10] kgdb: delay roundup if holding printk cpulock
From: Petr Mladek <pmladek@suse.com>
Date: 2021-08-04 12:31:41
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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); + } }