Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH] Convert struct pid count to refcount_t | Date | Mon, 1 Apr 2019 15:28:35 +0000 |
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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
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