On Tue, Dec 27, 2016 at 10:58 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> The other alternative is to keep the lock bit as bit #0, and just make
> the contention bit be the high bit. Then, on x86, you can do
>
> lock andb $0xfe,flags
> js contention
>
> which might be even better. Again, it would be a very special
> operation just for unlock. Something like
>
> bit_clear_and_branch_if_negative_byte(mem, label);
>
> and again, it would be trivial to do on most architectures.
>
> Let me try to write a patch or two for testing.
Ok, that was easy.
Of course, none of this is *tested*, but it looks superficially
correct, and allows other architectures to do the same optimization if
they want.
On x86, the unlock_page() code now generates
lock; andb $1,(%rdi) #, MEM[(volatile long int *)_7]
js .L114 #,
popq %rbp #
ret
for the actual unlock itself.
Now to actually compile the whole thing and see if it boots..
Linus
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