为什么在写锁定挂起时可以获取读锁？

Question

根据pthread_rwlock_rdlock的POSIX文档“如果编写器没有锁定并且锁上没有写入器阻塞，则调用线程获取读锁定。”我似乎发现即使编写器被阻止也可以获取读锁定。我写的一个小样本的输出如下所示：

first reader acquiring lock...
first reader lock acquired
first writer acquiring lock...
second reader acquiring lock...
second reader lock acquired
first reader releasing lock
second reader releasing lock
first writer lock acquired
first writer releasing lock

有关我的代码有什么问题，或者我没有正确理解的问题的任何建议？顺便说说：

$ make
gcc -g -I. -I../../isilib -c -Wpointer-arith -Wall -pedantic-errors
-D_POSIX_C_SOURCE=200809L -std=c99 -g rwlock_test1.c -o rwlock_test1.o
gcc rwlock_test1.o -L../../isilib -lisi -lrt -lm -pthread -o rwlock_test1
$ uname -a
Linux BLACKHEART 3.5.0-17-generic #28-Ubuntu SMP Tue Oct 9 19:31:23 UTC 
2012 x86_64 x86_64 x86_64 GNU/Linux
$ gcc --version
gcc (Ubuntu/Linaro 4.7.2-2ubuntu1) 4.7.2

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>

#define PTH_create( a, b, c, d ) 
    (pthread_create( (a), (b), (c), (d) ) != 0 ? abort() : (void)0 )

#define PTH_join( a, b ) 
    (pthread_join( (a), (b) ) != 0 ? abort() : (void)0 )

#define PTH_rwlock_rdlock( a ) 
    (pthread_rwlock_rdlock( (a) ) != 0 ? abort() : (void)0 )

#define PTH_rwlock_wrlock( a ) 
    (pthread_rwlock_wrlock( (a) ) != 0 ? abort() : (void)0 )

#define PTH_rwlock_unlock( a ) 
    (pthread_rwlock_unlock( (a) ) != 0 ? abort() : (void)0 )

static void *firstReader(
    void *arg
);

static void *firstWriter(
    void *arg
);

static void *secondReader(
    void *arg
);

static pthread_rwlock_t rwlock  = PTHREAD_RWLOCK_INITIALIZER;

int main( int argc, char **argv )
{
    pthread_t   thr1;
    pthread_t   thr2;
    pthread_t   thr3;

    PTH_create( &thr1, NULL, firstReader, NULL );
    PTH_create( &thr2, NULL, firstWriter, NULL );
    PTH_create( &thr3, NULL, secondReader, NULL );

    PTH_join( thr1, NULL );
    PTH_join( thr2, NULL );
    PTH_join( thr3, NULL );

    return 0;
}

static void *firstReader( void *arg )
{
    printf( "first reader acquiring lock... 
" );
    PTH_rwlock_rdlock( &rwlock );
    printf( "first reader lock acquired 
" );
    sleep( 10 );
    printf( "first reader releasing lock 
" );
    PTH_rwlock_unlock( &rwlock );
    return NULL;
}

static void *firstWriter( void *arg )
{
    sleep( 2 );
    printf( "first writer acquiring lock... 
" );
    PTH_rwlock_wrlock( &rwlock );
    printf( "first writer lock acquired 
" );
    sleep( 10 );
    printf( "first writer releasing lock 
" );
    PTH_rwlock_unlock( &rwlock );
    return NULL;
}

static void *secondReader( void *arg )
{
    sleep( 5 );
    printf( "second reader acquiring lock... 
" );
    PTH_rwlock_rdlock( &rwlock );
    printf( "second reader lock acquired 
" );
    sleep( 5 );
    printf( "second reader releasing lock 
" );
    PTH_rwlock_unlock( &rwlock );
    return NULL;
}

附加信息：

从posix标准：宏_POSIX_THREAD_PRIORITY_SCHEDULING指示是否支持线程执行调度选项。来自unistd.h：“如果定义了这些符号，相应的特征总是可用的......”然后列出_POSIX_THREAD_PRIORITY_SCHEDULING。再次来自posix：“如果支持线程执行调度选项，并且锁中涉及的线程正在使用调度策略SCHED_FIFO或SCHED_RR执行，则如果编写者持有锁，调用线程将不会获取锁定。”所以我有一个程序（下面）在我的Linux系统上显示_POSIX_THREAD_PRIORITY_SCHEDULING被定义，但是我无法强制线程策略到SCHED_RR（我也尝试过SCHED_FIFO，bt没有在程序中显示）。

额外的想法？谢谢大家...

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>

#define PTH_create( a, b, c, d ) 
    (pthread_create( (a), (b), (c), (d) ) != 0 ? abort() : (void)0 )

#define PTH_join( a, b ) 
    (pthread_join( (a), (b) ) != 0 ? abort() : (void)0 )

static void *driver(
    void *arg
);

int main( int argc, char **argv )
{
    pthread_attr_t  attr;
    pthread_attr_init( &attr );
    pthread_attr_setschedpolicy( &attr, SCHED_RR );

    pthread_t   thrID;

    PTH_create( &thrID, &attr, driver, NULL );
    printf( "%ld
", _POSIX_THREAD_PRIORITY_SCHEDULING );
    struct sched_param  param;
    int                 policy;
    pthread_getschedparam( thrID, &policy, &param );
    if ( policy == SCHED_FIFO )
        puts( "SCHED_FIFO" );
    else if ( policy == SCHED_RR )
        puts( "SCHED_RR" );
    else if ( policy == SCHED_FIFO )
        puts( "SCHED_FIFO" );
    else if ( policy == SCHED_OTHER )
        puts( "SCHED_OTHER" );
    else
        puts( "eh?" );

    PTH_join( thrID, NULL );

    return 0;
}

static void *driver( void *arg )
{
    sleep( 2 );
    return NULL;
}

$ ./sched_test
200809
SCHED_OTHER

Answer 1

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Answer 2

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