Re: Update of file offset on write() etc. is non-atomic with I/O

[Linus Torvalds]

On Mon, Mar 3, 2014 at 1:45 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> Um...  That's odd - we *could* get there with f.need_put and
> file_count(file) equal to 1, but why would we want to take
> f_pos_lock in that case?

Because that means that the file table is shared among threads. So
another thread can access the struct file pointer and do a concurrent
read() or write() on it, and so we need to lock f_pos.

Basically, there are two cases:

 - duplicated file pointers due to fork(). That's the "file_count()" test.

   Yes, this will trigger even if they didn't fork, just dup'ed the
file descriptor. We have no way of telling the difference, though.

 - concurrent access due to duplicated "struct file_table" pointers.
This is the "need_put" test, since __fget_light() will have tested the
proper files->count already.

Both need f_pos_lock.

               Linus
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