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    SubjectRe: linux-next ppc64: RCU mods cause __might_sleep BUGs
    On Tue, 1 May 2012, Paul E. McKenney wrote:
    > > > > > On Mon, 2012-04-30 at 15:37 -0700, Hugh Dickins wrote:
    > > > > > >
    > > > > > > BUG: sleeping function called from invalid context at include/linux/pagemap.h:354
    > > > > > > in_atomic(): 0, irqs_disabled(): 0, pid: 6886, name: cc1
    > > > > > > Call Trace:
    > > > > > > [c0000001a99f78e0] [c00000000000f34c] .show_stack+0x6c/0x16c (unreliable)
    > > > > > > [c0000001a99f7990] [c000000000077b40] .__might_sleep+0x11c/0x134
    > > > > > > [c0000001a99f7a10] [c0000000000c6228] .filemap_fault+0x1fc/0x494
    > > > > > > [c0000001a99f7af0] [c0000000000e7c9c] .__do_fault+0x120/0x684
    > > > > > > [c0000001a99f7c00] [c000000000025790] .do_page_fault+0x458/0x664
    > > > > > > [c0000001a99f7e30] [c000000000005868] handle_page_fault+0x10/0x30

    Got it at last. Embarrassingly obvious. __rcu_read_lock() and
    __rcu_read_unlock() are not safe to be using __this_cpu operations,
    the cpu may change in between the rmw's read and write: they should
    be using this_cpu operations (or, I put preempt_disable/enable in the
    __rcu_read_unlock below). __this_cpus there work out fine on x86,
    which was given good instructions to use; but not so well on PowerPC.

    I've been running successfully for an hour now with the patch below;
    but I expect you'll want to consider the tradeoffs, and may choose a
    different solution.

    Hugh

    --- 3.4-rc4-next-20120427/include/linux/rcupdate.h 2012-04-28 09:26:38.000000000 -0700
    +++ testing/include/linux/rcupdate.h 2012-05-02 11:46:06.000000000 -0700
    @@ -159,7 +159,7 @@ DECLARE_PER_CPU(struct task_struct *, rc
    */
    static inline void __rcu_read_lock(void)
    {
    - __this_cpu_inc(rcu_read_lock_nesting);
    + this_cpu_inc(rcu_read_lock_nesting);
    barrier(); /* Keep code within RCU read-side critical section. */
    }

    --- 3.4-rc4-next-20120427/kernel/rcupdate.c 2012-04-28 09:26:40.000000000 -0700
    +++ testing/kernel/rcupdate.c 2012-05-02 11:44:13.000000000 -0700
    @@ -72,6 +72,7 @@ DEFINE_PER_CPU(struct task_struct *, rcu
    */
    void __rcu_read_unlock(void)
    {
    + preempt_disable();
    if (__this_cpu_read(rcu_read_lock_nesting) != 1)
    __this_cpu_dec(rcu_read_lock_nesting);
    else {
    @@ -83,13 +84,14 @@ void __rcu_read_unlock(void)
    barrier(); /* ->rcu_read_unlock_special load before assign */
    __this_cpu_write(rcu_read_lock_nesting, 0);
    }
    -#ifdef CONFIG_PROVE_LOCKING
    +#if 1 /* CONFIG_PROVE_LOCKING */
    {
    int rln = __this_cpu_read(rcu_read_lock_nesting);

    - WARN_ON_ONCE(rln < 0 && rln > INT_MIN / 2);
    + BUG_ON(rln < 0 && rln > INT_MIN / 2);
    }
    #endif /* #ifdef CONFIG_PROVE_LOCKING */
    + preempt_enable();
    }
    EXPORT_SYMBOL_GPL(__rcu_read_unlock);

    --- 3.4-rc4-next-20120427/kernel/sched/core.c 2012-04-28 09:26:40.000000000 -0700
    +++ testing/kernel/sched/core.c 2012-05-01 22:40:46.000000000 -0700
    @@ -2024,7 +2024,7 @@ asmlinkage void schedule_tail(struct tas
    {
    struct rq *rq = this_rq();

    - rcu_switch_from(prev);
    + /* rcu_switch_from(prev); */
    rcu_switch_to();
    finish_task_switch(rq, prev);

    @@ -7093,6 +7093,10 @@ void __might_sleep(const char *file, int
    "BUG: sleeping function called from invalid context at %s:%d\n",
    file, line);
    printk(KERN_ERR
    + "cpu=%d preempt_count=%x preempt_offset=%x rcu_nesting=%x nesting_save=%x\n",
    + raw_smp_processor_id(), preempt_count(), preempt_offset,
    + rcu_preempt_depth(), current->rcu_read_lock_nesting_save);
    + printk(KERN_ERR
    "in_atomic(): %d, irqs_disabled(): %d, pid: %d, name: %s\n",
    in_atomic(), irqs_disabled(),
    current->pid, current->comm);

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