From: Peter Zijlstra <peterz@infradead.org>
To: "Paul Heidekrüger" <paul.heidekrueger@in.tum.de>
Cc: paulmck@kernel.org, will@kernel.org, boqun.feng@gmail.com,
stern@rowland.harvard.edu, parri.andrea@gmail.com,
linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
elver@google.com, charalampos.mainas@gmail.com,
pramod.bhatotia@in.tum.de
Subject: Re: Potentially Broken Address Dependency via test_bit() When Compiling With Clang
Date: Wed, 27 Oct 2021 14:34:29 +0200 [thread overview]
Message-ID: <20211027123429.GE174730@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20211027121747.GI174703@worktop.programming.kicks-ass.net>
On Wed, Oct 27, 2021 at 02:17:48PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 27, 2021 at 12:19:48PM +0200, Paul Heidekrüger wrote:
> I would personally not consider this a dependend load. The result
> depends on two loads, but there is no actual ordering between them.
>
> r1 = *x
> r2 = *y
> b = 1 & (r1 >> r2);
>
> (more or less)
melver pointed out on IRC that I missed the whole BIT_WORD(nr) thing.
And with that restored this should indeed be an address dependency.
Still, I wasn't actually expecting test_bit() to be one. Nice find.
next prev parent reply other threads:[~2021-10-27 12:34 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-27 10:19 Potentially Broken Address Dependency via test_bit() When Compiling With Clang Paul Heidekrüger
2021-10-27 11:56 ` David Laight
2021-10-27 12:17 ` Peter Zijlstra
2021-10-27 12:24 ` Marco Elver
2021-10-27 12:34 ` Peter Zijlstra [this message]
2021-10-27 14:27 ` Alan Stern
2021-10-28 12:37 ` Paul Heidekrüger
2021-10-28 14:34 ` Alan Stern
2021-11-02 18:35 ` Paul Heidekrüger
2021-11-02 19:01 ` Alan Stern
2021-11-04 18:16 ` Paul E. McKenney
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