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    SubjectRe: linux-next ppc64: RCU mods cause __might_sleep BUGs
    On Mon, Apr 30, 2012 at 03:37:10PM -0700, Hugh Dickins wrote:
    > Hi Paul,
    > On 3.4.0-rc4-next-20120427 and preceding linux-nexts (I've not tried
    > rc5-next-20120430 but expect it's the same), on PowerPC G5 quad with
    > CONFIG_PREEMPT=y and CONFIG_DEBUG_ATOMIC_SLEEP=y, I'm getting spurious
    > "BUG: sleeping function called from invalid context" messages from
    > __might_sleep().
    > Just once I saw such a message during startup. Once I saw such a
    > message when rebuilding the machine's kernel. Usually I see them
    > when I'm running a swapping load of kernel builds under memory
    > pressure (but that's what I'm habitually running there): perhaps
    > after a few minutes a flurry comes, then goes away, comes back
    > again later, and after perhaps a couple of hours of that I see
    > "INFO: rcu_preempt detected stalls" messages too, and soon it
    > freezes (or perhaps it's still running, but I'm so flooded by
    > messages that I reboot anyway).
    > Rather like from before you fixed schedule_tail() for your per-cpu
    > RCU mods, but not so easy to reproduce. I did a bisection and indeed
    > it converged as expected on the RCU changes. No such problem seen on
    > x86: it looks as if there's some further tweak required on PowerPC.
    > Here are my RCU config options (I don't usually have the TORTURE_TEST
    > in, but tried that for half an hour this morning, in the hope that it
    > would generate the issue: but it did not).
    > # RCU Subsystem
    > # CONFIG_RCU_FANOUT_EXACT is not set
    > # CONFIG_RCU_BOOST is not set
    > # CONFIG_SPARSE_RCU_POINTER is not set
    > # CONFIG_RCU_CPU_STALL_INFO is not set
    > Here's the message when I was rebuilding the G5's kernel:
    > 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
    > 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?

    Interesting... As you say, I saw this sort of thing before applying
    the changes to schedule_tail(), and it is all too possible that there
    is some other "sneak path" for context switches.

    Have you tried running with CONFIG_PROVE_RCU? This enables some
    additional debugging in rcu_switch_from() and rcu_switch_to() that
    helped track down the schedule_tail() problem.

    Thanx, Paul

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