On Fri, May 15, 2015 at 05:45:56PM -0700, Linus Torvalds wrote: > Al, do you have any ideas? Personally, I've wanted to make I_mutex a > rwsem for a long time, but right now pretty much everything uses it > for exclusion. For example, filename lookup is clearly just reading > the directory, so it should take a rwsem for reading, right? No. Not > the way it is done now. Filename lookup wants the directory inode > exclusively because that guarantees that we create just one dentry and > call the filesystem ->lookup only once on that dentry. rwsem by itself won't do us much good there. Look: for multiple lookups on the same existing entry we could try to teach d_splice_alias() to cope, etc. But what happens when a bunch of processes looks for the same inexistent entry? And no, "who cares about fuckloads of negatives with the same name" isn't a good answer - suppose we do mkdir() after that. OK, so we'll find a negative dentry in dcache. And tell the filesystem to create the sucker. Done. Made it positive. Now, do we hunt down all _other_ negative dentries for it? Or never keep negative ones at all. Or slap some kind of ->d_revalidate() there to catch all negative dentries creates before the last mkdir/creat/mknod/symlink/link in given parent? One possibility would be a new dentry state - "being looked up". Hashed, treated as "fall out of RCU mode" for lazy pathwalk purposes, and places where we call ->lookup() would (while still holding ->i_mutex on parent shared) wait for that state to end. Places where we call ->d_revalidate() (with or without ->i_mutex on parent) would also wait on those. It would need a careful analysis of tree-walkers, though. Doable, but there might be dragons. In case of e.g. ceph - swamp ones, with mirror in the line of sight... -- 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
