linux-kernel - Re: [PATCH] tools/memory-model Flag suspicious use of srcu cookies

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Date:   Tue, 24 Jan 2023 20:36:53 +0100
From:   Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To:     paulmck@...nel.org, Alan Stern <stern@...land.harvard.edu>
Cc:     parri.andrea@...il.com, will@...nel.org, peterz@...radead.org,
        boqun.feng@...il.com, npiggin@...il.com, dhowells@...hat.com,
        j.alglave@....ac.uk, luc.maranget@...ia.fr, akiyks@...il.com,
        dlustig@...dia.com, joel@...lfernandes.org, urezki@...il.com,
        quic_neeraju@...cinc.com, frederic@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tools/memory-model Flag suspicious use of srcu cookies



On 1/24/2023 8:15 PM, Paul E. McKenney wrote:
> On Tue, Jan 24, 2023 at 12:19:24PM -0500, Alan Stern wrote:
>> On Tue, Jan 24, 2023 at 03:39:51PM +0100, Jonas Oberhauser wrote:
>>> The herd model of LKMM deviates from actual implementations in the
>>> range of cookies that might be returned by srcu_lock() and similar
>>> functions.  As a consequence, code that relies on srcu_lock()
>>> returning specific values might pass on the herd model but fail in
>>> the real world.
>>>
>>> This patch flags any code that looks at the value of a cookie
>>> without passing it on to an srcu_unlock().  This indicates that the
>>> cookie value might be being used in ways that can lead herd to
>>> produce incorrect results, as in the following (contrived) case:
>>>
>>> P0(struct srcu_struct *ss)
>>> {
>>> 	int r = srcu_read_lock(ss);
>>> 	if (r==0)
>>> 		srcu_read_unlock(ss, r);
>>> }
>>>
>>> Without this patch, the code passes herd7 without any warnings.
>>>
>>> With this patch, this code is flagged with illegal-srcu-cookie-ctrl,
>>> indicating that a cookie is used to compute a control condition.
>>> Such scenarios potentially lead to other branches of the code that
>>> are possible in real usage not being evaluated by herd7.  In this
>>> example, this affects the branch where r!=0, which would lead to
>>> an unmatched read side critical section and thus to hangs of
>>> synchronize_srcu() calls.
>>>
>>> Besides use of cookies in control conditions, the patch also flags
>>> use in address computation and any time a cookie is inspected but
>>> not later passed to srcu_read_unlock().
>>>
>>> Signed-off-by: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
>>> ---
>> Acked-by: Alan Stern <stern@...land.harvard.edu>
> Thank you both!
>
> I wordsmithed the commit log as follows, but then realized that this
> depends on Alan's earlier patch.

Yeah, I don't know if I did this correctly. I based it on the 
lkmm-srcu.2023.01.20a branch.
Let me know if I should have done this differently.

Looking through your changes to learn for future submissions:

> [...]
>
> This patch flags any code that looks at the value of a cookie
> without passing it on to an srcu_unlock().

You missed this one : )

Have fun, jonas