path: root/fs
diff options
authorLinus Torvalds <torvalds@linux-foundation.org>2009-06-08 15:22:24 -0400
committerAl Viro <viro@zeniv.linux.org.uk>2009-06-17 00:36:35 -0400
commit9c64daff9d5afb102dfe64a26829e26725538e58 (patch)
tree6af331d16bf2b730716fd932d1747372fdf790a3 /fs
parent03347e2592078a90df818670fddf97a33eec70fb (diff)
ext3: avoid unnecessary spinlock in critical POSIX ACL path
If a filesystem supports POSIX ACL's, the VFS layer expects the filesystem to do POSIX ACL checks on any files not owned by the caller, and it does this for every single pathname component that it looks up. That obviously can be pretty expensive if the filesystem isn't careful about it, especially with locking. That's doubly sad, since the common case tends to be that there are no ACL's associated with the files in question. ext3 already caches the ACL data so that it doesn't have to look it up over and over again, but it does so by taking the inode->i_lock spinlock on every lookup. Which is a noticeable overhead even if it's a private lock, especially on CPU's where the serialization is expensive (eg Intel Netburst aka 'P4'). For the special case of not actually having any ACL's, all that locking is unnecessary. Even if somebody else were to be changing the ACL's on another CPU, we simply don't care - if we've seen a NULL ACL, we might as well use it. So just load the ACL speculatively without any locking, and if it was NULL, just use it. If it's non-NULL (either because we had a cached entry, or because the cache hasn't been filled in at all), it means that we'll need to get the lock and re-load it properly. This is noticeable even on Nehalem, which does locking quite well (much better than P4). From lmbench: Processor, Processes - times in microseconds - smaller is better -------------------------------------------------------------------- Host OS Mhz null null open slct fork exec sh call I/O stat clos TCP proc proc proc --------- ------------- ---- ---- ---- ---- ---- ---- ---- ---- ---- - before: nehalem.l Linux 2.6.30- 3193 0.04 0.09 0.95 1.45 2.18 69.1 273. 1141 nehalem.l Linux 2.6.30- 3193 0.04 0.09 0.95 1.48 2.28 69.9 253. 1140 nehalem.l Linux 2.6.30- 3193 0.04 0.10 0.95 1.42 2.19 68.6 284. 1141 - after: nehalem.l Linux 2.6.30- 3193 0.04 0.09 0.92 1.44 2.12 68.3 282. 1094 nehalem.l Linux 2.6.30- 3193 0.04 0.09 0.92 1.39 2.20 67.0 308. 1123 nehalem.l Linux 2.6.30- 3193 0.04 0.09 0.92 1.39 2.36 67.4 293. 1148 where you can see what appears to be a roughly 3% improvement in stat and open/close latencies from just the removal of the locking overhead. Of course, this only matters for files you don't own (the owner never needs to do the ACL checks), but that's the common case for libraries, header files, and executables. As well as for the base components of any absolute pathname, even if you are the owner of the final file. [ At some point we probably want to move this ACL caching logic entirely into the VFS layer (and only call down to the filesystem when uncached), but in the meantime this improves ext3 a bit. A similar fix to btrfs makes a much bigger difference (15x improvement in lmbench) due to broken caching. ] Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu> Acked-by: Jan Kara <jack@suse.cz> Cc: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Diffstat (limited to 'fs')
1 files changed, 8 insertions, 5 deletions
diff --git a/fs/ext3/acl.c b/fs/ext3/acl.c
index d81ef2fdb08..e0c74545171 100644
--- a/fs/ext3/acl.c
+++ b/fs/ext3/acl.c
@@ -129,12 +129,15 @@ fail:
static inline struct posix_acl *
ext3_iget_acl(struct inode *inode, struct posix_acl **i_acl)
- struct posix_acl *acl = EXT3_ACL_NOT_CACHED;
+ struct posix_acl *acl = ACCESS_ONCE(*i_acl);
- spin_lock(&inode->i_lock);
- if (*i_acl != EXT3_ACL_NOT_CACHED)
- acl = posix_acl_dup(*i_acl);
- spin_unlock(&inode->i_lock);
+ if (acl) {
+ spin_lock(&inode->i_lock);
+ acl = *i_acl;
+ if (acl != EXT3_ACL_NOT_CACHED)
+ acl = posix_acl_dup(acl);
+ spin_unlock(&inode->i_lock);
+ }
return acl;