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SubjectRe: [patch] fix infinite loop in generic_file_splice_read()
On Wed, 09 Apr 2008 17:57:56 +0200
Miklos Szeredi <miklos@szeredi.hu> wrote:

> generic_file_splice_read() goes into an infinite loop if it races with
> truncation. I've found this with fsx-linux on NFS over fuse.
>
> Perhaps the whole while() loop is bogus, but I can't tell from a
> cursory glance at __generic_file_splice_read() if it will return zero
> only on EOF, or it can do that for other reasons as well. In the
> latter case the loop is obviously needed.
>
> This simplistic patch fixes the issue for me.
>

We found suspicious-looking code in generic_file_splice_read() back in
February. See http://lkml.org/lkml/2008/2/29/443. I suspect that patch
(if it works) will address the truncate lockup as well - it zaps the loop
entirely.

Unfortunately Allard never got back to us (probably because he's running
2.6.24 which has a quite different generic_file_splice_read()) and the
patch didn't get anywhere.

Here it is again.

It needs a changelog.

Nobody has tested this at all, to my knowledge.

It's going to take some serious and sudden effort to get these bugs fixed
for 2.6.25.


fs/splice.c | 31 ++++++-------------------------
1 file changed, 6 insertions(+), 25 deletions(-)

diff -puN fs/splice.c~generic_file_splice_read-fix-lockups fs/splice.c
--- a/fs/splice.c~generic_file_splice_read-fix-lockups
+++ a/fs/splice.c
@@ -370,8 +370,10 @@ __generic_file_splice_read(struct file *
* for an in-flight io page
*/
if (flags & SPLICE_F_NONBLOCK) {
- if (TestSetPageLocked(page))
+ if (TestSetPageLocked(page)) {
+ error = -EAGAIN;
break;
+ }
} else
lock_page(page);

@@ -479,9 +481,8 @@ ssize_t generic_file_splice_read(struct
struct pipe_inode_info *pipe, size_t len,
unsigned int flags)
{
- ssize_t spliced;
- int ret;
loff_t isize, left;
+ int ret;

isize = i_size_read(in->f_mapping->host);
if (unlikely(*ppos >= isize))
@@ -491,29 +492,9 @@ ssize_t generic_file_splice_read(struct
if (unlikely(left < len))
len = left;

- ret = 0;
- spliced = 0;
- while (len && !spliced) {
- ret = __generic_file_splice_read(in, ppos, pipe, len, flags);
-
- if (ret < 0)
- break;
- else if (!ret) {
- if (spliced)
- break;
- if (flags & SPLICE_F_NONBLOCK) {
- ret = -EAGAIN;
- break;
- }
- }
-
+ ret = __generic_file_splice_read(in, ppos, pipe, len, flags);
+ if (ret > 0)
*ppos += ret;
- len -= ret;
- spliced += ret;
- }
-
- if (spliced)
- return spliced;

return ret;
}
_


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