linux-kernel - Re: Possible lock-less list race in scheduler

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Date:	Thu, 5 Mar 2015 17:02:06 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Huang Ying <ying.huang@...el.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Lai Jiangshan <eag0628@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: Possible lock-less list race in scheduler_ipi()

On Thu, Mar 5, 2015 at 3:48 PM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> llist_next() is pretty simple:
>
> static inline struct llist_node *llist_next(struct llist_node *node)
> {
>         return node->next;
> }
>
> It is so simple that I wonder if the compiler would be
> within its rights to reorder the load of node->next
> after some operations within ttwu_do_activate(), thus
> causing corruption of this linked-list due to a
> concurrent try_to_wake_up() performed by another core.
>
> Am I too paranoid about the possible compiler mishaps
> there, or are my concerns justified ?

I *think* you are too paranoid, because that would be a major compiler
bug anyway - gcc cannot reorder the load against anything that might
be changing the value.  Which obviously includes calling non-inlined
functions.

At least the code generation I see doesn't seem to say that gcc gets this wrong:

        ...
        leaq    -32(%rbx), %rsi #, p
        movq    (%rbx), %rbx    # MEM[(struct llist_node
*)__mptr_19].next, __mptr
        movq    %r12, %rdi      # tcp_ptr__,
        call    ttwu_do_activate.constprop.85   #
        ...

that "movq (%rbx), %rbx" is the "llist = llist_next(llist);" thing.

                            Linus
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