trylock_super(): replacement for grab_super_passive()
I've noticed significant locking contention in memory reclaimer around
sb_lock inside grab_super_passive(). Grab_super_passive() is called from
two places: in icache/dcache shrinkers (function super_cache_scan) and
from writeback (function __writeback_inodes_wb). Both are required for
progress in memory allocator.
Grab_super_passive() acquires sb_lock to increment sb->s_count and check
sb->s_instances. It seems sb->s_umount locked for read is enough here:
super-block deactivation always runs under sb->s_umount locked for write.
Protecting super-block itself isn't a problem: in super_cache_scan() sb
is protected by shrinker_rwsem: it cannot be freed if its slab shrinkers
are still active. Inside writeback super-block comes from inode from bdi
writeback list under wb->list_lock.
This patch removes locking sb_lock and checks s_instances under s_umount:
generic_shutdown_super() unlinks it under sb->s_umount locked for write.
New variant is called trylock_super() and since it only locks semaphore,
callers must call up_read(&sb->s_umount) instead of drop_super(sb) when
they're done.
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/super.c b/fs/super.c
index 65a53ef..2b7dc90 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -71,7 +71,7 @@
if (!(sc->gfp_mask & __GFP_FS))
return SHRINK_STOP;
- if (!grab_super_passive(sb))
+ if (!trylock_super(sb))
return SHRINK_STOP;
if (sb->s_op->nr_cached_objects)
@@ -105,7 +105,7 @@
freed += sb->s_op->free_cached_objects(sb, sc);
}
- drop_super(sb);
+ up_read(&sb->s_umount);
return freed;
}
@@ -118,7 +118,7 @@
sb = container_of(shrink, struct super_block, s_shrink);
/*
- * Don't call grab_super_passive as it is a potential
+ * Don't call trylock_super as it is a potential
* scalability bottleneck. The counts could get updated
* between super_cache_count and super_cache_scan anyway.
* Call to super_cache_count with shrinker_rwsem held
@@ -348,35 +348,31 @@
}
/*
- * grab_super_passive - acquire a passive reference
+ * trylock_super - try to grab ->s_umount shared
* @sb: reference we are trying to grab
*
- * Tries to acquire a passive reference. This is used in places where we
+ * Try to prevent fs shutdown. This is used in places where we
* cannot take an active reference but we need to ensure that the
- * superblock does not go away while we are working on it. It returns
- * false if a reference was not gained, and returns true with the s_umount
- * lock held in read mode if a reference is gained. On successful return,
- * the caller must drop the s_umount lock and the passive reference when
- * done.
+ * filesystem is not shut down while we are working on it. It returns
+ * false if we cannot acquire s_umount or if we lose the race and
+ * filesystem already got into shutdown, and returns true with the s_umount
+ * lock held in read mode in case of success. On successful return,
+ * the caller must drop the s_umount lock when done.
+ *
+ * Note that unlike get_super() et.al. this one does *not* bump ->s_count.
+ * The reason why it's safe is that we are OK with doing trylock instead
+ * of down_read(). There's a couple of places that are OK with that, but
+ * it's very much not a general-purpose interface.
*/
-bool grab_super_passive(struct super_block *sb)
+bool trylock_super(struct super_block *sb)
{
- spin_lock(&sb_lock);
- if (hlist_unhashed(&sb->s_instances)) {
- spin_unlock(&sb_lock);
- return false;
- }
-
- sb->s_count++;
- spin_unlock(&sb_lock);
-
if (down_read_trylock(&sb->s_umount)) {
- if (sb->s_root && (sb->s_flags & MS_BORN))
+ if (!hlist_unhashed(&sb->s_instances) &&
+ sb->s_root && (sb->s_flags & MS_BORN))
return true;
up_read(&sb->s_umount);
}
- put_super(sb);
return false;
}