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SubjectRe: [PATCH] reducing overheads in fget/fput
On Tue, Apr 29, 2003 at 01:28:36AM +0530, Dipankar Sarma wrote:
> On Mon, Apr 28, 2003 at 08:32:28PM +0100, viro@parcelfarce.linux.theplanet.co.uk wrote:
> > You are. Have a process share file table at the time of call and
> > have its sibling die in the middle. Oops - condition that had
> > been true at time of fget_light() (->files->count > 1) is false
> > at the time we fput_light(). Have fun - we had just leaked a
> > reference to struct file.
>
> That shouldn't be very difficult to fix. For the fget_light/fput_light
> pair in a syscall, we make the files->count == 1 check only once at the
> beginning. Do you see a problem with that ?

Here is a patch that fixes that. I re-did the measurements with
Andrew's experiment (dd if=/dev/zero of=foo bs=1 count=1M).
[4CPU P3 xeon with 1MB L2 cache and 512MB ram]

kernel sys time std-dev
------------ -------- -------
UP - vanilla 2.104 0.028
SMP - vanilla 2.976 0.023
UP - file 1.867 0.019
SMP - file 2.719 0.026

Thanks
Dipankar


diff -urN linux-2.5.66-base/fs/file_table.c linux-2.5.66-file/fs/file_table.c
--- linux-2.5.66-base/fs/file_table.c 2003-03-25 03:29:55.000000000 +0530
+++ linux-2.5.66-file/fs/file_table.c 2003-05-02 21:23:27.000000000 +0530
@@ -147,6 +147,13 @@
__fput(file);
}

+void fput_light(struct file * file, int flag)
+{
+ if (unlikely(flag))
+ if (atomic_dec_and_test(&file->f_count))
+ __fput(file);
+}
+
/* __fput is called from task context when aio completion releases the last
* last use of a struct file *. Do not use otherwise.
*/
@@ -190,6 +197,34 @@
return file;
}

+/*
+ * Lightweight file lookup - no refcnt increment if fd table isn't shared.
+ * You can use this only if it is guranteed that the current task already
+ * holds a refcnt to that file. That check has to be done at fget() only
+ * and a flag is returned to be passed to the corresponding fput_light().
+ * There must not be a cloning between an fget_light/fput_light pair.
+ */
+struct file *fget_light(unsigned int fd, int *flag)
+{
+ struct file *file;
+ struct files_struct *files = current->files;
+
+ *flag = 0;
+ if (likely((atomic_read(&files->count) == 1))) {
+ file = fcheck(fd);
+ } else {
+ read_lock(&files->file_lock);
+ file = fcheck(fd);
+ if (file) {
+ get_file(file);
+ *flag = 1;
+ }
+ read_unlock(&files->file_lock);
+ }
+ return file;
+}
+
+
void put_filp(struct file *file)
{
if (atomic_dec_and_test(&file->f_count)) {
diff -urN linux-2.5.66-base/fs/read_write.c linux-2.5.66-file/fs/read_write.c
--- linux-2.5.66-base/fs/read_write.c 2003-03-25 03:30:51.000000000 +0530
+++ linux-2.5.66-file/fs/read_write.c 2003-05-02 13:42:53.000000000 +0530
@@ -115,9 +115,10 @@
{
off_t retval;
struct file * file;
+ int flag;

retval = -EBADF;
- file = fget(fd);
+ file = fget_light(fd, &flag);
if (!file)
goto bad;

@@ -128,7 +129,7 @@
if (res != (loff_t)retval)
retval = -EOVERFLOW; /* LFS: should only happen on 32 bit platforms */
}
- fput(file);
+ fput_light(file, flag);
bad:
return retval;
}
@@ -141,9 +142,10 @@
int retval;
struct file * file;
loff_t offset;
+ int flag;

retval = -EBADF;
- file = fget(fd);
+ file = fget_light(fd, &flag);
if (!file)
goto bad;

@@ -161,7 +163,7 @@
retval = 0;
}
out_putf:
- fput(file);
+ fput_light(file, flag);
bad:
return retval;
}
@@ -251,11 +253,12 @@
{
struct file *file;
ssize_t ret = -EBADF;
+ int flag;

- file = fget(fd);
+ file = fget_light(fd, &flag);
if (file) {
ret = vfs_read(file, buf, count, &file->f_pos);
- fput(file);
+ fput_light(file, flag);
}

return ret;
@@ -265,11 +268,12 @@
{
struct file *file;
ssize_t ret = -EBADF;
+ int flag;

- file = fget(fd);
+ file = fget_light(fd, &flag);
if (file) {
ret = vfs_write(file, buf, count, &file->f_pos);
- fput(file);
+ fput_light(file, flag);
}

return ret;
@@ -280,14 +284,15 @@
{
struct file *file;
ssize_t ret = -EBADF;
+ int flag;

if (pos < 0)
return -EINVAL;

- file = fget(fd);
+ file = fget_light(fd, &flag);
if (file) {
ret = vfs_read(file, buf, count, &pos);
- fput(file);
+ fput_light(file, flag);
}

return ret;
@@ -298,14 +303,15 @@
{
struct file *file;
ssize_t ret = -EBADF;
+ int flag;

if (pos < 0)
return -EINVAL;

- file = fget(fd);
+ file = fget_light(fd, &flag);
if (file) {
ret = vfs_write(file, buf, count, &pos);
- fput(file);
+ fput_light(file, flag);
}

return ret;
@@ -479,11 +485,12 @@
{
struct file *file;
ssize_t ret = -EBADF;
+ int flag;

- file = fget(fd);
+ file = fget_light(fd, &flag);
if (file) {
ret = vfs_readv(file, vec, vlen, &file->f_pos);
- fput(file);
+ fput_light(file, flag);
}

return ret;
@@ -494,11 +501,12 @@
{
struct file *file;
ssize_t ret = -EBADF;
+ int flag;

- file = fget(fd);
+ file = fget_light(fd, &flag);
if (file) {
ret = vfs_writev(file, vec, vlen, &file->f_pos);
- fput(file);
+ fput_light(file, flag);
}

return ret;
@@ -511,12 +519,13 @@
struct inode * in_inode, * out_inode;
loff_t pos;
ssize_t retval;
+ int flag_in, flag_out;

/*
* Get input file, and verify that it is ok..
*/
retval = -EBADF;
- in_file = fget(in_fd);
+ in_file = fget_light(in_fd, &flag_in);
if (!in_file)
goto out;
if (!(in_file->f_mode & FMODE_READ))
@@ -539,7 +548,7 @@
* Get output file, and verify that it is ok..
*/
retval = -EBADF;
- out_file = fget(out_fd);
+ out_file = fget_light(out_fd, &flag_out);
if (!out_file)
goto fput_in;
if (!(out_file->f_mode & FMODE_WRITE))
@@ -579,9 +588,9 @@
retval = -EOVERFLOW;

fput_out:
- fput(out_file);
+ fput_light(out_file, flag_out);
fput_in:
- fput(in_file);
+ fput_light(in_file, flag_in);
out:
return retval;
}
diff -urN linux-2.5.66-base/include/linux/file.h linux-2.5.66-file/include/linux/file.h
--- linux-2.5.66-base/include/linux/file.h 2003-03-25 03:29:52.000000000 +0530
+++ linux-2.5.66-file/include/linux/file.h 2003-05-02 13:43:31.000000000 +0530
@@ -35,7 +35,9 @@

extern void FASTCALL(__fput(struct file *));
extern void FASTCALL(fput(struct file *));
+extern void FASTCALL(fput_light(struct file *, int));
extern struct file * FASTCALL(fget(unsigned int fd));
+extern struct file * FASTCALL(fget_light(unsigned int fd, int *flag));
extern void FASTCALL(set_close_on_exec(unsigned int fd, int flag));
extern void put_filp(struct file *);
extern int get_unused_fd(void);
-
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