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

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Message-ID: <20180126193954.GB13338@ZenIV.linux.org.uk>
Date:   Fri, 26 Jan 2018 19:39:55 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Joel Fernandes <joelaf@...gle.com>
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

On Fri, Jan 26, 2018 at 11:23:47AM -0800, Joel Fernandes wrote:

> I was just trying to be careful with the least intrusive solution
> since I'm not the original author of the driver.

Sure, but that needs a proof that it *is* a solution...

> 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.

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