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    SubjectRe: linux-next ppc64: RCU mods cause __might_sleep BUGs
    On Wed, May 02, 2012 at 01:25:30PM -0700, Hugh Dickins wrote:
    > 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.

    Thank you very much for tracking this down!!!

    > 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.

    The thing that puzzles me about this is that the normal path through
    the scheduler would save and restore these per-CPU variables to and
    from the task structure. There must be a sneak path through the
    scheduler that I failed to account for.

    But given your good work, this should be easy for me to chase down
    even on my x86-based laptop -- just convert from __this_cpu_inc() to a
    read-inc-delay-write sequence. And check that the underlying variable
    didn't change in the meantime. And dump an ftrace if it did. ;-)

    Thank you again, Hugh!

    Thanx, Paul

    > 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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