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SubjectRe: [RFC][PATCH -next 1/2] printk: move can_use_console out of console_trylock_for_printk
On Thu 2016-01-14 13:57:22, Sergey Senozhatsky wrote:
> vprintk_emit() disables preemption around console_trylock_for_printk()
> and console_unlock() calls for a strong reason -- can_use_console()
> check. The thing is that vprintl_emit() can be called on a CPU that
> is not fully brought up yet (!cpu_online()), which potentially can
> cause problems if console driver accesses per-cpu data. A console
> driver can explicitly state that it's safe to call it from !online
> cpu by setting CON_ANYTIME bit in console ->flags. That's why for
> !cpu_online() can_use_console() iterates all the console to find out
> if there is a CON_ANYTIME console, otherwise console_unlock() must be
> avoided.
>
> call_console_drivers(), called from console_cont_flush() and
> console_unlock(), does the same test during for_each_console() loop.
> However, we can have the following corner case. Assume that we have 2
> cpus -- CPU0 is online, CPU1 is !online; and no CON_ANYTIME consoles
> available.
>
> CPU0 online CPU1 !online
> console_trylock()
> ...
> console_unlock()

Please, where this console_unlock() comes from? If I get this
correctly, this CPU is not online and no CON_ANYTIME console exists
=> can_use_console() fails
=> console_trylock() fails
=> console_unlock() is not called from vprintk_emit().

Best Regards,
Petr

> console_cont_flush
> spin_lock logbuf_lock
> if (!cont.len) {
> spin_unlock logbuf_lock
> return
> }
> for (;;) {
> vprintk_emit
> spin_lock logbuf_lock
> log_store
> spin_unlock logbuf_lock
> spin_lock logbuf_lock
> !console_trylock_for_printk msg_print_text
> return console_idx = log_next()
> console_seq++
> console_prev = msg->flags
> spin_unlock logbuf_lock
>
> call_console_drivers()
> for_each_console(con) {
> if (!cpu_online() &&
> !(con->flags & CON_ANYTIME))
> continue;
> }
> /*
> * no message printed, we lost it
> */
> vprintk_emit
> spin_lock logbuf_lock
> log_store
> spin_unlock logbuf_lock
> !console_trylock_for_printk
> return
> /*
> * go to the beginning of the loop,
> * find out there are new messages,
> * lose it
> */
> }
>
> This patch moves can_use_console() check out of
> console_trylock_for_printk(). Instead it calls it in two
> non-preemptible sections:
> -- console_cont_flush()
> can_use_console() call happens with spin_lock logbuf_lock
> taken and local IRQs disabled, local IRQs are not enabled
> until call_console_drivers() returns.
>
> -- console_unlock()
> can_use_console() call happens with spin_lock logbuf_lock
> taken and local IRQs disabled, local IRQs are not enabled
> until call_console_drivers() returns.
>
> In both cases, we need to know that call_console_drivers() call
> was aborted because of !can_use_console() and we must exit
> console_unlock() function.
>
> This also lets to drop '!cpu_online() && !(con->flags & CON_ANYTIME)'
> test from call_console_drivers().
>
> At the same time this patch serves as a preparation to enable
> cond_resched for 'in-direct' console_unlock() callers -- the ones
> that come from vprintk_emit(). The 'direct' ones -- the ones that
> do console_lock()/console_unlock() -- already can cond_resched.
>
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> ---
> kernel/printk/printk.c | 71 +++++++++++++++++++++++++-------------------------
> 1 file changed, 36 insertions(+), 35 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 7ebcfea..5cc0ce6 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1452,9 +1452,6 @@ static void call_console_drivers(int level,
> continue;
> if (!con->write)
> continue;
> - if (!cpu_online(smp_processor_id()) &&
> - !(con->flags & CON_ANYTIME))
> - continue;
> if (con->flags & CON_EXTENDED)
> con->write(con, ext_text, ext_len);
> else
> @@ -1500,18 +1497,6 @@ static int have_callable_console(void)
> }
>
> /*
> - * Can we actually use the console at this time on this cpu?
> - *
> - * Console drivers may assume that per-cpu resources have been allocated. So
> - * unless they're explicitly marked as being able to cope (CON_ANYTIME) don't
> - * call them until this CPU is officially up.
> - */
> -static inline int can_use_console(unsigned int cpu)
> -{
> - return cpu_online(cpu) || have_callable_console();
> -}
> -
> -/*
> * Try to get console ownership to actually show the kernel
> * messages from a 'printk'. Return true (and with the
> * console_lock held, and 'console_locked' set) if it
> @@ -1519,21 +1504,7 @@ static inline int can_use_console(unsigned int cpu)
> */
> static int console_trylock_for_printk(void)
> {
> - unsigned int cpu = smp_processor_id();
> -
> - if (!console_trylock())
> - return 0;
> - /*
> - * If we can't use the console, we need to release the console
> - * semaphore by hand to avoid flushing the buffer. We need to hold the
> - * console semaphore in order to do this test safely.
> - */
> - if (!can_use_console(cpu)) {
> - console_locked = 0;
> - up_console_sem();
> - return 0;
> - }
> - return 1;
> + return console_trylock();
> }
>
> int printk_delay_msec __read_mostly;
> @@ -2177,10 +2148,23 @@ int is_console_locked(void)
> return console_locked;
> }
>
> -static void console_cont_flush(char *text, size_t size)
> +/*
> + * Can we actually use the console at this time on this cpu?
> + *
> + * Console drivers may assume that per-cpu resources have been allocated. So
> + * unless they're explicitly marked as being able to cope (CON_ANYTIME) don't
> + * call them until this CPU is officially up.
> + */
> +static inline int can_use_console(void)
> +{
> + return cpu_online(smp_processor_id()) || have_callable_console();
> +}
> +
> +static bool console_cont_flush(char *text, size_t size)
> {
> unsigned long flags;
> size_t len;
> + bool ret = true;
>
> raw_spin_lock_irqsave(&logbuf_lock, flags);
>
> @@ -2195,15 +2179,21 @@ static void console_cont_flush(char *text, size_t size)
> if (console_seq < log_next_seq && !cont.cons)
> goto out;
>
> + if (!can_use_console()) {
> + ret = false;
> + goto out;
> + }
> +
> len = cont_print_text(text, size);
> raw_spin_unlock(&logbuf_lock);
> stop_critical_timings();
> call_console_drivers(cont.level, NULL, 0, text, len);
> start_critical_timings();
> local_irq_restore(flags);
> - return;
> + return ret;
> out:
> raw_spin_unlock_irqrestore(&logbuf_lock, flags);
> + return ret;
> }
>
> /**
> @@ -2227,7 +2217,7 @@ void console_unlock(void)
> static u64 seen_seq;
> unsigned long flags;
> bool wake_klogd = false;
> - bool do_cond_resched, retry;
> + bool do_cond_resched, retry, aborted = false;
>
> if (console_suspended) {
> up_console_sem();
> @@ -2248,7 +2238,12 @@ void console_unlock(void)
> console_may_schedule = 0;
>
> /* flush buffered message fragment immediately to console */
> - console_cont_flush(text, sizeof(text));
> + if (!console_cont_flush(text, sizeof(text))) {
> + console_locked = 0;
> + up_console_sem();
> + return;
> + }
> +
> again:
> for (;;) {
> struct printk_log *msg;
> @@ -2257,6 +2252,12 @@ again:
> int level;
>
> raw_spin_lock_irqsave(&logbuf_lock, flags);
> +
> + if (!can_use_console()) {
> + aborted = true;
> + break;
> + }
> +
> if (seen_seq != log_next_seq) {
> wake_klogd = true;
> seen_seq = log_next_seq;
> @@ -2337,7 +2338,7 @@ skip:
> * flush, no worries.
> */
> raw_spin_lock(&logbuf_lock);
> - retry = console_seq != log_next_seq;
> + retry = !aborted && (console_seq != log_next_seq);
> raw_spin_unlock_irqrestore(&logbuf_lock, flags);
>
> if (retry && console_trylock())
> --
> 2.7.0
>

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