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Date:   Wed, 15 Jan 2020 07:50:55 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Waiman Long <longman@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
        stable@vger.kernel.org
Subject: Re: [PATCH] locking/rwsem: Fix kernel crash when spinning on
 RWSEM_OWNER_UNKNOWN
Message-ID: <20200115065055.GA21219@lst.de>
References: <20200114190303.5778-1-longman@redhat.com>
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On Tue, Jan 14, 2020 at 02:03:03PM -0500, Waiman Long wrote:
> The commit 91d2a812dfb9 ("locking/rwsem: Make handoff writer
> optimistically spin on owner") will allow a recently woken up waiting
> writer to spin on the owner. Unfortunately, if the owner happens to be
> RWSEM_OWNER_UNKNOWN, the code will incorrectly spin on it leading to a
> kernel crash. This is fixed by passing the proper non-spinnable bits
> to rwsem_spin_on_owner() so that RWSEM_OWNER_UNKNOWN will be treated
> as a non-spinnable target.
> 
> Fixes: 91d2a812dfb9 ("locking/rwsem: Make handoff writer optimistically spin on owner")
> 
> Reported-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Waiman Long <longman@redhat.com>

This survives all the tests that showed the problems with the original
code:

Tested-by: Christoph Hellwig <hch@lst.de>

>  		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.