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From:   Michel Lespinasse <walken@google.com>
Date:   Thu, 26 Mar 2020 22:09:09 -0700
Message-ID: <CANN689Hr972e_0+kujGxXPbCVTd7xnpBPZXDk2T3dwARnWENVQ@mail.gmail.com>
Subject: Re: [PATCH v2 07/10] mmap locking API: add mmap_read_release() and mmap_read_unlock_non_owner()
To:     Michel Lespinasse <walken@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Laurent Dufour <ldufour@linux.ibm.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Matthew Wilcox <willy@infradead.org>,
        Liam Howlett <Liam.Howlett@oracle.com>,
        Jerome Glisse <jglisse@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Hugh Dickins <hughd@google.com>, Ying Han <yinghan@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Markus Elfring <Markus.Elfring@web.de>
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On Thu, Mar 26, 2020 at 9:48 PM Davidlohr Bueso <dave@stgolabs.net> wrote:
>
> On Thu, 26 Mar 2020, Michel Lespinasse wrote:
>
> >Add a couple APIs to allow splitting mmap_read_unlock() into two calls:
> >- mmap_read_release(), called by the task that had taken the mmap lock;
> >- mmap_read_unlock_non_owner(), called from a work queue.
> >
> >These apis are used by kernel/bpf/stackmap.c only.
>
> I'm not crazy about the idea generalizing such calls into an mm api.
> We try to stay away from non-owner semantics in locking - granted
> the IS_ENABLED(CONFIG_PREEMPT_RT) warning, but still.
>
> Could this give future users the wrong impression? What about just
> using rwsem calls directly in bpf?

I see what you mean and I certainly don't want to encourage any new
non-owner call sites to appear.... This bpf stackmap site is a small
pain point in my larger range locking patchset too.

I am not sure what is the proper response to it; the opposite side of
your argument could be that using a direct rwsem call there hides the
issue and makes it less likely for someone to fix it ? I don't have a
very strong opinion on this, as I think it can be argued either way...

But at a minimum, I think it'd be worth adding a comment asking people
not to add new call sites to the mmap_read_release() and
mmap_read_unlock_non_owner() APIs ?

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.