On Tue, Jan 29, 2013 at 08:12:37PM +0900, Namhyung Kim wrote: > On Thu, 24 Jan 2013 10:00:04 +0530, Srivatsa S. Bhat wrote: > > On 01/24/2013 01:27 AM, Tejun Heo wrote: > >> On Thu, Jan 24, 2013 at 01:03:52AM +0530, Srivatsa S. Bhat wrote: > >>> CPU 0 CPU 1 > >>> > >>> read_lock(&rwlock) > >>> > >>> write_lock(&rwlock) //spins, because CPU 0 > >>> //has acquired the lock for read > >>> > >>> read_lock(&rwlock) > >>> ^^^^^ > >>> What happens here? Does CPU 0 start spinning (and hence deadlock) or will > >>> it continue realizing that it already holds the rwlock for read? > >> > >> I don't think rwlock allows nesting write lock inside read lock. > >> read_lock(); write_lock() will always deadlock. > >> > > > > Sure, I understand that :-) My question was, what happens when *two* CPUs > > are involved, as in, the read_lock() is invoked only on CPU 0 whereas the > > write_lock() is invoked on CPU 1. > > > > For example, the same scenario shown above, but with slightly different > > timing, will NOT result in a deadlock: > > > > Scenario 2: > > CPU 0 CPU 1 > > > > read_lock(&rwlock) > > > > > > read_lock(&rwlock) //doesn't spin > > > > write_lock(&rwlock) //spins, because CPU 0 > > //has acquired the lock for read > > > > > > So I was wondering whether the "fairness" logic of rwlocks would cause > > the second read_lock() to spin (in the first scenario shown above) because > > a writer is already waiting (and hence new readers should spin) and thus > > cause a deadlock. > > In my understanding, current x86 rwlock does basically this (of course, > in an atomic fashion): > > > #define RW_LOCK_BIAS 0x10000 > > rwlock_init(rwlock) > { > rwlock->lock = RW_LOCK_BIAS; > } > > arch_read_lock(rwlock) > { > retry: > if (--rwlock->lock >= 0) > return; > > rwlock->lock++; > while (rwlock->lock < 1) > continue; > > goto retry; > } > > arch_write_lock(rwlock) > { > retry: > if ((rwlock->lock -= RW_LOCK_BIAS) == 0) > return; > > rwlock->lock += RW_LOCK_BIAS; > while (rwlock->lock != RW_LOCK_BIAS) > continue; > > goto retry; > } > > > So I can't find where the 'fairness' logic comes from.. I looked through several of the rwlock implementations, and in all of them the writer backs off if it sees readers -- or refrains from asserting ownership of the lock to begin with. So it should be OK to use rwlock as shown in the underlying patch. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html