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Re: [BUG] ext2/3/4: dio reads stale data when we do some append dio writ

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [BUG] ext2/3/4: dio reads stale data when we do some append dio writes
From: Zheng Liu <gnehzuil.liu@xxxxxxxxx>
Date: Tue, 19 Nov 2013 19:51:22 +0800
Cc: linux-ext4@xxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
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In-reply-to: <20131119111826.GA20485@xxxxxxxxxxxxx>
Mail-followup-to: Christoph Hellwig <hch@xxxxxxxxxxxxx>, linux-ext4@xxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
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User-agent: Mutt/1.5.21 (2010-09-15)
On Tue, Nov 19, 2013 at 03:18:26AM -0800, Christoph Hellwig wrote:
> On Tue, Nov 19, 2013 at 07:19:47PM +0800, Zheng Liu wrote:
> > Yes, I know that XFS has a shared/exclusive lock.  I guess that is why
> > it can pass the test.  But another question is why xfs fails when we do
> > some append dio writes with doing buffered read.
> 
> Can you provide a test case for that issue?

Simple.  Reader just need to open this file without O_DIRECT flag.  I
paste the full code snippet below.  Please take care of this line:
        readfd = open(argv[1], /*O_DIRECT|*/O_RDONLY, S_IRWXU);

The result of this program on my own sand box looks like below:
        encounter an error: offset 0

                                                - Zheng

#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <memory.h>

#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>

#include <pthread.h>

#define BUF_ALIGN       1024

struct writer_data {
        int fd;
        size_t blksize;
        char *buf;
};

static void *writer(void *arg)
{
        struct writer_data *data = (struct writer_data *)arg;
        int ret;

        ret = write(data->fd, data->buf, data->blksize);
        if (ret < 0)
                fprintf(stderr, "write file failed: %s\n", strerror(errno));

        return NULL;
}

int main(int argc, char *argv[])
{
        pthread_t tid;
        struct writer_data wdata;
        size_t max_blocks = 10 * 1024;
        size_t blksize = 1 * 1024 * 1024;
        char *rbuf, *wbuf;
        int readfd, writefd;
        int i, j;

        if (argc < 2) {
                fprintf(stderr, "usage: %s [filename]\n", argv[0]);
                exit(1);
        }

        writefd = open(argv[1], O_CREAT|O_DIRECT|O_WRONLY|O_APPEND|O_TRUNC, 
S_IRWXU);
        if (writefd < 0) {
                fprintf(stderr, "failed to open wfile: %s\n", strerror(errno));
                exit(1);
        }
        readfd = open(argv[1], /*O_DIRECT|*/O_RDONLY, S_IRWXU);
        if (readfd < 0) {
                fprintf(stderr, "failed to open rfile: %s\n", strerror(errno));
                exit(1);
        }

        if (posix_memalign((void **)&wbuf, BUF_ALIGN, blksize)) {
                fprintf(stderr, "failed to alloc memory: %s\n", 
strerror(errno));
                exit(1);
        }

        if (posix_memalign((void **)&rbuf, 4096, blksize)) {
                fprintf(stderr, "failed to alloc memory: %s\n", 
strerror(errno));
                exit(1);
        }

        memset(wbuf, 'a', blksize);

        wdata.fd = writefd;
        wdata.blksize = blksize;
        wdata.buf = wbuf;

        for (i = 0; i < max_blocks; i++) {
                void *retval;
                int ret;

                ret = pthread_create(&tid, NULL, writer, &wdata);
                if (ret) {
                        fprintf(stderr, "create thread failed: %s\n", 
strerror(errno));
                        exit(1);
                }

                memset(rbuf, 'b', blksize);
                do {
                        ret = pread(readfd, rbuf, blksize, i * blksize);
                } while (ret <= 0);

                if (ret < 0) {
                        fprintf(stderr, "read file failed: %s\n", 
strerror(errno));
                        exit(1);
                }

                if (pthread_join(tid, &retval)) {
                        fprintf(stderr, "pthread join failed: %s\n", 
strerror(errno));
                        exit(1);
                }

                if (ret >= 0) {
                        for (j = 0; j < ret; j++) {
                                if (rbuf[i] != 'a') {
                                        fprintf(stderr, "encounter an error: 
offset %ld\n",
                                                i);
                                        goto err;
                                }
                        }
                }
        }

err:
        free(wbuf);
        free(rbuf);

        return 0;
}

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