lkml.org 
[lkml]   [2006]   [Sep]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH] Linux Kernel Markers 0.13 for 2.6.17
    * Mathieu Desnoyers (mathieu.desnoyers@polymtl.ca) wrote:
    > Yes, preempt_disable() has a barrier(), on gcc :
    > __asm__ __volatile__("": : :"memory").
    >
    >
    > > Either way, this doesn't prevent some otherwise unrelated
    > > non-memory-using code from being scheduled in there, which would not be
    > > executed. The gcc manual really strongly discourages jumping between
    > > inline asms, because they have basically unpredictable results.
    > >
    >
    > Ok, I will do the call in assembly then.
    >

    Before I rush on a solution too fast... I have a question for you :

    To protect code from being preempted, the macros preempt_disable and
    preempt_enable must normally be used. Logically, this macro must make sure gcc
    doesn't interleave preemptible code and non-preemptible code.

    Starting with this hypothesis, what makes gcc aware of this ? If we check
    preempt_disable (the disable call is almost symmetric) :

    linux/preempt.h:
    define add_preempt_count(val) do { preempt_count() += (val); } while (0)

    #define inc_preempt_count() add_preempt_count(1)

    #define preempt_disable() \
    do { \
    inc_preempt_count(); \
    barrier(); \
    } while (0)

    So the magic must be in the barrier() macro :

    linux/compiler-gcc.h:
    /* Optimization barrier */
    /* The "volatile" is due to gcc bugs */
    #define barrier() __asm__ __volatile__("": : :"memory")

    Which makes me think that if I put barriers around my asm, call, asm trio, no
    other code will be interleaved. Is it right ?

    Mathieu


    OpenPGP public key: http://krystal.dyndns.org:8080/key/compudj.gpg
    Key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2006-09-26 02:55    [W:0.024 / U:0.508 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site