linux-kernel - Re: [PATCH] ashmem: Fix lockdep issue during llseek

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Message-ID: <CAJWu+opST5wy=X9J0dhU6w4tByk5wxmUO=K5p5bewNJjuzERyg@mail.gmail.com>
Date:   Fri, 26 Jan 2018 12:45:34 -0800
From:   Joel Fernandes <joelaf@...gle.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     LKML <linux-kernel@...r.kernel.org>, Todd Kjos <tkjos@...gle.com>,
        Arve Hjonnevag <arve@...roid.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] ashmem: Fix lockdep issue during llseek

Hi Al,

On Fri, Jan 26, 2018 at 11:39 AM, Al Viro <viro@...iv.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