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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UK
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