From: J. Bruce Fields Date: Tue, 10 Feb 2015 15:55:53 +0000 (-0500) Subject: dcache: return -ESTALE not -EBUSY on distributed fs race X-Git-Url: http://git.lede-project.org./?a=commitdiff_plain;h=3d330dc175d3faf518cf035be5ae6a39bf256e6e;p=openwrt%2Fstaging%2Fblogic.git dcache: return -ESTALE not -EBUSY on distributed fs race On a distributed filesystem it's possible for lookup to discover that a directory it just found is already cached elsewhere in the directory heirarchy. The dcache won't let us keep the directory in both places, so we have to move the dentry to the new location from the place we previously had it cached. If the parent has changed, then this requires all the same locks as we'd need to do a cross-directory rename. But we're already in lookup holding one parent's i_mutex, so it's too late to acquire those locks in the right order. The (unreliable) solution in __d_unalias is to trylock() the required locks and return -EBUSY if it fails. I see no particular reason for returning -EBUSY, and -ESTALE is already the result of some other lookup races on NFS. I think -ESTALE is the more helpful error return. It also allows us to take advantage of the logic Jeff Layton added in c6a9428401c0 "vfs: fix renameat to retry on ESTALE errors" and ancestors, which hopefully resolves some of these errors before they're returned to userspace. I can reproduce these cases using NFS with: ssh root@$client ' mount -olookupcache=pos '$server':'$export' /mnt/ mkdir /mnt/TO mkdir /mnt/DIR touch /mnt/DIR/test.txt while true; do strace -e open cat /mnt/DIR/test.txt 2>&1 | grep EBUSY done ' ssh root@$server ' while true; do mv $export/DIR $export/TO/DIR mv $export/TO/DIR $export/DIR done ' It also helps to add some other concurrent use of the directory on the client (e.g., "ls /mnt/TO"). And you can replace the server-side mv's by client-side mv's that are repeatedly killed. (If the client is interrupted while waiting for the RENAME response then it's left with a dentry that has to go under one parent or the other, but it doesn't yet know which.) Acked-by: Jeff Layton Signed-off-by: J. Bruce Fields Signed-off-by: Al Viro --- diff --git a/fs/dcache.c b/fs/dcache.c index c71e3732e53b..d99736a63e3c 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -2690,7 +2690,7 @@ static int __d_unalias(struct inode *inode, struct dentry *dentry, struct dentry *alias) { struct mutex *m1 = NULL, *m2 = NULL; - int ret = -EBUSY; + int ret = -ESTALE; /* If alias and dentry share a parent, then no extra locks required */ if (alias->d_parent == dentry->d_parent)