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

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Message-ID: <45b976af3cf74555af7214993e7d614b@AcuMS.aculab.com>
Date:   Wed, 15 Jan 2020 15:16:41 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Waiman Long' <longman@...hat.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

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

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