> > /*
> > + * Guide to the rw_semaphore's count field for common values.
> > + * (32 bit case illustrated, similar for 64 bit)
>
> The values below are x86-specific; the actual defines are arch-dependent.
> Do other archs use different values?
This is also the value used also for generic case. I don't see other
arch specific values defined.
>
> > + *
> > + * 0x0000000X (1) X readers active or attempting lock, no writer
> > waiting
> > + * X = #active_readers + #readers attempting to lock
> > + * (X*ACTIVE_BIAS)
>
> Not sure it matters, but maybe you want to note that it's possible for 0
> readers
> to be active with this value, and all of the other readers may have initially
> failed to claim the lock but may be successful if one can claim the wait_lock
> while
> the count is still > 0.
I'll add the explanation for the down_read_failed scenario in
the note section below.
>
> > + *
> > + * 0x00000000 rwsem is unlocked, and no one is waiting for the lock or
> > + * attempting to read lock or write lock.
> > + *
> > + * 0xffff000X (1) X readers active or attempt lock, there are waiters
> > for lock
> > + * X = #active readers + # readers attempting lock
> > + * (X*ACTIVE_BIAS + WAITING_BIAS)
> > + * (2) 1 writer attempting lock, no waiters for lock
> > + * X-1 = #active readers + #readers attempting lock
> > + * ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
> > + * (3) 1 writer active, no waiters for lock
> > + * X-1 = #active readers + #readers attempting lock
> > + * ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
> > + *
> > + * 0xffff0001 (1) 1 reader active or attempting lock, waiters for lock
> > + * (WAITING_BIAS + ACTIVE_BIAS)
> > + * (2) 1 writer active or attempt lock, no waiters for lock
> > + * (ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
> > + *
> > + * 0xffff0000 (1) There are writers or readers queued but none active
> > + * or in the process of attempting lock.
> > + * (WAITING_BIAS)
> > + * Note: writer can attempt to steal lock for this count by adding
> > + * ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
> > + *
> > + * 0xfffe0001 (1) 1 writer active, or attempting lock. Waiters on
> > queue.
> > + * (ACTIVE_WRITE_BIAS + WAITING_BIAS)
>
> The count can have more values than just 0xfffe0001 because multiple
> failed write lock attempts plus failed read lock attempts can produce other
> values than those listed.
You're correct. The values are not comprehensive. I tried to show the common
ones and how they arose. How about I replace the 0xfffe0001 case with
count < WAITING_BIAS
(1) X writer active, Y writers attempting lock, Z readers attempting
lock, no waiters
where X = 0 or 1, (X+Y) >= 2, Z >= 0
(X+Y) * ACTIVE_WRITE_BIAS + Z * ACTIVE_BIAS
(2) X writer active, Y writers attempting lock, Z readers attempting
lock, with waiters
where X = 0 or 1, (X+Y) >= 1, Z >= 0
(X+Y) * ACTIVE_WRITE_BIAS + Z * ACTIVE_BIAS + WAITING_BIAS
Thanks.
Tim
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