On Fri, 31 Jul 2015, Dominique Martinet wrote: > Hugh Dickins wrote on Fri, Jul 31, 2015: > > It will indeed be weird and odd if it confirms that DCACHE_DISCONNECTED > > revert is good. I agree that Dominique's 4bf46a272647 seems now more > > likely, if still unlikely; but that was included in v4.1, and I saw > > no problem with v4.1 once the rmap_walk() skip was fixed. > > I think it could, actually, and that neither commits are actually bad -- > just that they affect timing enough to raise an issue between d_delete > (I guess?) and link_path_walk (see last mail in other thread[1]) > > It's probably an old race that was very hard to hit because of cache > coherency. > Basically, before the wmb/rmb, the dentry was always updated closely to > its flags, so the other CPU would "usually" get both updates at the same > time; the barriers make it so the updates are split and it's possible to > get it, and would explain why I could pick 4bf46a2726 as "the one" > > > I'm not sure why the problem wouldn't arise on tmpfs though. > > Hugh, could you try the reproducer I gave in the other thread[2] on both > filesystems maybe? Sorry, I probably won't get around to that, to be honest: it shouldn't need me to run it anyway. > I need to let the thing run for a while, might need to tune params as > well. I was trying to fine tune cpu affinity with less threads but it's > not getting anywhere. > > I'll also check if it's getting even easier to reproduce with > 75a6f82a0d10 (or a recent kernel), who knows... How fast do you hit the > bug with the commit? "A number of hours". I don't have my records in front of me at the moment, but I think when I was lucky it happened within two hours, but more commonly around ten or twelve hours. I just leave it going and get on with other things: yours may be a _much_ better reproducer. Though once there's a potential fix, we shall both need to try it, to report back if our separate cases are fixed. Hugh > > > Thanks, > -- > Dominique > > [1] https://marc.info/?l=linux-fsdevel&m=143835651005259&w=2 > [2] https://marc.info/?l=linux-fsdevel&m=143825706609188&w=2 -- 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
