Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by kernel parameter

From: Luck, Tony
Date: Wed Dec 11 2019 - 13:44:21 EST

On Wed, Dec 11, 2019 at 06:52:02PM +0100, Peter Zijlstra wrote:
> Sure, but we're talking two cpus here.
> u32 var = 0;
> u8 *ptr = &var;
> xchg(ptr, 1)
> xchg((ptr+1, 1);
> r = READ_ONCE(var);

It looks like our current implementation of set_bit() would already run
into this if some call sites for a particular bitmap `pass in constant
bit positions (which get optimized to byte wide "orb") while others pass
in a variable bit (which execute as 64-bit "bts").

I'm not a h/w architect ... but I've assumed that a LOCK operation
on something contained entirely within a cache line gets its atomicity
by keeping exclusive ownership of the cache line. Split lock happens
because you can't keep ownership for two cache lines, so it gets
escalated to a bus lock.