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    SubjectRe: mmotm 2014-04-24-13-07 uploaded
    On Sat, Apr 26, 2014 at 11:26:50PM -0700, Paul E. McKenney wrote:
    > On Fri, Apr 25, 2014 at 01:52:30PM -0700, Randy Dunlap wrote:
    > > On 04/25/14 13:44, Randy Dunlap wrote:
    > > > On 04/25/14 07:04, Rik van Riel wrote:
    > > >> On Thu, 24 Apr 2014 15:40:29 -0700
    > > >> Randy Dunlap <rdunlap@infradead.org> wrote:
    > > >>
    > > >>
    > > >>> uml on x86_64 defconfig:
    > > >>>
    > > >>> drivers/built-in.o: In function `__handle_sysrq':
    > > >>> drivers/tty/sysrq.c:514: undefined reference to `rcu_sysrq_start'
    > > >>> drivers/tty/sysrq.c:558: undefined reference to `rcu_sysrq_end'
    > > >>
    > > >> Randy, does the patch below fix it?
    > >
    > > CONFIG_TINY_RCU=y
    > >
    > > tiny.c does not provide these functions... Paul...
    > >
    > > > Hm, no, it does not. I'll look into it.
    > > >
    > > >> Mike, this patch should also address your concerns.
    > > >>
    > > >> Andrew, this can be a -fix patch for the sysctl-rcu patch,
    > > >> assuming it fixes things :)
    > > >>
    > > >> Signed-off-by: Rik van Riel <riel@redhat.com
    > > >> ---
    > > >> drivers/tty/sysrq.c | 1 +
    > > >> kernel/rcu/update.c | 3 ++-
    > > >> 2 files changed, 3 insertions(+), 1 deletion(-)
    > > >>
    > > >> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
    > > >> index dd53af9..0867433 100644
    > > >> --- a/drivers/tty/sysrq.c
    > > >> +++ b/drivers/tty/sysrq.c
    > > >> @@ -46,6 +46,7 @@
    > > >> #include <linux/jiffies.h>
    > > >> #include <linux/syscalls.h>
    > > >> #include <linux/of.h>
    > > >> +#include <linux/rcupdate.h>
    > > >>
    > > >> #include <asm/ptrace.h>
    > > >> #include <asm/irq_regs.h>
    > > >> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
    > > >> index 2ac3289..d22309c 100644
    > > >> --- a/kernel/rcu/update.c
    > > >> +++ b/kernel/rcu/update.c
    > > >> @@ -322,7 +322,8 @@ int rcu_jiffies_till_stall_check(void)
    > > >>
    > > >> void rcu_sysrq_start(void)
    > > >> {
    > > >> - rcu_cpu_stall_suppress = 2;
    > > >> + if (!rcu_cpu_stall_suppress)
    > > >> + rcu_cpu_stall_suppress = 2;
    > > >> }
    > > >>
    > > >> void rcu_sysrq_end(void)
    >
    > You would think that I would have learned by now that RCU requires a bit
    > more care... Anyway, patch containing only the APIs that -should- work
    > is below. Currently testing it on the usual set of configs.

    Right... Adding a "static inline" would help, wouldn't it? Updated
    patch below.

    Thanx, Paul

    ------------------------------------------------------------------------

    rcu: Provide API to suppress stall warnings while sysrc runs

    Some sysrq handlers can run for a long time, because they dump a lot
    of data onto a serial console. Having RCU stall warnings pop up in
    the middle of them only makes the problem worse.

    This commit provides rcu_sysrq_start() and rcu_sysrq_end() APIs to
    temporarily suppress RCU CPU stall warnings while a sysrq request is
    handled.

    Signed-off-by: Rik van Riel <riel@redhat.com>
    [ paulmck: Fix TINY_RCU build error. ]
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
    index a6f2664a1b77..ca6fe55913b7 100644
    --- a/include/linux/rcupdate.h
    +++ b/include/linux/rcupdate.h
    @@ -248,6 +248,18 @@ void rcu_idle_exit(void);
    void rcu_irq_enter(void);
    void rcu_irq_exit(void);

    +#ifdef CONFIG_RCU_STALL_COMMON
    +void rcu_sysrq_start(void);
    +void rcu_sysrq_end(void);
    +#else /* #ifdef CONFIG_RCU_STALL_COMMON */
    +static inline void rcu_sysrq_start(void)
    +{
    +}
    +static inline void rcu_sysrq_end(void)
    +{
    +}
    +#endif /* #else #ifdef CONFIG_RCU_STALL_COMMON */
    +
    #ifdef CONFIG_RCU_USER_QS
    void rcu_user_enter(void);
    void rcu_user_exit(void);
    diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
    index ed7a0d72562c..a2aeb4df0f60 100644
    --- a/kernel/rcu/update.c
    +++ b/kernel/rcu/update.c
    @@ -320,6 +320,18 @@ int rcu_jiffies_till_stall_check(void)
    return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
    }

    +void rcu_sysrq_start(void)
    +{
    + if (!rcu_cpu_stall_suppress)
    + rcu_cpu_stall_suppress = 2;
    +}
    +
    +void rcu_sysrq_end(void)
    +{
    + if (rcu_cpu_stall_suppress == 2)
    + rcu_cpu_stall_suppress = 0;
    +}
    +
    static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
    {
    rcu_cpu_stall_suppress = 1;


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