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    SubjectRe: suspicious RCU usage warnings in 3.3.0
    On Fri, Apr 13, 2012 at 02:55:12PM +0300, mroos@linux.ee wrote:
    > > sparc64: Eliminate obsolete __handle_softirq() function
    > >
    > > The invocation of softirq is now handled by irq_exit(), so there is no
    > > need for sparc64 to invoke it on the trap-return path. In fact, doing so
    > > is a bug because if the trap occurred in the idle loop, this invocation
    > > can result in lockdep-RCU failures. The problem is that RCU ignores idle
    > > CPUs, and the sparc64 trap-return path to the softirq handlers fails to
    > > tell RCU that the CPU must be considered non-idle while those handlers
    > > are executing. This means that RCU is ignoring any RCU read-side critical
    > > sections in those handlers, which in turn means that RCU-protected data
    > > can be yanked out from under those read-side critical sections.
    > >
    > > The shiny new lockdep-RCU ability to detect RCU read-side critical sections
    > > that RCU is ignoring located this problem.
    > >
    > > The fix is straightforward: Make sparc64 stop manually invoking the
    > > softirq handlers.
    > >
    > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    >
    > It works for me on Sun Fire V100 - no more RCU warnings under ping
    > flood.
    >
    > Tested-by: Meelis Roos <mroos@linux.ee>

    OK, if this thing is going to actually work, I guess I need to update
    the changelog to give credit where it is due, please see below.

    My main concern about my patch is my removal of this line:

    bne,pn %icc, __handle_softirq

    It is quite possible that this should instead change to look as follows:

    bne,pn %icc, __handle_preemption

    This code is under #ifndef CONFIG_SMP, so Meelis's testing would not
    reach it.

    Anyway, patch with updated changelog below.

    Thanx, Paul

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

    sparc64: Eliminate obsolete __handle_softirq() function

    The invocation of softirq is now handled by irq_exit(), so there is no
    need for sparc64 to invoke it on the trap-return path. In fact, doing so
    is a bug because if the trap occurred in the idle loop, this invocation
    can result in lockdep-RCU failures. The problem is that RCU ignores idle
    CPUs, and the sparc64 trap-return path to the softirq handlers fails to
    tell RCU that the CPU must be considered non-idle while those handlers
    are executing. This means that RCU is ignoring any RCU read-side critical
    sections in those handlers, which in turn means that RCU-protected data
    can be yanked out from under those read-side critical sections.

    The shiny new lockdep-RCU ability to detect RCU read-side critical sections
    that RCU is ignoring located this problem.

    The fix is straightforward: Make sparc64 stop manually invoking the
    softirq handlers.

    Reported-by: Meelis Roos <mroos@linux.ee>
    Suggested-by: David Miller <davem@davemloft.net>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Tested-by: Meelis Roos <mroos@linux.ee>

    diff --git a/arch/sparc/kernel/rtrap_64.S b/arch/sparc/kernel/rtrap_64.S
    index 77f1b95..9171fc2 100644
    --- a/arch/sparc/kernel/rtrap_64.S
    +++ b/arch/sparc/kernel/rtrap_64.S
    @@ -20,11 +20,6 @@

    .text
    .align 32
    -__handle_softirq:
    - call do_softirq
    - nop
    - ba,a,pt %xcc, __handle_softirq_continue
    - nop
    __handle_preemption:
    call schedule
    wrpr %g0, RTRAP_PSTATE, %pstate
    @@ -89,9 +84,7 @@ rtrap:
    cmp %l1, 0

    /* mm/ultra.S:xcall_report_regs KNOWS about this load. */
    - bne,pn %icc, __handle_softirq
    ldx [%sp + PTREGS_OFF + PT_V9_TSTATE], %l1
    -__handle_softirq_continue:
    rtrap_xcall:
    sethi %hi(0xf << 20), %l4
    and %l1, %l4, %l4


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