On 01/23/2013 12:15 AM, Stephen Hemminger wrote: > On Tue, 22 Jan 2013 13:03:22 +0530 > "Srivatsa S. Bhat" <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> wrote: > >> A straight-forward (and obvious) algorithm to implement Per-CPU Reader-Writer >> locks can also lead to too many deadlock possibilities which can make it very >> hard/impossible to use. This is explained in the example below, which helps >> justify the need for a different algorithm to implement flexible Per-CPU >> Reader-Writer locks. >> >> We can use global rwlocks as shown below safely, without fear of deadlocks: >> >> Readers: >> >> CPU 0 CPU 1 >> ------ ------ >> >> 1. spin_lock(&random_lock); read_lock(&my_rwlock); >> >> >> 2. read_lock(&my_rwlock); spin_lock(&random_lock); >> >> >> Writer: >> >> CPU 2: >> ------ >> >> write_lock(&my_rwlock); >> >> >> We can observe that there is no possibility of deadlocks or circular locking >> dependencies here. Its perfectly safe. >> >> Now consider a blind/straight-forward conversion of global rwlocks to per-CPU >> rwlocks like this: >> >> The reader locks its own per-CPU rwlock for read, and proceeds. >> >> Something like: read_lock(per-cpu rwlock of this cpu); >> >> The writer acquires all per-CPU rwlocks for write and only then proceeds. >> >> Something like: >> >> for_each_online_cpu(cpu) >> write_lock(per-cpu rwlock of 'cpu'); >> >> >> Now let's say that for performance reasons, the above scenario (which was >> perfectly safe when using global rwlocks) was converted to use per-CPU rwlocks. >> >> >> CPU 0 CPU 1 >> ------ ------ >> >> 1. spin_lock(&random_lock); read_lock(my_rwlock of CPU 1); >> >> >> 2. read_lock(my_rwlock of CPU 0); spin_lock(&random_lock); >> >> >> Writer: >> >> CPU 2: >> ------ >> >> for_each_online_cpu(cpu) >> write_lock(my_rwlock of 'cpu'); >> >> >> Consider what happens if the writer begins his operation in between steps 1 >> and 2 at the reader side. It becomes evident that we end up in a (previously >> non-existent) deadlock due to a circular locking dependency between the 3 >> entities, like this: >> >> >> (holds Waiting for >> random_lock) CPU 0 -------------> CPU 2 (holds my_rwlock of CPU 0 >> for write) >> ^ | >> | | >> Waiting| | Waiting >> for | | for >> | V >> ------ CPU 1 <------ >> >> (holds my_rwlock of >> CPU 1 for read) >> >> >> >> So obviously this "straight-forward" way of implementing percpu rwlocks is >> deadlock-prone. One simple measure for (or characteristic of) safe percpu >> rwlock should be that if a user replaces global rwlocks with per-CPU rwlocks >> (for performance reasons), he shouldn't suddenly end up in numerous deadlock >> possibilities which never existed before. The replacement should continue to >> remain safe, and perhaps improve the performance. >> >> Observing the robustness of global rwlocks in providing a fair amount of >> deadlock safety, we implement per-CPU rwlocks as nothing but global rwlocks, >> as a first step. >> >> >> Cc: David Howells <dhowells@xxxxxxxxxx> >> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> > > We got rid of brlock years ago, do we have to reintroduce it like this? > The problem was that brlock caused starvation. > Um? I still see it in include/linux/lglock.h and its users in fs/ directory. BTW, I'm not advocating that everybody start converting their global reader-writer locks to per-cpu rwlocks, because such a conversion probably won't make sense in all scenarios. The thing is, for CPU hotplug in particular, the "preempt_disable() at the reader; stop_machine() at the writer" scheme had some very desirable properties at the reader side (even though people might hate stop_machine() with all their heart ;-)), namely : At the reader side: o No need to hold locks to prevent CPU offline o Extremely fast/optimized updates (the preempt count) o No need for heavy memory barriers o Extremely flexible nesting rules So this made perfect sense at the reader for CPU hotplug, because it is expected that CPU hotplug operations are very infrequent, and it is well-known that quite a few atomic hotplug readers are in very hot paths. The problem was that the stop_machine() at the writer was not only a little too heavy, but also inflicted real-time latencies on the system because it needed cooperation from _all_ CPUs synchronously, to take one CPU down. So the idea is to get rid of stop_machine() without hurting the reader side. And this scheme of per-cpu rwlocks comes close to ensuring that. (You can look at the previous versions of this patchset [links given in cover letter] to see what other schemes we hashed out before coming to this one). The only reason I exposed this as a generic locking scheme was because Tejun pointed out that, complex locking schemes implemented in individual subsystems is not such a good idea. And also this comes at a time when per-cpu rwsemaphores have just been introduced in the kernel and Oleg had ideas about converting the cpu hotplug (sleepable) locking to use them. Regards, Srivatsa S. Bhat -- 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
