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authorKonstantin Khlebnikov <khlebnikov@yandex-team.ru>2015-02-19 20:19:35 +0300
committerAl Viro <viro@zeniv.linux.org.uk>2015-02-22 11:38:42 -0500
commiteb6ef3df4faa5424cf2a24b4e4f3eeceb1482a8e (patch)
tree760db5df872be2122597841c33479c41cf2d7194
parent54f2a2f42759b11ada761013a12f0e743702219a (diff)
downloadlinux-eb6ef3df4faa5424cf2a24b4e4f3eeceb1482a8e.tar.gz
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>
-rw-r--r--fs/fs-writeback.c6
-rw-r--r--fs/internal.h2
-rw-r--r--fs/super.c40
3 files changed, 22 insertions, 26 deletions
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 073657f..e907052 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -769,9 +769,9 @@ static long __writeback_inodes_wb(struct bdi_writeback *wb,
struct inode *inode = wb_inode(wb->b_io.prev);
struct super_block *sb = inode->i_sb;
- if (!grab_super_passive(sb)) {
+ if (!trylock_super(sb)) {
/*
- * grab_super_passive() may fail consistently due to
+ * trylock_super() may fail consistently due to
* s_umount being grabbed by someone else. Don't use
* requeue_io() to avoid busy retrying the inode/sb.
*/
@@ -779,7 +779,7 @@ static long __writeback_inodes_wb(struct bdi_writeback *wb,
continue;
}
wrote += writeback_sb_inodes(sb, wb, work);
- drop_super(sb);
+ up_read(&sb->s_umount);
/* refer to the same tests at the end of writeback_sb_inodes */
if (wrote) {
diff --git a/fs/internal.h b/fs/internal.h
index 30459da..01dce1d 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -84,7 +84,7 @@ extern struct file *get_empty_filp(void);
* super.c
*/
extern int do_remount_sb(struct super_block *, int, void *, int);
-extern bool grab_super_passive(struct super_block *sb);
+extern bool trylock_super(struct super_block *sb);
extern struct dentry *mount_fs(struct file_system_type *,
int, const char *, void *);
extern struct super_block *user_get_super(dev_t);
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 @@ static unsigned long super_cache_scan(struct shrinker *shrink,
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 @@ static unsigned long super_cache_scan(struct shrinker *shrink,
freed += sb->s_op->free_cached_objects(sb, sc);
}
- drop_super(sb);
+ up_read(&sb->s_umount);
return freed;
}
@@ -118,7 +118,7 @@ static unsigned long super_cache_count(struct shrinker *shrink,
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 @@ static int grab_super(struct super_block *s) __releases(sb_lock)
}
/*
- * 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;
}