Messages in this thread | | | Date | Fri, 02 May 2014 06:10:48 -0700 | From | Randy Dunlap <> | Subject | Re: [PATCH] rwsem: Comments to explain the meaning of the rwsem's count field |
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On 05/01/2014 04:05 PM, Tim Chen wrote: > On Thu, 2014-05-01 at 16:18 -0400, Peter Hurley wrote: >> On 05/01/2014 01:50 PM, Tim Chen wrote: >>> It takes me a while to understand how rwsem's count field mainifest >>> 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/holding 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. >> >> Except there are a lot of transition states for the count that look like >> stable states for some other condition, and vice versa. >> >> For example, 0xffff000X could be: >> 1. stable state as described below. >> 2. 1 or more (but not X) readers active, >> 1 writer which failed to acquire and has not yet backed out the adjustment >> 0 or more readers which failed to acquire because of the waiting writer >> and have not yet backed out >> 3. 1 writer active, >> 1 or more readers which failed to acquire because of the active writer and >> have not yet backed out >> 4. maybe more states where a owning writer has just dropped the lock > > Thanks for the feedback. Yes, one thing I missed was to account for the > readers and writers who are actively attempting to lock by adding > ACTIVE_BIAS or ACTIVE_WRITE_BIAS to the count. Once we account for > those we should take care of the transition states. > The revised comments also look at the readers and writers actively > attempting the lock. > >> >> Because of this, it's hazardous to infer lock state except for the specific >> existing tests (eg., the count observed by a failed reader after it has >> acquired the wait_lock). > > Thanks. > > Tim > > --- > > 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@linux.intel.com> > --- > 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) > + * > + * 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
> + * acquisition (i.e. nobody else has lock or attempts lock). If > + * unsuccessful, in rwsem_down_write_failed, we'll check to see if there > + * are only waiters but none active (5th case above), and attempt to > + * steal the lock. > + * > + */ > + > +/* > * Initialize an rwsem: > */ > void __init_rwsem(struct rw_semaphore *sem, const char *name, >
-- ~Randy
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