2009-09-09 11:14:33

by Sergey Senozhatsky

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Subject: aio_read const struct iovec *

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Hello,
I have the following question:
struct file_operations {
ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
At the same time:
ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);

'const struct iovec *' both for aio_write and aio_read.
And "char __user *" and "const char __user *" for read/write.

For example,
pipe_read(struct kiocb *iocb, const struct iovec *_iov,
unsigned long nr_segs, loff_t ppos)
{
struct iovec *iov = (struct iovec *)_iov;

Could we avoid 'struct iovec *iov = (struct iovec *)_iov;' by changing aio_read 'const struct iovec *'
to 'struct iovec *'?


Sergey

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2009-09-09 11:53:50

by Matthew Wilcox

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Subject: Re: aio_read const struct iovec *

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On Wed, Sep 09, 2009 at 02:18:38PM +0300, Sergey Senozhatsky wrote:
> Hello,
> I have the following question:
> struct file_operations {
> ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
> ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
> At the same time:
> ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
> ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
>
> 'const struct iovec *' both for aio_write and aio_read.
> And "char __user *" and "const char __user *" for read/write.
>
> For example,
> pipe_read(struct kiocb *iocb, const struct iovec *_iov,
> unsigned long nr_segs, loff_t ppos)
> {
> struct iovec *iov = (struct iovec *)_iov;
>
> Could we avoid 'struct iovec *iov = (struct iovec *)_iov;' by changing aio_read 'const struct iovec *'
> to 'struct iovec *'?

If you try it, it'll be educational for you to see where it fails ;-)

But if you're too impatient, aio_rw_vect_retry() will be (one of)
the problems:

ssize_t (*rw_op)(struct kiocb *, const struct iovec *,
unsigned long, loff_t);
[...]
rw_op = file->f_op->aio_read;
[...]
rw_op = file->f_op->aio_write;


so aio_read and aio_write need to have the same prototype.

Now, I'm not sure why it's more beneficial to have a lying const in the
aio_read prototype than simply omitting the const from the aio_write
prototype ... but I'm sure if you try that, you'll find a good reason too.

--
Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."