linux-kernel - RE: [PATCH] list: Prevent compiler reloads inside 'safe' list iteration

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Message-ID: <2e936d8fd2c445beb08e6dd3ee1f3891@AcuMS.aculab.com>
Date:   Tue, 10 Mar 2020 11:36:41 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Chris Wilson' <chris@...is-wilson.co.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH] list: Prevent compiler reloads inside 'safe' list
 iteration

From: Chris Wilson
> Sent: 10 March 2020 09:21
> Instruct the compiler to read the next element in the list iteration
> once, and that it is not allowed to reload the value from the stale
> element later. This is important as during the course of the safe
> iteration, the stale element may be poisoned (unbeknownst to the
> compiler).

Eh?
I thought any function call will stop the compiler being allowed
to reload the value.
The 'safe' loop iterators are only 'safe' against called
code removing the current item from the list.

> This helps prevent kcsan warnings over 'unsafe' conduct in releasing the
> list elements during list_for_each_entry_safe() and friends.

Sounds like kcsan is buggy ????

	David

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