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    SubjectRe: linux-next ppc64: RCU mods cause __might_sleep BUGs
    On Tue, 1 May 2012, Benjamin Herrenschmidt 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
    >
    > Hrm ... in_atomic and irqs_disabled are both 0 ... so yeah it smells
    > like a preempt count problem... odd.
    >
    > Did you get a specific bisect target yet ?

    Oh, I went as far as we need, I think, but I didn't bother quite to
    complete it because, once in that area, we know the schedule_tail()
    omission would muddy the waters: the tail of my bisect log was

    # bad: [e798cf3385d3aa7c84afa65677eb92e0c0876dfd] rcu: Add exports for per-CPU variables used for inlining
    git bisect bad e798cf3385d3aa7c84afa65677eb92e0c0876dfd
    # good: [90aec3b06194393c909e3e5a47b6ed99bb8caba5] rcu: Make exit_rcu() more precise and consolidate
    git bisect good 90aec3b06194393c909e3e5a47b6ed99bb8caba5

    from which I concluded that the patch responsible is

    commit ab8fc41a8545d40a4b58d745876c125af72a8a5c
    Author: Paul E. McKenney <paul.mckenney@linaro.org>
    Date: Fri Apr 13 14:32:01 2012 -0700

    rcu: Move __rcu_read_lock() and __rcu_read_unlock() to per-CPU variables

    This commit is another step towards inlinable __rcu_read_lock() and
    __rcu_read_unlock() functions for preemptible RCU. This keeps these two
    functions out of line, but switches them to use the per-CPU variables
    that are required to export their definitions without requiring that
    all RCU users include sched.h. These per-CPU variables are saved and
    restored at context-switch time.

    >
    > Cheers,
    > Ben.
    >
    > > 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
    > >
    > > I've plenty more examples, most of them from page faults or from kswapd;
    > > but I don't think there's any more useful information in them.
    > >
    > > Anything I can try later on?

    I'd forgotten about CONFIG_PROVE_RCU (and hadn't been using PROVE_LOCKING
    on that machine), but following Paul's suggestion have now turned them on.

    But not much light shed, I'm afraid. Within minutes it showed a trace
    exactly like the one above, but the only thing PROVE_LOCKING and PROVE_RCU
    had to say was that we're holding mmap_sem at that point, which is no
    surprise and not a problem, just something lockdep is right to note.

    That was an isolated occurrence, it continued quietly for maybe 20 minutes,
    then output lots to the console screen - but garbled in a way I've not
    seen before - the 0s came out just right (or perhaps all the hex digits
    were being shown as 0s), but most everything else was grayly unreadable.
    Then after a few minutes, spontaneously rebooted.

    Perhaps I should remind myself of netdump; but getting the trace above
    without complaint from PROVE_RCU tells me that it is not helping.

    Hugh


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