kmalloc returning the same adress over and over again [Linux 2.4]

I working on some code in the linux kernel (2.4) and for some reason kmalloc returns the same address (I believe it only happens after the middle of the test). I checked that no calls to kfree were made between the calls to kmalloc (i.e memory is still in use).

maybe I'm out of memory ? (kmalloc didn't return NULL...)

any ideas on how such a thing can happen ?

thanks in advance for the help!

code:

typedef struct
{
    char* buffer;
    int read_count;
    int write_count;
    struct semaphore read_sm;
    struct semaphore write_sm;
    int reader_ready;
    int writer_ready;
    int createTimeStamp;
} data_buffer_t ;

typedef struct vsf_t vsf_t;

struct vsf_t
{
    int minor;
    int type;
    int open_count;
    int waiting_pid;
    data_buffer_t* data;
    list_t proc_list;
    vsf_t* otherSide_vsf;
    int real_create_time_stamp;
};

int create_vsf(struct inode *inode, struct file *filp, struct vsf_command_parameters* parms)
{
...
    buff_data = allocate_buffer();
    if (buff_data == NULL)
    {
        kfree(this_vsfRead);
        kfree(this_vsfWrite);
        return -ENOMEM;
    }
...
}

data_buffer_t* allocate_buffer()
{
...
    data_buffer_t* this_buff = (data_buffer_t*)kmalloc(sizeof(data_buffer_t), GFP_KERNEL);
    if (this_buff == NULL)
    {
        printk( KERN_WARNING "failure at allocating memory\n" );
        return NULL;
    }
...
return this_buff;
}

*I print after every kmalloc and kfree,I'm absolutely sure that no kfree is called between kmalloc's (that return the same adress)

Answers:-

I don't know what kmalloc's data structures look like but you could imagine this happening if a previous double free caused a cycle in a linked list of buffers. Further frees could still chain on additional distinct buffers (able to be reallocated) but once those were exhausted that last buffer would be returned indefinitely.

Posted in: Embedded System