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Subject[PATCH linux-2.6.0-test10-mm1] dio-read-race-fix
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Janet,

I think I found the problem that is causing the dio reads to get
unintialized data with buffer writes. i_size is sampled after
i_sem is dropped and after the i/o have completed. i_size could
have changed in the mean time. This patch samples i_size before
dropping i_sem. I'll leave the test running over the weekend to
verify.

Daniel

On Tue, 2003-12-02 at 18:33, Janet Morgan wrote:
> Hi Suparna, Daniel,
>
> Just wanted to let you know that I seem to be hitting a failure when I
> run the testcase (attached) that Stephen Tweedie wrote to expose the DIO
> races (the testcase issues buffered writes and dio reads). I'm trying to
> debug the failure, but I know it will be slow going for me.
>
> I hit the problem when I run multiple instances of the test against
> the same filesystem (I do specify a unique file for each instance of
> the test). I normally run about 6 instances of the test, e.g.,
> "direct_read_under foo1", "direct_read_under foo2", etc. The test
> runs in an infinite loop until a data mis-compare is detected.
> I have not been able to reproduce the failure when I restrict each
> instance of the test to its own filesystem.
>
> I've tried running with the combinations below and I eventually
> get a failure in each case. I assume all but the last combo includes
> the critical aio/dio fixes for the associated base.
>
> Combinations tested:
>
> Daniel's latest:
> 2.6.0-test9-mm5 +
> aio-dio-fallback-bio_count-race.patch +
> direct-io-memleak-fix.patch
>
> Suparna's latest:
> 2.6.0-test9-mm5 +
> Suparna's current patches:
>
> http://marc.theaimsgroup.com/?l=linux-aio&m=106983304420570&w=2 +
>
> http://marc.theaimsgroup.com/?l=linux-kernel&m=106904658121299&w=2 +
> direct-io-memleak-fix.patch
>
> stock linux-2.6.0-test9 plus:
> aio-refcnt.patch +
> dio-aio-fixes.patch +
> dio-aio-fixes-fixes.patch +
> O_DIRECT-race-fixes-rollup.patch
>
> stock linux-2.6.0-test11 plus:
> O_DIRECT-race-fixes-rollup.patch
>
> Thanks,
> -Janet
>
> ______________________________________________________________________
>
> #define _XOPEN_SOURCE 600
> #define _GNU_SOURCE
>
> #include <unistd.h>
> #include <stdlib.h>
> #include <stdio.h>
> #include <string.h>
> #include <errno.h>
> #include <sys/fcntl.h>
> #include <sys/mman.h>
> #include <sys/wait.h>
>
> #define BIGSIZE 128*1024*1024
> #define READSIZE 32*1024*1024
> #define WRITESIZE 32*1024*1024
>
> int pagesize;
> char *iobuf;
>
> void assert(const char *what, int assertion)
> {
> if (assertion)
> return;
> perror(what);
> exit(1);
> }
>
> void do_buffered_writes(int fd, int pattern)
> {
> int rc;
> int offset;
>
> memset(iobuf, pattern, WRITESIZE);
> for (offset = 0; offset+WRITESIZE <= BIGSIZE; offset += WRITESIZE) {
> rc = pwrite(fd, iobuf, WRITESIZE, offset);
> assert("pwrite", rc >= 0);
> if (rc != WRITESIZE) {
> fprintf(stderr, "short write (%d out of %d)\n",
> rc, WRITESIZE);
> exit(1);
> }
> fsync(fd);
> }
> }
>
> int do_direct_reads(char *filename)
> {
> int fd;
> int offset;
> int rc, i;
> int *p;
>
> fd = open(filename, O_DIRECT|O_RDONLY, 0);
> assert("open", fd >= 0);
>
> for (offset = 0; offset+READSIZE <= BIGSIZE; offset += READSIZE) {
> rc = pread(fd, iobuf, READSIZE, offset);
> assert("pread", rc >= 0);
> if (rc != READSIZE) {
> fprintf(stderr, "short read (%d out of %d)\n",
> rc, READSIZE);
> exit(1);
> }
> for (i=0, p=(int *)iobuf; i<READSIZE; i+=4) {
> if (*p) {
> fprintf(stderr,
> "Found data (%08x) at offset %d+%d\n",
> *p, offset, i);
> return 1;
> }
> p++;
> }
> }
> return 0;
> }
>
> int main(int argc, char *argv[])
> {
> char *filename;
> int fd;
> int pid;
> int err;
> int pass = 0;
> int bufsize;
>
> if (argc != 2) {
> fprintf(stderr, "Needs a filename as an argument.\n");
> exit(1);
> }
>
> filename = argv[1];
>
> pagesize = getpagesize();
> bufsize = READSIZE;
> if (WRITESIZE > READSIZE)
> bufsize = WRITESIZE;
> err = posix_memalign((void**) &iobuf, pagesize, bufsize);
> if (err) {
> fprintf(stderr, "Error allocating %d aligned bytes.\n", bufsize);
> exit(1);
> }
>
> fd = open(filename, O_CREAT|O_TRUNC|O_RDWR, 0666);
> assert("open", fd >= 0);
>
> do {
> printf("Pass %d...\n", ++pass);
>
> assert("ftruncate", ftruncate(fd, BIGSIZE) == 0);
> fsync(fd);
>
> pid = fork();
> assert("fork", pid >= 0);
>
> if (!pid) {
> do_buffered_writes(fd, 0);
> exit(0);
> }
>
> err = do_direct_reads(filename);
>
> wait4(pid, NULL, WNOHANG, 0);
>
> if (err)
> break;
>
> /* Fill the file with a known pattern so that the blocks
> * on disk can be detected if they become exposed. */
> do_buffered_writes(fd, 1);
> fsync(fd);
>
> assert("ftruncate", ftruncate(fd, 0) == 0);
> fsync(fd);
> } while (1);
>
> return err;
> }
>
--- linux-2.6.0-test10-mm1.ddm/fs/direct-io.c 2003-12-05 17:14:57.704920048 -0800
+++ linux-2.6.0-test10-mm1.ddm.DIO/fs/direct-io.c 2003-12-05 16:55:56.000000000 -0800
@@ -909,6 +909,7 @@ direct_io_worker(int rw, struct kiocb *i
int ret = 0;
int ret2;
size_t bytes;
+ loff_t i_size;

dio->bio = NULL;
dio->inode = inode;
@@ -1017,9 +1018,15 @@ direct_io_worker(int rw, struct kiocb *i
* All block lookups have been performed. For READ requests
* we can let i_sem go now that its achieved its purpose
* of protecting us from looking up uninitialized blocks.
+ *
+ * We also need sample i_size before we release i_sem to prevent
+ * a racing write from changing i_size causing us to return
+ * uninitialized data.
*/
- if ((rw == READ) && dio->needs_locking)
+ if ((rw == READ) && dio->needs_locking) {
+ i_size = i_size_read(inode);
up(&dio->inode->i_sem);
+ }

/*
* OK, all BIOs are submitted, so we can decrement bio_count to truly
@@ -1064,7 +1071,6 @@ direct_io_worker(int rw, struct kiocb *i
if (ret == 0)
ret = dio->page_errors;
if (ret == 0 && dio->result) {
- loff_t i_size = i_size_read(inode);

ret = dio->result;
/*
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