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    SubjectRe: [1/4] rcu: Detect uses of rcu read side in extended quiescent states
    On Sun, Jun 05, 2011 at 10:44:33PM -0500, Milton Miller wrote:
    > On Mon, 06 Jun 2011 about 03:10:55 -0000, Frederic Weisbecker wrote:
    > > @@ -3994,8 +3995,13 @@ void lockdep_rcu_dereference(const char *file, const int line)
    > > printk("\n===================================================\n");
    > > printk( "[ INFO: suspicious rcu_dereference_check() usage. ]\n");
    > > printk( "---------------------------------------------------\n");
    > > - printk("%s:%d invoked rcu_dereference_check() without protection!\n",
    > > - file, line);
    > > + printk("%s:%d invoked rcu_dereference_check() ", file, line);
    > > +
    > > + if (type == RCU_WARN_UNPROTECTED)
    > > + printk("without protection!\n");
    > > + else if (type == RCU_WARN_EXT_QS)
    > > + printk("while in RCU extended quiescent state!\n");
    > > +
    > > printk("\nother info that might help us debug this:\n\n");
    > > printk("\nrcu_scheduler_active = %d, debug_locks = %d\n", rcu_scheduler_active, debug_locks);
    > > lockdep_print_held_locks(curr);
    >
    > Can we keep the above in one printk? That way the printing is
    > guaranteed to come out on one line. Probably the easiest way would
    > be add char *why = "" then assign a string based on the current
    > conditions. Do all of that before the first printk which gets the
    > a %s added.

    I have the following queued the -rcu tree which does add the string.

    Frederic, would it be possible to base on this patch?

    Thanx, Paul

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

    commit c15d76f26712bd5228aa0c6af7a7e7c492a812c9
    Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Date: Tue May 24 08:31:09 2011 -0700

    rcu: Restore checks for blocking in RCU read-side critical sections

    Long ago, using TREE_RCU with PREEMPT would result in "scheduling
    while atomic" diagnostics if you blocked in an RCU read-side critical
    section. However, PREEMPT now implies TREE_PREEMPT_RCU, which defeats
    this diagnostic. This commit therefore adds a replacement diagnostic
    based on PROVE_RCU.

    Because rcu_lockdep_assert() and lockdep_rcu_dereference() are now being
    used for things that have nothing to do with rcu_dereference(), rename
    lockdep_rcu_dereference() to lockdep_rcu_suspicious() and add a third
    argument that is a string indicating what is suspicious. This third
    argument is passed in from a new third argument to rcu_lockdep_assert().
    Update all calls to rcu_lockdep_assert() to add an informative third
    argument.

    Finally, add a pair of rcu_lockdep_assert() calls from within
    rcu_note_context_switch(), one complaining if a context switch occurs
    in an RCU-bh read-side critical section and another complaining if a
    context switch occurs in an RCU-sched read-side critical section.
    These are present only if the PROVE_RCU kernel parameter is enabled.

    Again, you must enable PROVE_RCU to see these new diagnostics. But you
    are enabling PROVE_RCU to check out new RCU uses in any case, aren't you?

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
    index 4aef1dd..bfa66e7 100644
    --- a/include/linux/lockdep.h
    +++ b/include/linux/lockdep.h
    @@ -545,7 +545,7 @@ do { \
    #endif

    #ifdef CONFIG_PROVE_RCU
    -extern void lockdep_rcu_dereference(const char *file, const int line);
    +void lockdep_rcu_suspicious(const char *file, const int line, const char *s);
    #endif

    #endif /* __LINUX_LOCKDEP_H */
    diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
    index 58b13f1..fb2933d 100644
    --- a/include/linux/rcupdate.h
    +++ b/include/linux/rcupdate.h
    @@ -297,19 +297,20 @@ extern int rcu_my_thread_group_empty(void);
    /**
    * rcu_lockdep_assert - emit lockdep splat if specified condition not met
    * @c: condition to check
    + * @s: informative message
    */
    -#define rcu_lockdep_assert(c) \
    +#define rcu_lockdep_assert(c, s) \
    do { \
    static bool __warned; \
    if (debug_lockdep_rcu_enabled() && !__warned && !(c)) { \
    __warned = true; \
    - lockdep_rcu_dereference(__FILE__, __LINE__); \
    + lockdep_rcu_suspicious(__FILE__, __LINE__, s); \
    } \
    } while (0)

    #else /* #ifdef CONFIG_PROVE_RCU */

    -#define rcu_lockdep_assert(c) do { } while (0)
    +#define rcu_lockdep_assert(c, s) do { } while (0)

    #endif /* #else #ifdef CONFIG_PROVE_RCU */

    @@ -338,14 +339,16 @@ extern int rcu_my_thread_group_empty(void);
    #define __rcu_dereference_check(p, c, space) \
    ({ \
    typeof(*p) *_________p1 = (typeof(*p)*__force )ACCESS_ONCE(p); \
    - rcu_lockdep_assert(c); \
    + rcu_lockdep_assert(c, "suspicious rcu_dereference_check()" \
    + " usage"); \
    rcu_dereference_sparse(p, space); \
    smp_read_barrier_depends(); \
    ((typeof(*p) __force __kernel *)(_________p1)); \
    })
    #define __rcu_dereference_protected(p, c, space) \
    ({ \
    - rcu_lockdep_assert(c); \
    + rcu_lockdep_assert(c, "suspicious rcu_dereference_protected()" \
    + " usage"); \
    rcu_dereference_sparse(p, space); \
    ((typeof(*p) __force __kernel *)(p)); \
    })
    @@ -359,7 +362,9 @@ extern int rcu_my_thread_group_empty(void);
    #define __rcu_dereference_index_check(p, c) \
    ({ \
    typeof(p) _________p1 = ACCESS_ONCE(p); \
    - rcu_lockdep_assert(c); \
    + rcu_lockdep_assert(c, \
    + "suspicious rcu_dereference_index_check()" \
    + " usage"); \
    smp_read_barrier_depends(); \
    (_________p1); \
    })
    diff --git a/kernel/lockdep.c b/kernel/lockdep.c
    index 53a6895..9789028 100644
    --- a/kernel/lockdep.c
    +++ b/kernel/lockdep.c
    @@ -3798,7 +3798,7 @@ void lockdep_sys_exit(void)
    }
    }

    -void lockdep_rcu_dereference(const char *file, const int line)
    +void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
    {
    struct task_struct *curr = current;

    @@ -3807,9 +3807,10 @@ void lockdep_rcu_dereference(const char *file, const int line)
    return;
    #endif /* #ifdef CONFIG_PROVE_RCU_REPEATEDLY */
    /* Note: the following can be executed concurrently, so be careful. */
    - printk("\n===================================================\n");
    - printk( "[ INFO: suspicious rcu_dereference_check() usage. ]\n");
    - printk( "---------------------------------------------------\n");
    + printk("\n===============================\n");
    + printk( "[ INFO: suspicious RCU usage. ]\n");
    + printk( "-------------------------------\n");
    + printk( "%s\n", s);
    printk("%s:%d invoked rcu_dereference_check() without protection!\n",
    file, line);
    printk("\nother info that might help us debug this:\n\n");
    @@ -3818,4 +3819,4 @@ void lockdep_rcu_dereference(const char *file, const int line)
    printk("\nstack backtrace:\n");
    dump_stack();
    }
    -EXPORT_SYMBOL_GPL(lockdep_rcu_dereference);
    +EXPORT_SYMBOL_GPL(lockdep_rcu_suspicious);
    diff --git a/kernel/pid.c b/kernel/pid.c
    index 57a8346..a7577b3 100644
    --- a/kernel/pid.c
    +++ b/kernel/pid.c
    @@ -419,7 +419,9 @@ EXPORT_SYMBOL(pid_task);
    */
    struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
    {
    - rcu_lockdep_assert(rcu_read_lock_held());
    + rcu_lockdep_assert(rcu_read_lock_held(),
    + "find_task_by_pid_ns() needs rcu_read_lock()"
    + " protection");
    return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
    }

    diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    index 88547c8..8b4b3da 100644
    --- a/kernel/rcutree.c
    +++ b/kernel/rcutree.c
    @@ -153,6 +153,12 @@ void rcu_bh_qs(int cpu)
    */
    void rcu_note_context_switch(int cpu)
    {
    + rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map),
    + "Illegal context switch in RCU-bh"
    + " read-side critical section");
    + rcu_lockdep_assert(!lock_is_held(&rcu_sched_lock_map),
    + "Illegal context switch in RCU-sched"
    + " read-side critical section");
    rcu_sched_qs(cpu);
    rcu_preempt_note_context_switch(cpu);
    }

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