On Thu, Jan 15, 2009 at 04:42:19PM +0900, Hisashi Hifumi wrote:
> I changed i_mutex usage on generic_file_llseek.
> This function is inside i_mutex, but I think there is room for optimization in some cases.
> When SEEK_END is specified from caller, in this case we should handle
> inode->i_size so i_mutex is needed. But in other cases such as SEEK_CUR or
> SEEK_SET, i_mutex is not needed because just changing file->f_pos value without
> touching i_size.
Of course if you have multiple threads, they will share a struct file,
and you're updating f_pos and f_version without locking. Maybe that's
OK, but it's soemthing you didn't discuss.
I think it's the only reason to have the mutex here. Otherwise we could
simply use i_size_read() in generic_file_llseek_unlocked() and there
would be no need for a mutex at all.
> - mutex_lock(&file->f_dentry->d_inode->i_mutex);
> - rval = generic_file_llseek_unlocked(file, offset, origin);
> - mutex_unlock(&file->f_dentry->d_inode->i_mutex);
> + if (origin == SEEK_END) {
> + mutex_lock(&file->f_dentry->d_inode->i_mutex);
> + rval = generic_file_llseek_unlocked(file, offset, origin);
> + mutex_unlock(&file->f_dentry->d_inode->i_mutex);
> + } else
> + rval = generic_file_llseek_unlocked(file, offset, origin);
I'm pretty sure the spinning mutex work will have a significant effect
on the performance here.
--
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."
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