On 08/19/2016 09:36 PM, Vegard Nossum wrote:
> On 08/19/2016 07:25 AM, Michael Kerrisk (man-pages) wrote:
>> This is an optional patch, to provide a small performance improvement.
>> Alter account_pipe_buffers() so that it returns the new value in
>> user->pipe_bufs. This means that we can refactor too_many_pipe_buffers_soft()
>> and too_many_pipe_buffers_hard() to avoid the costs of repeated use of
>> atomic_long_read() to get the value user->pipe_bufs.
> [...]
>> @@ -627,17 +625,18 @@ struct pipe_inode_info *alloc_pipe_info(void)
>> struct pipe_inode_info *pipe;
>> unsigned long pipe_bufs = PIPE_DEF_BUFFERS;
>> struct user_struct *user = get_current_user();
>> + unsigned long num_bufs;
>
> Maybe user_bufs would be more descriptive since num_bufs is a bit
> ambiguous without the context.
Okay -- changed.
>> pipe = kzalloc(sizeof(struct pipe_inode_info), GFP_KERNEL_ACCOUNT);
>> if (pipe == NULL)
>> goto out_free_uid;
>>
>> - if (too_many_pipe_buffers_soft(user))
>> + if (too_many_pipe_buffers_soft(atomic_long_read(&user->pipe_bufs)))
>> pipe_bufs = 1;
>>
>> - account_pipe_buffers(user, 0, pipe_bufs);
>> + num_bufs = account_pipe_buffers(user, 0, pipe_bufs);
>>
>> - if (too_many_pipe_buffers_hard(user))
>> + if (too_many_pipe_buffers_hard(num_bufs))
>> goto out_revert_acct;
>>
>> pipe->bufs = kcalloc(pipe_bufs, sizeof(struct pipe_buffer),
>
> Why not structure it like this?
>
> num_bufs = account_pipe_buffers(user, 0, pipe_bufs);
> if (too_many_pipe_buffers_soft(num_bufs)) {
> num_bufs = account_pipe_buffers(user, pipe_bufs, 1);
> pipe_bufs = 1;
> }
> if (too_many_pipe_buffers_hard(num_bufs))
> goto out_revert_acct;
>
> Otherwise you still have the case that somebody makes it past
> too_many_pipe_buffers_soft() before the accounting is done.
Ahh -- thanks! I knew there was a small glitch there, but decided to
ignore it because didn't see the obvious solution. Fixed in 6/8 patch.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
