Date: Mon, 1 Apr 2019 15:28:35 +0000 From: David Laight <David.Laight@...LAB.COM> To: 'Alan Stern' <stern@...land.harvard.edu>, Oleg Nesterov <oleg@...hat.com> CC: "Paul E. McKenney" <paulmck@...ux.ibm.com>, Jann Horn <jannh@...gle.com>, Joel Fernandes <joel@...lfernandes.org>, Kees Cook <keescook@...omium.org>, "Eric W. Biederman" <ebiederm@...ssion.com>, LKML <linux-kernel@...r.kernel.org>, Android Kernel Team <kernel-team@...roid.com>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Andrew Morton <akpm@...ux-foundation.org>, Matthew Wilcox <willy@...radead.org>, "Michal Hocko" <mhocko@...e.com>, "Reshetova, Elena" <elena.reshetova@...el.com> Subject: RE: [PATCH] Convert struct pid count to refcount_t From: Alan Stern > Sent: 29 March 2019 19:45 ... > There is a big difference between WRITE_ONCE() and plain assignment. > Given "WRITE_ONCE(X, 2)", the compiler will emit a simple store > instruction. But given "X = 2", the compiler is allowed to emit > instructions equivalent to: > > if (X != 2) > X = 2; Worse for you, it can also emit: X = 0; X = 2; Many years ago I fell foul of a compiler (not C) that implemented a write to a 2 bit wide bitfield as: X &= ~3 X |= value even when 'value' was a compile time constant of 3. Took a while to find out why the linked list got f*cked. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
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