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authorNick Piggin <npiggin@kernel.dk>2011-01-07 09:49:49 +0300
committerNick Piggin <npiggin@kernel.dk>2011-01-07 09:50:26 +0300
commitfa0d7e3de6d6fc5004ad9dea0dd6b286af8f03e9 (patch)
tree203e0f73883e4c26b5597e36042386a1237dab35 /Documentation/filesystems/porting
parent77812a1ef139d84270d27faacc0630c887411013 (diff)
downloadlinux-fa0d7e3de6d6fc5004ad9dea0dd6b286af8f03e9.tar.xz
fs: icache RCU free inodes
RCU free the struct inode. This will allow: - Subsequent store-free path walking patch. The inode must be consulted for permissions when walking, so an RCU inode reference is a must. - sb_inode_list_lock to be moved inside i_lock because sb list walkers who want to take i_lock no longer need to take sb_inode_list_lock to walk the list in the first place. This will simplify and optimize locking. - Could remove some nested trylock loops in dcache code - Could potentially simplify things a bit in VM land. Do not need to take the page lock to follow page->mapping. The downsides of this is the performance cost of using RCU. In a simple creat/unlink microbenchmark, performance drops by about 10% due to inability to reuse cache-hot slab objects. As iterations increase and RCU freeing starts kicking over, this increases to about 20%. In cases where inode lifetimes are longer (ie. many inodes may be allocated during the average life span of a single inode), a lot of this cache reuse is not applicable, so the regression caused by this patch is smaller. The cache-hot regression could largely be avoided by using SLAB_DESTROY_BY_RCU, however this adds some complexity to list walking and store-free path walking, so I prefer to implement this at a later date, if it is shown to be a win in real situations. I haven't found a regression in any non-micro benchmark so I doubt it will be a problem. Signed-off-by: Nick Piggin <npiggin@kernel.dk>
Diffstat (limited to 'Documentation/filesystems/porting')
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diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting
index 1eb76959d096..ccf0ce7866b9 100644
--- a/Documentation/filesystems/porting
+++ b/Documentation/filesystems/porting
@@ -346,3 +346,17 @@ look at examples of other filesystems) for guidance.
for details of what locks to replace dcache_lock with in order to protect
particular things. Most of the time, a filesystem only needs ->d_lock, which
protects *all* the dcache state of a given dentry.
+
+--
+[mandatory]
+
+ Filesystems must RCU-free their inodes, if they can have been accessed
+via rcu-walk path walk (basically, if the file can have had a path name in the
+vfs namespace).
+
+ i_dentry and i_rcu share storage in a union, and the vfs expects
+i_dentry to be reinitialized before it is freed, so an:
+
+ INIT_LIST_HEAD(&inode->i_dentry);
+
+must be done in the RCU callback.