On Tuesday 07 October 2008 21:29, Andi Kleen wrote: > > Maybe cmpxchg8b is good for i486 or later x86, but i386 or other > > architectures that do not have similar instruction needs some locking > > primitive. I think lazy > > We have a cmpxchg emulation on 386. That works because only UP 386s are > supported, so it can be done in software. > > > seqlock is one option for making file->f_pos access atomic. > > The question is if it's the right option. At least all the common > operations on fds (read/write) are all writers, not readers. Common operations are read, do something, write. So seqlocks then cost one atomic operation, a couple of memory barriers (all noops on x86), and some predictable branches etc. cmpxchg based would require 2 lock ; cmpxchg8b on 32-bit. Fairly heavy. Also I don't think we have generic accessors to do this, so I think that is for another project. Anyway, I think importantly this creates some usable accessors for the f_pos problem. I think we actually need to touch a _lot_ of code to cover all f_pos accesses in the kernel, but I guess this gets the ball rolling. So.. is everyone agreed that corrupting f_pos is a bad thing? (serious question) If so, then we should get something like this merged sooner rather than later. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
