Mikulas Patocka <mpatocka@xxxxxxxxxx> writes: > On Tue, 17 Jul 2012, Jeff Moyer wrote: > >> > This is the patch that fixes this crash: it takes a rw-semaphore around >> > all direct-IO path. >> > >> > (note that if someone is concerned about performance, the rw-semaphore >> > could be made per-cpu --- take it for read on the current CPU and take it >> > for write on all CPUs). >> >> Here we go again. :-) I believe we had at one point tried taking a rw >> semaphore around GUP inside of the direct I/O code path to fix the fork >> vs. GUP race (that still exists today). When testing that, the overhead >> of the semaphore was *way* too high to be considered an acceptable >> solution. I've CC'd Larry Woodman, Andrea, and Kosaki Motohiro who all >> worked on that particular bug. Hopefully they can give better >> quantification of the slowdown than my poor memory. >> >> Cheers, >> Jeff > > Both down_read and up_read together take 82 ticks on Core2, 69 ticks on > AMD K10, 62 ticks on UltraSparc2 if the target is in L1 cache. So, if > percpu rw_semaphores were used, it would slow down only by this amount. Sorry, I'm not familiar with per-cpu rw semaphores. Where are they implemented? > I hope that Linux developers are not so obsessed with performance that > they want a fast crashing kernel rather than a slow reliable kernel. > Note that anything that changes a device block size (for example > mounting a filesystem with non-default block size) may trigger a crash > if lvm or udev reads the device simultaneously; the crash really > happened in business environment). I wasn't suggesting that we leave the problem unfixed (though I can see how you might have gotten that idea, sorry for not being more clear). I was merely suggesting that we should try to fix the problem in a way that does not kill performance. Cheers, Jeff -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
