Andrew Morton <akpm@xxxxxxxx> wrote:
> Is the cond_resched_lock() here safe? Once we've dropped that lock, the
> list cursor `loop' is invalidated?
Yes. Nothing else is permitted to modify the d_subdirs list. Not the VFS,
not the filesystem, not the memory pressure shrinker.
> If all lookup paths to all entries on this list are removed at this time
> then OK - but these dentries are still on the LRU.
Ah... but the LRU shrinker may not touch dentries belonging to a filesystem
that's got s_umount writelocked (eg: one that's in the process of being
unmounted). See prune_dcache():
/*
* ...otherwise we need to be sure this filesystem isn't being
* unmounted, otherwise we could race with
* generic_shutdown_super(), and end up holding a reference to
* an inode while the filesystem is unmounted.
* So we try to get s_umount, and make sure s_root isn't NULL.
* (Take a local copy of s_umount to avoid a use-after-free of
* `dentry').
*/
s_umount = &dentry->d_sb->s_umount;
if (down_read_trylock(s_umount)) {
if (dentry->d_sb->s_root != NULL) {
prune_one_dentry(dentry);
up_read(s_umount);
continue;
}
up_read(s_umount);
}
spin_unlock(&dentry->d_lock);
> (An answer-via-comment-patch would suit ;))
It is commented already:
/*
* destroy the dentries attached to a superblock on unmounting
* - we don't need to use dentry->d_lock, and only need dcache_lock when
* removing the dentry from the system lists and hashes because:
* - the superblock is detached from all mountings and open files, so the
* dentry trees will not be rearranged by the VFS
* - s_umount is write-locked, so the memory pressure shrinker will ignore
* any dentries belonging to this superblock that it comes across
* - the filesystem itself is no longer permitted to rearrange the dentries
* in this superblock
*/
void shrink_dcache_for_umount(struct super_block *sb)
{
Note the point about the memory pressure shrinker.
David
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