On Mon, Dec 19, 2005 at 09:27:44AM +0000, Russell King wrote:
> > mov r0, #1
> > swp r1, r0, [%0]
> > cmp r1, #0
> > bne __contention
> That's over-simplified, and is the easy bit. Now work out how you handle
> the unlock operation.
>
> You don't know whether the lock is contended or not in the unlock path,
> so you always have to do the "wake up" thing. (You can't rely on the
> value of the lock since another thread may well be between this swp
> instruction and entering the __contention function. Hence you can't
> use the value of the lock to determine whether there's anyone sleeping
> on it.)
Here's a slightly less efficient way to determine if anyone else has
swp'd behind your back (apologies if I get my ARM assembly wrong, it's
been a few years):
swp r1, r13, [%0] # load r1 from addr and store r13 there
cmp r1, #0 # is r1 0?
streq r13, [%0, #4] # if it is, store a copy of r13 at the
# next address
blne __lock_contention
I'm assuming that r13 (the stack pointer) will be different for each
task, and (with this being a mutex), we won't try to double-acquire it.
Unlock is then:
mov r0, #0 # put 0 in r0
swp r1, r0, [%0] # release the lock
ldr r0, [%0, #4] # load the copy
cmp r0, r1 # did it change?
blne __unlock_contention
In __unlock_contention, thread1 would try to re-acquire the mutex (at
[%0]), and if it does, wake up the first waiter. Obviously it's unfair
as thread3 could come along and grab the mutex, but that's not a huge
deal.
Note that we're not checking the value of r13 at the unlock site as lock
and unlock can be done at different stack levels. We're checking to see
if the value last written to the lock was the one by the successful
acquirer.
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