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    SubjectRe: [-mm PATCH] RCU: debug sleep check
    This adds an overhead of a function call and local_irq_save/restore
    in the rcu reader path when CONFIG_DEBUG_SPINLOCK_SLEEP is set.
    Hopefully that is not much of a concern. Comments are welcome.
    Applies on top of my earlier patches. The full patchset is
    at http://www.hill9.org/linux/kernel/patches/2.6.18-mm1/.

    Add a debug check for rcu read-side critical section code calling
    a function that might sleep which is illegal. The check is enabled only
    if CONFIG_DEBUG_SPINLOCK_SLEEP is set.

    Signed-off-by: Dipankar Sarma <dipankar@in.ibm.com>
    ---


    include/linux/rcupdate.h | 34 ++++++++++++++++++++++++++++++----
    kernel/rcupdate.c | 33 +++++++++++++++++++++++++++++++++
    kernel/sched.c | 2 +-
    3 files changed, 64 insertions(+), 5 deletions(-)

    diff -puN include/linux/rcupdate.h~rcu-reader-sleep-check include/linux/rcupdate.h
    --- linux-2.6.18-mm1-rcu/include/linux/rcupdate.h~rcu-reader-sleep-check 2006-09-24 23:18:35.000000000 +0530
    +++ linux-2.6.18-mm1-rcu-dipankar/include/linux/rcupdate.h 2006-09-24 23:18:35.000000000 +0530
    @@ -64,6 +64,16 @@ struct rcu_head {
    (ptr)->next = NULL; (ptr)->func = NULL; \
    } while (0)

    +#ifdef CONFIG_DEBUG_SPINLOCK_SLEEP
    +extern int rcu_read_in_atomic(void);
    +extern void rcu_add_read_count(void);
    +extern void rcu_sub_read_count(void);
    +#else
    +static inline int rcu_read_in_atomic(void) { return 0;}
    +static inline void rcu_add_read_count(void) {}
    +static inline void rcu_sub_read_count(void) {}
    +#endif
    +
    /**
    * rcu_read_lock - mark the beginning of an RCU read-side critical section.
    *
    @@ -93,14 +103,22 @@ struct rcu_head {
    *
    * It is illegal to block while in an RCU read-side critical section.
    */
    -#define rcu_read_lock() __rcu_read_lock()
    +#define rcu_read_lock() \
    + do { \
    + rcu_add_read_count(); \
    + __rcu_read_lock(); \
    + } while (0)

    /**
    * rcu_read_unlock - marks the end of an RCU read-side critical section.
    *
    * See rcu_read_lock() for more information.
    */
    -#define rcu_read_unlock() __rcu_read_unlock()
    +#define rcu_read_unlock() \
    + do { \
    + __rcu_read_unlock(); \
    + rcu_sub_read_count(); \
    + } while (0)

    /*
    * So where is rcu_write_lock()? It does not exist, as there is no
    @@ -123,14 +141,22 @@ struct rcu_head {
    * can use just rcu_read_lock().
    *
    */
    -#define rcu_read_lock_bh() __rcu_read_lock_bh()
    +#define rcu_read_lock_bh() \
    + do { \
    + rcu_add_read_count(); \
    + __rcu_read_lock_bh(); \
    + } while (0)

    /**
    * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
    *
    * See rcu_read_lock_bh() for more information.
    */
    -#define rcu_read_unlock_bh() __rcu_read_unlock_bh()
    +#define rcu_read_unlock_bh() \
    + do { \
    + __rcu_read_unlock_bh(); \
    + rcu_sub_read_count(); \
    + } while (0)

    /**
    * rcu_dereference - fetch an RCU-protected pointer in an
    diff -puN kernel/rcupdate.c~rcu-reader-sleep-check kernel/rcupdate.c
    --- linux-2.6.18-mm1-rcu/kernel/rcupdate.c~rcu-reader-sleep-check 2006-09-24 23:18:35.000000000 +0530
    +++ linux-2.6.18-mm1-rcu-dipankar/kernel/rcupdate.c 2006-09-24 23:18:35.000000000 +0530
    @@ -127,5 +127,38 @@ void __init rcu_init(void)
    __rcu_init();
    }

    +#ifdef CONFIG_DEBUG_SPINLOCK_SLEEP
    +DEFINE_PER_CPU(int, rcu_read_count);
    +int rcu_read_in_atomic(void)
    +{
    + int val;
    + int cpu = get_cpu();
    + val = per_cpu(rcu_read_count, cpu);
    + put_cpu();
    + return val;
    +}
    +
    +void rcu_add_read_count(void)
    +{
    + int cpu, flags;
    + local_irq_save(flags);
    + cpu = smp_processor_id();
    + per_cpu(rcu_read_count, cpu)++;
    + local_irq_restore(flags);
    +}
    +
    +void rcu_sub_read_count(void)
    +{
    + int cpu, flags;
    + local_irq_save(flags);
    + cpu = smp_processor_id();
    + per_cpu(rcu_read_count, cpu)--;
    + local_irq_restore(flags);
    +}
    +EXPORT_SYMBOL_GPL(rcu_read_in_atomic);
    +EXPORT_SYMBOL_GPL(rcu_add_read_count);
    +EXPORT_SYMBOL_GPL(rcu_sub_read_count);
    +#endif
    +
    EXPORT_SYMBOL_GPL(rcu_barrier);
    EXPORT_SYMBOL_GPL(synchronize_rcu);
    diff -puN kernel/sched.c~rcu-reader-sleep-check kernel/sched.c
    --- linux-2.6.18-mm1-rcu/kernel/sched.c~rcu-reader-sleep-check 2006-09-24 23:18:35.000000000 +0530
    +++ linux-2.6.18-mm1-rcu-dipankar/kernel/sched.c 2006-09-24 23:18:35.000000000 +0530
    @@ -6974,7 +6974,7 @@ void __might_sleep(char *file, int line)
    #ifdef in_atomic
    static unsigned long prev_jiffy; /* ratelimiting */

    - if ((in_atomic() || irqs_disabled()) &&
    + if ((in_atomic() || irqs_disabled() || rcu_read_in_atomic()) &&
    system_state == SYSTEM_RUNNING && !oops_in_progress) {
    if (time_before(jiffies, prev_jiffy + HZ) && prev_jiffy)
    return;
    _
    -
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