linux-kernel - Re: Process Hang in __read_seqcount

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Message-ID: <CAN8Q1EeF3g8O7=Vn9ufj-3Kd9Qhg4xcTPKkcqBmbsXWW-BnHcQ@mail.gmail.com>
Date:	Wed, 24 Oct 2012 09:30:49 -0700
From:	Peter LaDow <petela@...ougs.wsu.edu>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Process Hang in __read_seqcount_begin

On Tue, Oct 23, 2012 at 9:32 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Could you try following patch ?

Thanks for the suggestion. But I have a question about the patch below.

> +       /* Note : cmpxchg() is a memory barrier, we dont need smp_wmb() */
> +       if (old != new && cmpxchg(&ptr->sequence, old, new) == old)
> +               return 1;
> +       return 0;

Looking at arch/powerpc/include/asm/system.h, cmpxchg is defined as a
call to __cmpxchg_u32 (we are 32-bit, and I presume the size is 32
bits):

static __always_inline unsigned long
__cmpxchg_u32(volatile unsigned int *p, unsigned long old, unsigned long new)
{
        unsigned int prev;

        __asm__ __volatile__ (
        PPC_RELEASE_BARRIER
"1:     lwarx   %0,0,%2         # __cmpxchg_u32\n\
        cmpw    0,%0,%3\n\
        bne-    2f\n"
        PPC405_ERR77(0,%2)
"       stwcx.  %4,0,%2\n\
        bne-    1b"
        PPC_ACQUIRE_BARRIER
        "\n\
2:"
        : "=&r" (prev), "+m" (*p)
        : "r" (p), "r" (old), "r" (new)
        : "cc", "memory");

        return prev;
}

And the interesting part is PPC_RELEASE_BARRIER and
PPC_ACQUIRE_BARRIER.  Both of these are noops in non SMP systems.
>From arch/powerpc/include/asm/sync.h:

#ifdef CONFIG_SMP
#define __PPC_ACQUIRE_BARRIER                           \
        START_LWSYNC_SECTION(97);                       \
        isync;                                          \
        MAKE_LWSYNC_SECTION_ENTRY(97, __lwsync_fixup);
#define PPC_ACQUIRE_BARRIER     "\n" stringify_in_c(__PPC_ACQUIRE_BARRIER)
#define PPC_RELEASE_BARRIER     stringify_in_c(LWSYNC) "\n"
#else
#define PPC_ACQUIRE_BARRIER
#define PPC_RELEASE_BARRIER
#endif

So, if these are noops, does this really become an atomic operation?

Thanks,
Pete
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