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Date: Fri, 19 Aug 2016 15:32:36 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To: Petr Mladek <pmladek@...e.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
Viresh Kumar <viresh.kumar@...aro.org>,
Jan Kara <jack@...e.cz>,
Andrew Morton <akpm@...ux-foundation.org>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Jan Kara <jack@...e.com>, Tejun Heo <tj@...nel.org>,
Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Byungchul Park <byungchul.park@....com>, vlevenetz@...sol.com,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v10 1/2] printk: Make printk() completely async
On (08/18/16 12:56), Petr Mladek wrote:
> On Thu 2016-08-18 18:51:44, Sergey Senozhatsky wrote:
> > On (08/18/16 11:33), Petr Mladek wrote:
> > [..]
> > > > > 2. Force synchronous printk inside WARN()/BUG() macros.
> > > >
> > > > will it help? semaphore up() calls wake_up_process() regardless the context.
> > > > not to mention that we still may have spin_dump() enabled.
> > >
> > > Good point. That changes my preferences :-)
> > >
> > > >
> > > > > 3. Force printk_deferred() inside WARN()/BUG() macros via the per-CPU
> > > > > printk_func.
> > > > >
> > > > > It might be elegant. But we do not want this outside the scheduler
> > > > > code. Therefore we would need special variants of WARN_*_SCHED()
> > > > > BUG_*_SCHED() macros.
> > >
> > > Also we need to make sure that everything will be done on a single CPU
> > > as the printk_func is per-CPU variable.
> >
> > lib/dump_stack.c dump_stack() disables local IRQs, so we will stay
> > on the same CPU.
>
> It is less important to stay of the same CPU if you use
> printk_deferred() directly. But it us must to have if you
> use it via the printk_func per-CPU variable.
>
> The advantage of the printk_func trick is that it is transparent.
> You do not need to modify any existing functions used by WARN()/BUG()
> macros.
good point.
so something like below, perhaps. I'm less sure about
deferred BUG()/BUG_ON():
#define DEFERRED_BUG() do { \
printk_deferred_enter(); \
BUG(); \
printk_deferred_exit(); \
} while (0) \
#define DEFERRED_BUG_ON(condition) do { \
printk_deferred_enter(); \
BUG_ON(condition); \
printk_deferred_exit(); \
} while (0)
depending on .config BUG() may never return back -- passing control
to do_exit(), so printk_deferred_exit() won't be executed. thus we
probably need to have a per-cpu variable that would indicate that
we are in deferred_bug. hm... but do we really need deferred BUG()
in the first place?
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
---
include/asm-generic/bug.h | 50 +++++++++++++++++++++++++++++++++++++++++++++++
include/linux/printk.h | 6 ++++++
kernel/printk/internal.h | 18 +----------------
kernel/printk/printk.c | 42 ++++++++++++++++++++++++++++++++++-----
4 files changed, 94 insertions(+), 22 deletions(-)
diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
index 6f96247..d72ee1e 100644
--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -60,6 +60,10 @@ struct bug_entry {
* significant issues that need prompt attention if they should ever
* appear at runtime. Use the versions with printk format strings
* to provide better diagnostics.
+ *
+ * DEFERRED_WARN macros call printk_deferred() to print the messages
+ * and are meant to be used from the contexts where direct printk()
+ * can deadlock the system.
*/
#ifndef __WARN_TAINT
extern __printf(3, 4)
@@ -145,6 +149,52 @@ void __warn(const char *file, int line, void *caller, unsigned taint,
unlikely(__ret_warn_once); \
})
+#define DEFERRED_WARN_ON(condition) ({ \
+ int __ret_warn_on = !!(condition); \
+ if (unlikely(__ret_warn_on)) { \
+ printk_deferred_enter(); \
+ __WARN(); \
+ printk_deferred_exit(); \
+ } \
+ unlikely(__ret_warn_on); \
+})
+
+#define DEFERRED_WARN(condition, format...) ({ \
+ int __ret_warn_on = !!(condition); \
+ if (unlikely(__ret_warn_on)) { \
+ printk_deferred_enter(); \
+ __WARN_printf(format); \
+ printk_deferred_exit(); \
+ } \
+ unlikely(__ret_warn_on); \
+})
+
+#define DEFERRED_WARN_ON_ONCE(condition) ({ \
+ static bool __section(.data.unlikely) __warned; \
+ int __ret_warn_once = !!(condition); \
+ \
+ if (unlikely(__ret_warn_once && !__warned)) { \
+ __warned = true; \
+ printk_deferred_enter(); \
+ WARN_ON(1); \
+ printk_deferred_exit(); \
+ } \
+ unlikely(__ret_warn_once); \
+})
+
+#define DEFERRED_WARN_ONCE(condition, format...) ({ \
+ static bool __section(.data.unlikely) __warned; \
+ int __ret_warn_once = !!(condition); \
+ \
+ if (unlikely(__ret_warn_once && !__warned)) { \
+ __warned = true; \
+ printk_deferred_enter(); \
+ WARN(1, format); \
+ printk_deferred_exit(); \
+ } \
+ unlikely(__ret_warn_once); \
+})
+
#else /* !CONFIG_BUG */
#ifndef HAVE_ARCH_BUG
#define BUG() do {} while (1)
diff --git a/include/linux/printk.h b/include/linux/printk.h
index 696a56b..5142654 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -193,6 +193,9 @@ void __init setup_log_buf(int early);
__printf(1, 2) void dump_stack_set_arch_desc(const char *fmt, ...);
void dump_stack_print_info(const char *log_lvl);
void show_regs_print_info(const char *log_lvl);
+
+extern void printk_deferred_enter(void);
+extern void printk_deferred_exit(void);
#else
static inline __printf(1, 0)
int vprintk(const char *s, va_list args)
@@ -252,6 +255,9 @@ static inline void dump_stack_print_info(const char *log_lvl)
static inline void show_regs_print_info(const char *log_lvl)
{
}
+
+static void printk_deferred_enter(void) {}
+static void printk_deferred_exit(void) {}
#endif
extern asmlinkage void dump_stack(void) __cold;
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 7fd2838..70f1cf2 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -19,23 +19,12 @@
typedef __printf(1, 0) int (*printk_func_t)(const char *fmt, va_list args);
int __printf(1, 0) vprintk_default(const char *fmt, va_list args);
+DECLARE_PER_CPU(printk_func_t, printk_func);
#ifdef CONFIG_PRINTK_NMI
extern raw_spinlock_t logbuf_lock;
-/*
- * printk() could not take logbuf_lock in NMI context. Instead,
- * it temporary stores the strings into a per-CPU buffer.
- * The alternative implementation is chosen transparently
- * via per-CPU variable.
- */
-DECLARE_PER_CPU(printk_func_t, printk_func);
-static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
-{
- return this_cpu_read(printk_func)(fmt, args);
-}
-
extern atomic_t nmi_message_lost;
static inline int get_nmi_message_lost(void)
{
@@ -44,11 +33,6 @@ static inline int get_nmi_message_lost(void)
#else /* CONFIG_PRINTK_NMI */
-static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
-{
- return vprintk_default(fmt, args);
-}
-
static inline int get_nmi_message_lost(void)
{
return 0;
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 0d3e026..6e260a0 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2026,7 +2026,17 @@ asmlinkage __visible int printk(const char *fmt, ...)
int r;
va_start(args, fmt);
- r = vprintk_func(fmt, args);
+ /*
+ * printk() could not take logbuf_lock in NMI context. Instead,
+ * it temporary stores the strings into a per-CPU buffer. The
+ * alternative implementation is chosen transparently via per-CPU
+ * variable.
+ *
+ * We also switch printk_func from default to vprintk_deferred()
+ * when in DEFERRED_WARN()/DEFERRED_BUG(), because it may be
+ * unsafe to execute vprintk_default().
+ */
+ r = this_cpu_read(printk_func)(fmt, args);
va_end(args);
return r;
@@ -3037,6 +3047,17 @@ void wake_up_klogd(void)
preempt_enable();
}
+static int vprintk_deferred(const char *fmt, va_list args)
+{
+ int r;
+
+ r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, 0, fmt, args);
+ __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
+ irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
+
+ return r;
+}
+
int printk_deferred(const char *fmt, ...)
{
va_list args;
@@ -3044,16 +3065,27 @@ int printk_deferred(const char *fmt, ...)
preempt_disable();
va_start(args, fmt);
- r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, 0, fmt, args);
+ r = vprintk_deferred(fmt, args);
va_end(args);
-
- __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
- irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
preempt_enable();
return r;
}
+void printk_deferred_enter(void)
+{
+ preempt_disable();
+ this_cpu_write(printk_func, vprintk_deferred);
+}
+EXPORT_SYMBOL(printk_deferred_enter);
+
+void printk_deferred_exit(void)
+{
+ this_cpu_write(printk_func, vprintk_default);
+ preempt_enable();
+}
+EXPORT_SYMBOL(printk_deferred_exit);
+
/*
* printk rate limiting, lifted from the networking subsystem.
*
--
2.9.3
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