On 2016年04月08日 01:31, Sergey Senozhatsky wrote: > Change `synchronous' printk param to be RW, so user space > can change printk mode back and forth to/from sync mode > (which is considered to be more reliable). > > Signed-off-by: Sergey Senozhatsky <sergey.senozhat...@gmail.com> > --- > kernel/printk/printk.c | 63 > ++++++++++++++++++++++++++++++++++++++++++-------- > 1 file changed, 53 insertions(+), 10 deletions(-) > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index 89f5441..5ae6f73 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -288,9 +288,6 @@ static u32 log_buf_len = __LOG_BUF_LEN; > > /* Control whether printing to console must be synchronous. */ > static bool __read_mostly printk_sync = true; > -module_param_named(synchronous, printk_sync, bool, S_IRUGO); > -MODULE_PARM_DESC(synchronous, "make printing to console synchronous"); > - > /* Printing kthread for async printk */ > static struct task_struct *printk_kthread; > /* When `true' printing thread has messages to print */ > @@ -1785,7 +1782,7 @@ asmlinkage int vprintk_emit(int facility, int level, > * operate in sync mode once panic() occurred. > */ > if (console_loglevel != CONSOLE_LOGLEVEL_MOTORMOUTH && > - printk_kthread) { > + !printk_sync && printk_kthread) { > /* Offload printing to a schedulable context. */ > printk_kthread_need_flush_console = true; > wake_up_process(printk_kthread); > @@ -2757,6 +2754,16 @@ static int __init printk_late_init(void) > late_initcall(printk_late_init); > > #if defined CONFIG_PRINTK > +/* > + * kernel_param_ops.set is called from two places: > + * - from parse_args()->printk_sync_set(), when we can't kthread_run(), so > + * we just set the param value. The actual initalization happens later, > + * from late_initcall(). > + * > + * - from user space via sysfs knob; we can kthread_run() there (if needed). > + */ > +static bool printk_initcall_done; > + > static int printk_kthread_func(void *data) > { > while (1) { > @@ -2780,11 +2787,7 @@ static int printk_kthread_func(void *data) > return 0; > } > > -/* > - * Init async printk via late_initcall, after core/arch/device/etc. > - * initialization. > - */ > -static int __init init_printk_kthread(void) > +static int __init_printk_kthread(void) > { > struct task_struct *thread; > struct sched_param param = { > @@ -2794,6 +2797,9 @@ static int __init init_printk_kthread(void) > if (printk_sync) > return 0; > > + if (printk_kthread) > + return 0; > + > thread = kthread_run(printk_kthread_func, NULL, "printk"); > if (IS_ERR(thread)) { > pr_err("printk: unable to create printing thread\n"); > @@ -2805,6 +2811,43 @@ static int __init init_printk_kthread(void) > printk_kthread = thread; > return 0; > } > + > +static int printk_sync_set(const char *val, const struct kernel_param *kp) > +{ > + static DEFINE_MUTEX(printk_sync_lock); > + int ret; > + > + mutex_lock(&printk_sync_lock); > + ret = param_set_bool(val, kp); > + if (ret) { > + mutex_unlock(&printk_sync_lock); > + return ret; > + } > + > + if (printk_initcall_done) > + ret = __init_printk_kthread(); > + mutex_unlock(&printk_sync_lock); > + return ret; > +} > + > +static const struct kernel_param_ops param_ops_printk_sync = { > + .set = printk_sync_set, > + .get = param_get_bool, > +}; > + > +module_param_cb(synchronous, ¶m_ops_printk_sync, &printk_sync, > + S_IRUGO | S_IWUSR); > +MODULE_PARM_DESC(synchronous, "make printing to console synchronous"); > + > +/* > + * Init async printk via late_initcall, after core/arch/device/etc. > + * initialization. > + */ > +static __init int init_printk_kthread(void) > +{ > + printk_initcall_done = true; > + return __init_printk_kthread(); hello,
One confusion, Why not use a lock to protect __init_printk_kthread from parallel call? Otherwise I think there is a race. But for simplicity, maybe you could write codes as below. + int ret = __init_printk_kthread(); + printk_initcall_done = true; + return ret; In my opinion, using a lock is better. thanks xinhui > +} > late_initcall(init_printk_kthread); > > /* > @@ -2820,7 +2863,7 @@ static void wake_up_klogd_work_func(struct irq_work > *irq_work) > int pending = __this_cpu_xchg(printk_pending, 0); > > if (pending & PRINTK_PENDING_OUTPUT) { > - if (printk_kthread) { > + if (!printk_sync && printk_kthread) { > wake_up_process(printk_kthread); > } else { > /* >