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    SubjectRe: [PATCH v2 1/1] printk: suppress rcu stall warnings caused by slow console devices
    On Thu, Nov 11, 2021 at 04:59:04PM -0300, Wander Lairson Costa wrote:
    > If we have a reasonable large dataset to flush in the printk ring
    > buffer in the presence of a slow console device (like a serial port
    > with a low baud rate configured), the RCU stall detector may report
    > warnings.
    >
    > This patch suppresses RCU stall warnings while flushing the ring buffer
    > to the console.
    >
    > Signed-off-by: Wander Lairson Costa <wander@redhat.com>

    From an RCU perspective:

    Acked-by: Paul E. McKenney <paulmck@kernel.org>

    > ---
    > kernel/printk/printk.c | 24 +++++++++++++++++++++++-
    > 1 file changed, 23 insertions(+), 1 deletion(-)
    >
    > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
    > index cbc35d586afb..42b622454314 100644
    > --- a/kernel/printk/printk.c
    > +++ b/kernel/printk/printk.c
    > @@ -2588,6 +2588,20 @@ static inline int can_use_console(void)
    > return cpu_online(raw_smp_processor_id()) || have_callable_console();
    > }
    >
    > +extern int rcu_cpu_stall_suppress;
    > +
    > +static void rcu_console_stall_suppress(void)
    > +{
    > + if (!rcu_cpu_stall_suppress)
    > + rcu_cpu_stall_suppress = 4;
    > +}
    > +
    > +static void rcu_console_stall_unsuppress(void)
    > +{
    > + if (rcu_cpu_stall_suppress == 4)
    > + rcu_cpu_stall_suppress = 0;
    > +}
    > +
    > /**
    > * console_unlock - unlock the console system
    > *
    > @@ -2634,6 +2648,9 @@ void console_unlock(void)
    > * and cleared after the "again" goto label.
    > */
    > do_cond_resched = console_may_schedule;
    > +
    > + rcu_console_stall_suppress();
    > +
    > again:
    > console_may_schedule = 0;
    >
    > @@ -2645,6 +2662,7 @@ void console_unlock(void)
    > if (!can_use_console()) {
    > console_locked = 0;
    > up_console_sem();
    > + rcu_console_stall_unsuppress();
    > return;
    > }
    >
    > @@ -2716,8 +2734,10 @@ void console_unlock(void)
    >
    > handover = console_lock_spinning_disable_and_check();
    > printk_safe_exit_irqrestore(flags);
    > - if (handover)
    > + if (handover) {
    > + rcu_console_stall_unsuppress();
    > return;
    > + }
    >
    > if (do_cond_resched)
    > cond_resched();
    > @@ -2738,6 +2758,8 @@ void console_unlock(void)
    > retry = prb_read_valid(prb, next_seq, NULL);
    > if (retry && console_trylock())
    > goto again;
    > +
    > + rcu_console_stall_unsuppress();
    > }
    > EXPORT_SYMBOL(console_unlock);
    >
    > --
    > 2.27.0
    >

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