linux-kernel - RE: [PATCH v2] The value may overflow

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Message-ID: <c312066b2cc44919bd11b6cd938cb05f@AcuMS.aculab.com>
Date:   Tue, 5 Sep 2023 14:15:25 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'paulmck@...nel.org'" <paulmck@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:     Denis Arefev <arefev@...mel.ru>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "lvc-project@...uxtesting.org" <lvc-project@...uxtesting.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "trufanov@...mel.ru" <trufanov@...mel.ru>,
        "vfh@...mel.ru" <vfh@...mel.ru>
Subject: RE: [PATCH v2] The value may overflow

...
> That would instead be more than 512-16=496 CPUs, correct?  496 CPUs would
> only require a 31-bit shift, which should be OK, but 497 would require
> a 32-bit shift, which would result in sign extension.  If it turns out
> that sign extension is OK, then we should get in trouble at 513 CPUs,
> which would result in a 33-bit shift (and is that even defined in C?).

Not quite right :-)

(1 << 31) is int and negative, that gets sign extended before
being converted to 'unsigned long' - so has the top 33 bits set.

(1 << 32) is undefined, the current x86 cpu ignore the high
shift bits so it is (1 << 0).

If the mask is being used to optimise a search the code might
just happen to work!

	David

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