Re: [patch] voluntary-preempt-2.6.9-rc3-mm2-T0

From: Hugh Dickins (hugh_at_veritas.com)
Date: 10/05/04

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    Date:	Tue, 5 Oct 2004 13:07:34 +0100 (BST)
To: Ingo Molnar <mingo@redhat.com>

    On Tue, 5 Oct 2004, Ingo Molnar wrote:
    > On Tue, 5 Oct 2004, Rui Nuno Capela wrote:
    >
    > i think this is the clearest indication that there's something is
    > fundamentally wrong - ksoftirqd must never use that much CPU time on an
    > idle system.

    Please, would you try this patch below that I posted yesterday?
    At the time I thought the trylock was hardly used so not urgent.

    I've just now discovered that the standard SMP PREEMPT read_lock
    - as in do_wait's read_lock(&tasklist_lock) for example - uses it
    via one of those dreaded expansions that grep misses:
                    if (likely(_raw_##op##_trylock(lock)))

    I've been suffering the occasional leftover zombie from multiple
    kernel builds precisely since the preempt-smp.patch went in; been
    hunting it unsuccessfully in spare moments, yesterday noticed that
    bug, today realize it's probably what I've been hunting - I'm
    about to start my own tests again, can't be sure until tomorrow.

    Hugh

    The i386 and x86_64 _raw_read_trylocks in preempt-smp.patch
    are too successful: atomic_read() returns a signed integer.

    Signed-off-by: Hugh Dickins <hugh@veritas.com>

    --- 2.6.9-rc3-mm2/include/asm-i386/spinlock.h 2004-10-04 12:00:14.000000000 +0100
    +++ linux/include/asm-i386/spinlock.h 2004-10-04 18:50:32.752864600 +0100
    @@ -235,7 +235,7 @@ static inline int _raw_read_trylock(rwlo
     {
             atomic_t *count = (atomic_t *)lock;
             atomic_dec(count);
    - if (atomic_read(count) < RW_LOCK_BIAS)
    + if (atomic_read(count) >= 0)
                     return 1;
             atomic_inc(count);
             return 0;
    --- 2.6.9-rc3-mm2/include/asm-x86_64/spinlock.h 2004-10-04 12:00:15.000000000 +0100
    +++ linux/include/asm-x86_64/spinlock.h 2004-10-04 18:50:32.752864600 +0100
    @@ -236,7 +236,7 @@ static inline int _raw_read_trylock(rwlo
     {
             atomic_t *count = (atomic_t *)lock;
             atomic_dec(count);
    - if (atomic_read(count) < RW_LOCK_BIAS)
    + if (atomic_read(count) >= 0)
                     return 1;
             atomic_inc(count);
             return 0;

    -
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