linux-kernel - Re: [PATCH 1/2] lockdep: Avoid to modify chain keys in validate

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Message-ID: <20201110172838.GP2594@hirez.programming.kicks-ass.net>
Date:   Tue, 10 Nov 2020 18:28:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Qian Cai <cai@...hat.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH 1/2] lockdep: Avoid to modify chain keys in
 validate_chain()

On Mon, Nov 02, 2020 at 01:37:41PM +0800, Boqun Feng wrote:
> Chris Wilson reported a problem spotted by check_chain_key(): a chain
> key got changed in validate_chain() because we modify the ->read in
> validate_chain() to skip checks for dependency adding, and ->read is
> taken into calculation for chain key since commit f611e8cf98ec
> ("lockdep: Take read/write status in consideration when generate
> chainkey").
> 
> Fix this by avoiding to modify ->read in validate_chain() based on two
> facts: a) since we now support recursive read lock detection, there is
> no need to skip checks for dependency adding for recursive readers, b)
> since we have a), there is only one case left (nest_lock) where we want
> to skip checks in validate_chain(), we simply remove the modification
> for ->read and rely on the return value of check_deadlock() to skip the
> dependency adding.
> 
> Reported-by: Chris Wilson <chris@...is-wilson.co.uk>
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> Cc: Peter Zijlstra <peterz@...radead.org>

Thanks Boqun!