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Subject[rfc] non-blocking event loop for network & disk I/O
In designing a streaming media application, I want to use a single event
processing loop for both disk and network I/O. The idea is that the I/O
done in this loop should never block. Doing this for network I/O is pretty
standard (i.e. set O_NONBLOCK and use a poll variant).

Asynchronous/nonblocking file I/O has its limitations, however. AIO can
only be used in conjunction with O_DIRECT, and I'd rather be able to
utilize the page cache. Even so, it is difficult to mix io_getevents with
poll in a single event loop. You could use mmap/mincore, but there is
still a race between the mincore call and the actual read of the data.

So, have I missed any other tools which would help to solve this problem?

It seems that if we introduced a nonblocking flag for file I/O, this
problem would be addressed. Call it O_ATOMICREAD or some such thing. It's
actually quite easy to do (patch attached).

I even went so far as to modify squid's aufs store type to use this, so
there is use outside of just the streaming media example given above
(though I haven't had time to benchmark it).

Comments?

Thanks in advance,

Jeff

--- linux-2.6.11/fs/fcntl.c.orig 2005-03-17 13:48:28.302745792 -0500
+++ linux-2.6.11/fs/fcntl.c 2005-03-17 13:48:48.203720384 -0500
@@ -181,7 +181,7 @@ asmlinkage long sys_dup(unsigned int fil
return ret;
}

-#define SETFL_MASK (O_APPEND | O_NONBLOCK | O_NDELAY | FASYNC | O_DIRECT | O_NOATIME)
+#define SETFL_MASK (O_APPEND | O_NONBLOCK | O_NDELAY | FASYNC | O_DIRECT | O_NOATIME | O_ATOMICREAD)

static int setfl(int fd, struct file * filp, unsigned long arg)
{
--- linux-2.6.11/include/asm-mips/fcntl.h.orig 2005-03-17 14:01:08.039248288 -0500
+++ linux-2.6.11/include/asm-mips/fcntl.h 2005-03-17 14:01:30.365854128 -0500
@@ -27,6 +27,7 @@
#define O_DIRECTORY 0x10000 /* must be a directory */
#define O_NOFOLLOW 0x20000 /* don't follow links */
#define O_NOATIME 0x40000
+#define O_ATOMICREAD 0x80000 /* non-blocking file i/o */

#define O_NDELAY O_NONBLOCK

--- linux-2.6.11/include/asm-parisc/fcntl.h.orig 2005-03-17 14:02:01.120178760 -0500
+++ linux-2.6.11/include/asm-parisc/fcntl.h 2005-03-17 14:02:02.160020680 -0500
@@ -20,6 +20,7 @@
#define O_DSYNC 01000000 /* HPUX only */
#define O_RSYNC 02000000 /* HPUX only */
#define O_NOATIME 04000000
+#define O_ATOMICREAD 10000000 /* non-blocking file i/o */

#define FASYNC 00020000 /* fcntl, for BSD compatibility */
#define O_DIRECT 00040000 /* direct disk access hint - currently ignored */
--- linux-2.6.11/include/asm-ppc64/fcntl.h.orig 2005-03-17 13:57:16.464453008 -0500
+++ linux-2.6.11/include/asm-ppc64/fcntl.h 2005-03-17 13:57:26.291959000 -0500
@@ -28,6 +28,7 @@
#define O_LARGEFILE 0200000
#define O_DIRECT 0400000 /* direct disk access hint */
#define O_NOATIME 01000000
+#define O_ATOMICREAD 02000000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
--- linux-2.6.11/include/asm-x86_64/fcntl.h.orig 2005-03-17 13:56:19.711080832 -0500
+++ linux-2.6.11/include/asm-x86_64/fcntl.h 2005-03-17 13:56:24.575341352 -0500
@@ -21,6 +21,7 @@
#define O_DIRECTORY 0200000 /* must be a directory */
#define O_NOFOLLOW 0400000 /* don't follow links */
#define O_NOATIME 01000000
+#define O_ATOMICREAD 02000000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
--- linux-2.6.11/include/asm-arm/fcntl.h.orig 2005-03-17 13:58:49.073374312 -0500
+++ linux-2.6.11/include/asm-arm/fcntl.h 2005-03-17 13:58:50.032228544 -0500
@@ -21,6 +21,7 @@
#define O_DIRECT 0200000 /* direct disk access hint - currently ignored */
#define O_LARGEFILE 0400000
#define O_NOATIME 01000000
+#define O_ATOMICREAD 02000000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
--- linux-2.6.11/include/asm-m68k/fcntl.h.orig 2005-03-17 14:00:32.230692016 -0500
+++ linux-2.6.11/include/asm-m68k/fcntl.h 2005-03-17 14:00:33.107558712 -0500
@@ -21,6 +21,7 @@
#define O_DIRECT 0200000 /* direct disk access hint - currently ignored */
#define O_LARGEFILE 0400000
#define O_NOATIME 01000000
+#define O_ATOMICREAD 02000000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
--- linux-2.6.11/include/asm-ppc/fcntl.h.orig 2005-03-17 13:56:56.661463520 -0500
+++ linux-2.6.11/include/asm-ppc/fcntl.h 2005-03-17 13:57:07.509814320 -0500
@@ -21,6 +21,7 @@
#define O_LARGEFILE 0200000
#define O_DIRECT 0400000 /* direct disk access hint */
#define O_NOATIME 01000000
+#define O_ATOMICREAD 02000000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
--- linux-2.6.11/include/asm-sparc/fcntl.h.orig 2005-03-17 14:02:57.827557928 -0500
+++ linux-2.6.11/include/asm-sparc/fcntl.h 2005-03-17 14:02:58.682427968 -0500
@@ -22,6 +22,7 @@
#define O_LARGEFILE 0x40000
#define O_DIRECT 0x100000 /* direct disk access hint */
#define O_NOATIME 0x200000
+#define O_ATOMICREAD 0x400000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
--- linux-2.6.11/include/asm-alpha/fcntl.h.orig 2005-03-17 13:58:22.624395168 -0500
+++ linux-2.6.11/include/asm-alpha/fcntl.h 2005-03-17 13:58:23.599246968 -0500
@@ -22,6 +22,7 @@
#define O_LARGEFILE 0400000 /* will be set by the kernel on every open */
#define O_DIRECT 02000000 /* direct disk access - should check with OSF/1 */
#define O_NOATIME 04000000
+#define O_ATOMICREAD 10000000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
--- linux-2.6.11/include/asm-v850/fcntl.h.orig 2005-03-17 14:03:45.439319840 -0500
+++ linux-2.6.11/include/asm-v850/fcntl.h 2005-03-17 14:03:46.312187144 -0500
@@ -21,6 +21,7 @@
#define O_DIRECT 0200000 /* direct disk access hint - currently ignored */
#define O_LARGEFILE 0400000
#define O_NOATIME 01000000
+#define O_ATOMICREAD 02000000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
--- linux-2.6.11/include/asm-i386/fcntl.h.orig 2005-03-17 13:55:43.761545992 -0500
+++ linux-2.6.11/include/asm-i386/fcntl.h 2005-03-17 13:56:02.172747064 -0500
@@ -21,6 +21,7 @@
#define O_DIRECTORY 0200000 /* must be a directory */
#define O_NOFOLLOW 0400000 /* don't follow links */
#define O_NOATIME 01000000
+#define O_ATOMICREAD 02000000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
--- linux-2.6.11/include/asm-frv/fcntl.h.orig 2005-03-17 13:59:33.734584776 -0500
+++ linux-2.6.11/include/asm-frv/fcntl.h 2005-03-17 13:59:34.580456184 -0500
@@ -21,6 +21,7 @@
#define O_DIRECTORY 0200000 /* must be a directory */
#define O_NOFOLLOW 0400000 /* don't follow links */
#define O_NOATIME 01000000
+#define O_ATOMICREAD 02000000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
--- linux-2.6.11/include/asm-m32r/fcntl.h.orig 2005-03-17 14:00:13.786495960 -0500
+++ linux-2.6.11/include/asm-m32r/fcntl.h 2005-03-17 14:00:14.661362960 -0500
@@ -25,6 +25,7 @@
#define O_DIRECTORY 0200000 /* must be a directory */
#define O_NOFOLLOW 0400000 /* don't follow links */
#define O_NOATIME 01000000
+#define O_ATOMICREAD 02000000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
--- linux-2.6.11/include/asm-h8300/fcntl.h.orig 2005-03-17 13:59:56.604108080 -0500
+++ linux-2.6.11/include/asm-h8300/fcntl.h 2005-03-17 13:59:57.522968392 -0500
@@ -21,6 +21,7 @@
#define O_DIRECT 0200000 /* direct disk access hint - currently ignored */
#define O_LARGEFILE 0400000
#define O_NOATIME 01000000
+#define O_ATOMICREAD 02000000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
--- linux-2.6.11/include/asm-s390/fcntl.h.orig 2005-03-17 13:57:42.654471520 -0500
+++ linux-2.6.11/include/asm-s390/fcntl.h 2005-03-17 13:57:43.860288208 -0500
@@ -28,6 +28,7 @@
#define O_DIRECTORY 0200000 /* must be a directory */
#define O_NOFOLLOW 0400000 /* don't follow links */
#define O_NOATIME 01000000
+#define O_ATOMICREAD 02000000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
--- linux-2.6.11/include/asm-ia64/fcntl.h.orig 2005-03-17 13:56:44.119370208 -0500
+++ linux-2.6.11/include/asm-ia64/fcntl.h 2005-03-17 13:56:46.889949016 -0500
@@ -29,6 +29,7 @@
#define O_DIRECTORY 0200000 /* must be a directory */
#define O_NOFOLLOW 0400000 /* don't follow links */
#define O_NOATIME 01000000
+#define O_ATOMICREAD 02000000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
--- linux-2.6.11/include/asm-cris/fcntl.h.orig 2005-03-17 13:59:20.996521256 -0500
+++ linux-2.6.11/include/asm-cris/fcntl.h 2005-03-17 13:59:22.037363024 -0500
@@ -23,6 +23,7 @@
#define O_DIRECTORY 0200000 /* must be a directory */
#define O_NOFOLLOW 0400000 /* don't follow links */
#define O_NOATIME 01000000
+#define O_ATOMICREAD 02000000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get f_flags */
--- linux-2.6.11/include/asm-sparc64/fcntl.h.orig 2005-03-17 14:03:18.313443600 -0500
+++ linux-2.6.11/include/asm-sparc64/fcntl.h 2005-03-17 14:03:19.482265912 -0500
@@ -22,6 +22,7 @@
#define O_LARGEFILE 0x40000
#define O_DIRECT 0x100000 /* direct disk access hint */
#define O_NOATIME 0x200000
+#define O_ATOMICREAD 0x400000 /* non-blocking file i/o */


#define F_DUPFD 0 /* dup */
--- linux-2.6.11/include/asm-arm26/fcntl.h.orig 2005-03-17 13:59:04.970957512 -0500
+++ linux-2.6.11/include/asm-arm26/fcntl.h 2005-03-17 13:59:06.485727232 -0500
@@ -21,6 +21,7 @@
#define O_DIRECT 0200000 /* direct disk access hint - currently ignored */
#define O_LARGEFILE 0400000
#define O_NOATIME 01000000
+#define O_ATOMICREAD 02000000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
--- linux-2.6.11/include/asm-sh/fcntl.h.orig 2005-03-17 14:02:16.734804976 -0500
+++ linux-2.6.11/include/asm-sh/fcntl.h 2005-03-17 14:02:24.028696136 -0500
@@ -21,6 +21,7 @@
#define O_DIRECTORY 0200000 /* must be a directory */
#define O_NOFOLLOW 0400000 /* don't follow links */
#define O_NOATIME 01000000
+#define O_ATOMICREAD 02000000 /* non-blocking file i/o */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
--- linux-2.6.11/mm/filemap.c.orig 2005-03-17 13:37:48.270045552 -0500
+++ linux-2.6.11/mm/filemap.c 2005-03-17 13:48:20.594917560 -0500
@@ -696,7 +696,7 @@ void do_generic_mapping_read(struct addr
unsigned long prev_index;
loff_t isize;
struct page *cached_page;
- int error;
+ int error, nonblock = filp->f_flags & O_ATOMICREAD;
struct file_ra_state ra = *_ra;

cached_page = NULL;
@@ -728,7 +728,7 @@ void do_generic_mapping_read(struct addr
nr = nr - offset;

cond_resched();
- if (index == next_index && req_size) {
+ if (index == next_index && req_size && !nonblock) {
ret_size = page_cache_readahead(mapping, &ra,
filp, index, req_size);
next_index += ret_size;
@@ -738,11 +738,21 @@ void do_generic_mapping_read(struct addr
find_page:
page = find_get_page(mapping, index);
if (unlikely(page == NULL)) {
+ if (nonblock) {
+ desc->error = -EWOULDBLOCK;
+ break;
+ }
handle_ra_miss(mapping, &ra, index);
goto no_cached_page;
}
- if (!PageUptodate(page))
+ if (!PageUptodate(page)) {
+ if (nonblock) {
+ page_cache_release(page);
+ desc->error = -EWOULDBLOCK;
+ break;
+ }
goto page_not_up_to_date;
+ }
page_ok:

/* If users can be writing to this page using arbitrary
@@ -1000,7 +1010,7 @@ __generic_file_aio_read(struct kiocb *io
desc.error = 0;
do_generic_file_read(filp,ppos,&desc,file_read_actor);
retval += desc.written;
- if (!retval) {
+ if (!retval || desc.error) {
retval = desc.error;
break;
}
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