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Date:   Thu, 6 Aug 2020 15:48:28 +0200
From:   peterz@infradead.org
To:     Peter Oskolkov <posk@google.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        linux-kernel@vger.kernel.org, Paul Turner <pjt@google.com>,
        Chris Kennelly <ckennelly@google.com>,
        Peter Oskolkov <posk@posk.io>
Subject: Re: [PATCH 1/2] membarrier: add
 MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU
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On Wed, Aug 05, 2020 at 05:08:58PM -0700, Peter Oskolkov wrote:

Thanks for the Cc!

> + * @MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU:
> + *                          If a thread belonging to the current process
> + *                          is currently in an RSEQ critical section on the
> + *                          CPU identified by flags parameter, restart it.
> + *                          @flags: if @flags >= 0, identifies the CPU to
> + *                                  restart RSEQ CS on; if == -1, restarts
> + *                                  RSEQ CSs on all CPUs.

> +	} else if (cpu_id == -1) {
> +		on_each_cpu(membarrier_rseq_ipi,
> +			    current->group_leader, true);

This is an unpriv IPI the world. That's a big no-no.

Double so because all you want to target is the current process, which
you're defining as CLONE_THREAD, where the rest of this file uses
CLONE_VM to define a process.