linux-kernel - Re: [PATCH] locking/rwsem: Fix kernel crash when spinning on RWSEM_OWNER

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Date:   Wed, 15 Jan 2020 10:47:49 -0500
From:   Waiman Long <longman@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>,
        Christoph Hellwig <hch@....de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] locking/rwsem: Fix kernel crash when spinning on
 RWSEM_OWNER_UNKNOWN

On 1/15/20 10:16 AM, David Laight wrote:
> From: linux-kernel-owner@...r.kernel.org <linux-kernel-owner@...r.kernel.org> On Behalf Of Waiman Long
>> Sent: 15 January 2020 14:27
> ...
>>>>  		if ((wstate == WRITER_HANDOFF) &&
>>>> -		    (rwsem_spin_on_owner(sem, 0) == OWNER_NULL))
>>>> +		    rwsem_spin_on_owner(sem, RWSEM_NONSPINNABLE) == OWNER_NULL)
>>> Nit: the inner braces in the first half of the conditional aren't required
>>> either.
>> I typically over-parenthesize the code to make it easier to read as we
>> don't need to think too much about operator precedence to see if it is
>> doing the right thing.
> The problem is it actually makes it harder to read.
> It is difficult for the 'mark 1 eyeball' to follow lots of sets of brackets.
> Since == (etc) are the lowest priority operators (apart from ?:) they
> never need ().
>
> 	David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
>
It depends. I find it hard to read an expression with "&" and "&&"
without parentheses. Anyway, I will admit that the above code is
inconsistent in term of how parentheses are used. So I will change that.

Cheers,
Longman