On (12/31/15 12:13), Sergey Senozhatsky wrote:
[..]
> cond_resched() does its job there, of course. well, a user process still can
> do a lot of call_console_drivers() calls. may be we can check who is calling
> console_unlock() and if we have "!printk_sync && !oops_in_progress" (or just
> printk_sync
> test) AND a user process then return from console_unlock() doing
> irq_work_queue()
> and set PRINTK_PENDING_OUTPUT pending bit, the way vprintk_emit() does it.
attached two patches, I ended up having on top of yours. just in case.
printk: factor out can_printk_async()
console_unlock() can be called directly or indirectly by a user
space process, so it can end up doing call_console_drivers() loop,
which will hold it from returning back to user-space from a syscall
for unpredictable amount of time.
Factor out can_printk_async() function, which queues an irq work and
sets a PRINTK_PENDING_OUTPUT pending bit (if we can do async printk).
vprintk_emit() already does it, add can_printk_async() call to
console_unlock() for !PF_KTHREAD processes.
and
printk: introduce console_sync_mode
console_sync_mode() should be called early in panic() to switch
printk from async mode to sync. Otherwise, STOP IPIs can arrive
to other CPUs too late and those CPUs will see oops_in_progress
being 0 again.
-ss
>From c3fc955809adab8f497cdc7581e67e1fa29d6517 Mon Sep 17 00:00:00 2001
From: Sergey Senozhatsky <[email protected]>
Date: Wed, 30 Dec 2015 20:39:12 +0900
Subject: [PATCH 1/2] printk: introduce console_sync_mode
console_sync_mode() should be called early in panic() to switch
printk from async mode to sync. Otherwise, STOP IPIs can arrive
to other CPUs too late and those CPUs will see oops_in_progress
being 0 again.
---
include/linux/console.h | 1 +
kernel/panic.c | 1 +
kernel/printk/printk.c | 5 +++++
3 files changed, 7 insertions(+)
diff --git a/include/linux/console.h b/include/linux/console.h
index bd19434..f068985 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -150,6 +150,7 @@ extern int console_trylock(void);
extern void console_unlock(void);
extern void console_conditional_schedule(void);
extern void console_unblank(void);
+extern void console_sync_mode(void);
extern struct tty_driver *console_device(int *);
extern void console_stop(struct console *);
extern void console_start(struct console *);
diff --git a/kernel/panic.c b/kernel/panic.c
index b333380..04c8ff4 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -117,6 +117,7 @@ void panic(const char *fmt, ...)
if (old_cpu != PANIC_CPU_INVALID && old_cpu != this_cpu)
panic_smp_self_stop();
+ console_sync_mode();
console_verbose();
bust_spinlocks(1);
va_start(args, fmt);
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index de9d31b..47a70a2 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2501,6 +2501,11 @@ void console_unblank(void)
console_unlock();
}
+void console_sync_mode(void)
+{
+ printk_sync = true;
+}
+
/*
* Return the console tty driver structure and its associated index
*/
--
2.6.4
>From 92f2c0f2a5ed015caa2757dcfec4407d708f8628 Mon Sep 17 00:00:00 2001
From: Sergey Senozhatsky <[email protected]>
Date: Thu, 31 Dec 2015 13:39:58 +0900
Subject: [PATCH 2/2] printk: factor out can_printk_async()
console_unlock() can be called directly or indirectly by a user
space process, so it can end up doing call_console_drivers() loop,
which will hold it from returning back to user-space from a syscall
for unpredictable amount of time.
Factor out can_printk_async() function, which queues an irq work and
sets a PRINTK_PENDING_OUTPUT pending bit (if we can do async printk).
vprintk_emit() already does it, add can_printk_async() call to
console_unlock() for !PF_KTHREAD processes.
---
kernel/printk/printk.c | 42 ++++++++++++++++++++++++++++--------------
1 file changed, 28 insertions(+), 14 deletions(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 47a70a2..7d3a8e1 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -355,6 +355,26 @@ int printk_deferred(const char *fmt, ...)
return r;
}
+static bool can_printk_async(bool sync)
+{
+ /*
+ * By default we print message to console asynchronously so that kernel
+ * doesn't get stalled due to slow serial console. That can lead to
+ * softlockups, lost interrupts, or userspace timing out under heavy
+ * printing load.
+ *
+ * However we resort to synchronous printing of messages during early
+ * boot, when oops is in progress, or when synchronous printing was
+ * explicitely requested by kernel parameter.
+ */
+ if (keventd_up() && !oops_in_progress && !sync) {
+ __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
+ irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
+ return true;
+ }
+ return false;
+}
+
/* Return log buffer address */
char *log_buf_addr_get(void)
{
@@ -1889,20 +1909,7 @@ asmlinkage int vprintk_emit(int facility, int level,
logbuf_cpu = UINT_MAX;
raw_spin_unlock(&logbuf_lock);
lockdep_on();
- /*
- * By default we print message to console asynchronously so that kernel
- * doesn't get stalled due to slow serial console. That can lead to
- * softlockups, lost interrupts, or userspace timing out under heavy
- * printing load.
- *
- * However we resort to synchronous printing of messages during early
- * boot, when oops is in progress, or when synchronous printing was
- * explicitely requested by kernel parameter.
- */
- if (keventd_up() && !oops_in_progress && !sync_print) {
- __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
- irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
- } else
+ if (!can_printk_async(sync_print))
sync_print = true;
local_irq_restore(flags);
@@ -2328,6 +2335,13 @@ void console_unlock(void)
return;
}
+ if (!(current->flags & PF_KTHREAD) &&
+ can_printk_async(printk_sync)) {
+ console_locked = 0;
+ up_console_sem();
+ return;
+ }
+
/*
* Console drivers are called under logbuf_lock, so
* @console_may_schedule should be cleared before; however, we may
--
2.6.4
