Re: [PATCH] rwsem: Comments to explain the meaning of the rwsem's count field

From: Tim Chen
Date: Fri May 02 2014 - 12:09:21 EST


On Fri, 2014-05-02 at 06:10 -0700, Randy Dunlap wrote:

> >
> > It takes me quite a while to understand how rwsem's count field mainifest
>
> manifests
>
> > itself in different scenarios. I'm adding comments to provide a quick
> > reference on the the rwsem's count field for each scenario where readers
> > and writers are contending for the lock. Hopefully it will be useful
> > for future maintenance of the code and for people to get up to speed on
> > how the logic in the code works.
> >
> > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > ---
> > kernel/locking/rwsem-xadd.c | 48 +++++++++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 48 insertions(+)
> >
> > diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> > index 1d66e08..b92a403 100644
> > --- a/kernel/locking/rwsem-xadd.c
> > +++ b/kernel/locking/rwsem-xadd.c
> > @@ -12,6 +12,54 @@
> > #include <linux/export.h>
> >
> > /*
> > + * Guide to the rw_semaphore's count field for common values.
> > + * (32 bit case illustrated, similar for 64 bit)
>
> 32-bit 64-bit
>
> > + *
> > + * 0x0000000X (1) X readers active or attempting lock, no writer waiting
> > + * X = #active_readers + #readers attempting to lock
> > + * (X*ACTIVE_BIAS)
> > + *
> > + * 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
>
> attempting
>
> > + * 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
>
> attempting
>
> > + * (ACTIVE_BIAS + ACTIVE_WRITE_BIAS)

Oops, should be (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)
> > + *
> > + * Note: Reader attempt to lock by adding ACTIVE_BIAS in down_read and checking
> > + * the count becomes more than 0, i.e. the case where there are only
> > + * readers or no body has lock. (1st and 2nd case above)
>
> nobody
>
> > + *
> > + * Writer attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
> > + * checking the count becomes ACTIVE_WRITE_BIAS for succesful lock
>
> successful

Thanks for correcting my grammar mistakes. Will update the changes.

Tim



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