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Date: Mon, 20 Aug 2012 16:59:19 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Jones <davej@redhat.com>, Linux Kernel <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, rostedt <rostedt@goodmis.org>,
        dhowells <dhowells@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: lockdep trace from posix timers
Message-ID: <20120820145919.GB18499@redhat.com>
References: <20120724203613.GA9637@redhat.com> <1345140478.29668.54.camel@twins> <20120817151447.GA7918@redhat.com> <1345446957.23018.14.camel@twins> <1345463081.23018.34.camel@twins>
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On 08/20, Peter Zijlstra wrote:
>
>  task_work_cancel(struct task_struct *task, task_work_func_t func)
>  {
> ...
> +
> +again:
> +	workp = &task->task_works;
> +	work = *workp;
> +	while (work) {
> +		if (work->func == func) {

But this all can race with task_work_run() if task != current.

This "work" can be freed/reused. And it should only return !NULL
if twork->func() was not called.

Oleg.