linux-kernel - Re: [PATCH 2/2] xen: privcmd: Add support for ioeventfd

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Message-ID: <6d313c8a-0d3e-40bd-a3f4-86b2ebc5f59a@suse.com>
Date:   Mon, 9 Oct 2023 12:58:24 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>,
        Alex Bennée <alex.bennee@...aro.org>
Cc:     Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        stratos-dev@...lists.linaro.org,
        Erik Schilling <erik.schilling@...aro.org>,
        Manos Pitsidianakis <manos.pitsidianakis@...aro.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Subject: Re: [PATCH 2/2] xen: privcmd: Add support for ioeventfd

On 09.10.23 12:53, Viresh Kumar wrote:
> On 09-10-23, 10:40, Alex Bennée wrote:
>> I thought generally sync points act as full barriers. Doing a bunch of
>> grepping I think ends at:
>>
>>    static __always_inline bool __mutex_unlock_fast(struct mutex *lock)
>>    {
>>            unsigned long curr = (unsigned long)current;
>>
>>            return atomic_long_try_cmpxchg_release(&lock->owner, &curr, 0UL);
>>    }
>>
>> so you should already have completed your writes by that point.
> 
> I am not sure if depending on such indirect mechanisms to implement
> barriers for you is a good idea.
> 
> The situation here probably requires explicit barriers to make sure it
> doesn't break in future ?
> 

Depending on lock implementations to include the needed barriers is fine IMO.

That is one central objective locks must ensure: to make sure any updates in
a locked region are operating on consistent data and being observable by
others after leaving the locked region.


Juergen

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