On Sat, Mar 03, 2012 at 06:37:22PM +0100, Stefan Richter wrote: > On Feb 16 Chris Boot wrote: [ . . . ] > > I guess I need to protect access to > > the entire session really. Possibly even rwlocks due to only the > > management processes ever changing anything, but lots of reads during > > command handling. > > The use case for rwlocks is not really the case where infrequent write > access meets frequent read access. Rather, the use case is when it is > important to reduce or prevent contention between readers. > > Rwlocks come with their own downsides though. I guess the somewhat > costlier lock implementation could counter the benefit of allowing > concurrent readers. Or maybe latency spikes around a write access could be > an issue. > > I believe I have read somewhere that one should rather use a simple > spinlock unless exhaustive tests prove that an rwlock really performs > better. Furthermore, in many if not all use cases of rwlocks, RCU is > available as another alternative. RCU comes with its own set of downsides > though, for example not being as well and widely understood by programmers > compared to locking, being less easy to debug (may have improved > recently), and posing some challenges to RT-PREEMPT kernels. The preemptible RCU implementations (TREE_PREEMPT_RCU and TINY_PREEMPT_RCU) handle the -rt tree. > AFAIU the above considerations cannot be applied 100 % to able-to-sleep > reader-writer locks, i.e. the kernel's rwsem. Still, the use case of an > rwsem (in contrast to a mutex) is not particularly where a datum is rarely > written and often read, but where it is desirable to let multiple readers > not block each other. > > [Somebody correct me where I'm wrong.] And you can use SRCU if readers need to block. That said, even I agree that RCU is not always the right tool for the job. ;-), Paul > PS, I cloned your git tree not long ago, but again various distractions > kept me from having a broader look at your code... > -- > Stefan Richter > -=====-===-- --== ---== > http://arcgraph.de/sr/ > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > -- To unsubscribe from this list: send the line "unsubscribe target-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
