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libfs: Remove parent dentry locking in offset_iterate_dir() 

Since offset_iterate_dir() does not walk the parent's d_subdir list
nor does it manipulate the parent's d_child, there doesn't seem to
be a reason to hold the parent's d_lock. The offset_ctx's xarray can
be sufficiently protected with just the RCU read lock.

Flame graph data captured during the git regression run shows a
20% reduction in CPU cycles consumed in offset_find_next().

Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202307171640.e299f8d5-oliver.sang@intel.com
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Message-Id: <169030957098.157536.9938425508695693348.stgit@manet.1015granger.net>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Chuck Lever 2023-07-25 14:31:04 -04:00 committed by Christian Brauner
parent bbaef7973d
commit 2be4f05af7
1 changed files with 4 additions and 7 deletions
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11
fs/libfs.c
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@ -407,7 +407,7 @@ static struct dentry *offset_find_next(struct xa_state *xas)
child = xas_next_entry(xas, U32_MAX);
if (!child)
goto out;
spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED);
spin_lock(&child->d_lock);
if (simple_positive(child))
found = dget_dlock(child);
spin_unlock(&child->d_lock);
@ -425,17 +425,14 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
inode->i_ino, fs_umode_to_dtype(inode->i_mode));
}
static void offset_iterate_dir(struct dentry *dir, struct dir_context *ctx)
static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
{
struct inode *inode = d_inode(dir);
struct offset_ctx *so_ctx = inode->i_op->get_offset_ctx(inode);
XA_STATE(xas, &so_ctx->xa, ctx->pos);
struct dentry *dentry;
while (true) {
spin_lock(&dir->d_lock);
dentry = offset_find_next(&xas);
spin_unlock(&dir->d_lock);
if (!dentry)
break;
@ -465,7 +462,7 @@ static void offset_iterate_dir(struct dentry *dir, struct dir_context *ctx)
* output buffer.
*
* On return, @ctx->pos contains an offset that will read the next entry
* in this directory when shmem_readdir() is called again with @ctx.
* in this directory when offset_readdir() is called again with @ctx.
*
* Return values:
* %0 - Complete
@ -479,7 +476,7 @@ static int offset_readdir(struct file *file, struct dir_context *ctx)
if (!dir_emit_dots(file, ctx))
return 0;
offset_iterate_dir(dir, ctx);
offset_iterate_dir(d_inode(dir), ctx);
return 0;
}