Re: [PATCH] ashmem: Fix lockdep issue during llseek

Joel Fernandes
2018-01-26T20:45:34Z

Hi Al,

On Fri, Jan 26, 2018 at 11:39 AM, Al Viro <viro@zeniv.linux.org.uk> wrote:
[..]
>
>> But one usecase for the mutex is with concurrent lseeks, you can end
>> up with a file->f_pos that is different from the latest update to
>> asma->file->f_pos. A barrier could fix this it too though. Any
>> thoughts?
>
> lseek(2) is serialized against lseek(2) and read(2) on the same struct
> file - see fdget_pos() for details.

Ah right, Ok. Thanks.

> ashmem_mutex really does look like an overkill - something much lighter
> should serve just fine...

There's also the issue of asma->size getting updated from while being
in read ashmem_llseek (although this is a theoretical concern):

if (asma->size == 0) {
ret = -EINVAL;
goto out;
}

Which could just use READ_ONCE and WRITE_ONCE instead of the mutex I suppose?

Other than that, I don't see any other issues at the moment with
dropping of the ashmem_mutex from ashmem_llseek.

thanks,

- Joel