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Date: Thu, 28 Mar 2019 17:26:42 +0100
From: Oleg Nesterov <>
To: Jann Horn <>
Cc: Joel Fernandes <>,
	"Paul E. McKenney" <>,
	Kees Cook <>,
	"Eric W. Biederman" <>,
	LKML <>,
	Android Kernel Team <>,
	Kernel Hardening <>,
	Andrew Morton <>,
	Matthew Wilcox <>,
	Michal Hocko <>,
	"Reshetova, Elena" <>
Subject: Re: [PATCH] Convert struct pid count to refcount_t

On 03/28, Jann Horn wrote:
> Since we're just talking about RCU stuff now, adding Paul McKenney to
> the thread.

Since you added Paul let me add more confusion to this thread ;)

There were some concerns about the lack of barriers in put_pid(), but I can't
find that old discussion and I forgot the result of that discussion...

Paul, could you confirm that this code

	CPU_0		CPU_1

	X = 1;		if (READ_ONCE(Y))
	mb();			X = 2;
	Y = 1;		BUG_ON(X != 2);

is correct? I think it is, control dependency pairs with mb(), right?

If not, then put_pid() needs atomic_read_acquire() as it was proposed in that


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