From eb6ef3df4faa5424cf2a24b4e4f3eeceb1482a8e Mon Sep 17 00:00:00 2001 From: Konstantin Khlebnikov Date: Thu, 19 Feb 2015 20:19:35 +0300 Subject: 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 Signed-off-by: Al Viro --- fs/super.c | 40 ++++++++++++++++++---------------------- 1 file changed, 18 insertions(+), 22 deletions(-) (limited to 'fs/super.c') diff --git a/fs/super.c b/fs/super.c index 65a53efc1cf4..2b7dc90ccdbb 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; } -- cgit