From 8129ed29644bf56ed17ec1bbbeed5c568b43d6a0 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Tue, 11 Aug 2015 17:05:04 +0200 Subject: change sb_writers to use percpu_rw_semaphore We can remove everything from struct sb_writers except frozen and add the array of percpu_rw_semaphore's instead. This patch doesn't remove sb_writers->wait_unfrozen yet, we keep it for get_super_thawed(). We will probably remove it later. This change tries to address the following problems: - Firstly, __sb_start_write() looks simply buggy. It does __sb_end_write() if it sees ->frozen, but if it migrates to another CPU before percpu_counter_dec(), sb_wait_write() can wrongly succeed if there is another task which holds the same "semaphore": sb_wait_write() can miss the result of the previous percpu_counter_inc() but see the result of this percpu_counter_dec(). - As Dave Hansen reports, it is suboptimal. The trivial microbenchmark that writes to a tmpfs file in a loop runs 12% faster if we change this code to rely on RCU and kill the memory barriers. - This code doesn't look simple. It would be better to rely on the generic locking code. According to Dave, this change adds the same performance improvement. Note: with this change both freeze_super() and thaw_super() will do synchronize_sched_expedited() 3 times. This is just ugly. But: - This will be "fixed" by the rcu_sync changes we are going to merge. After that freeze_super()->percpu_down_write() will use synchronize_sched(), and thaw_super() won't use synchronize() at all. This doesn't need any changes in fs/super.c. - Once we merge rcu_sync changes, we can also change super.c so that all wb_write->rw_sem's will share the single ->rss in struct sb_writes, then freeze_super() will need only one synchronize_sched(). Signed-off-by: Oleg Nesterov Reviewed-by: Jan Kara --- include/linux/fs.h | 19 ++++++------------- 1 file changed, 6 insertions(+), 13 deletions(-) (limited to 'include') diff --git a/include/linux/fs.h b/include/linux/fs.h index 4bed78966c6b..ce356f66cc2a 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -1,7 +1,6 @@ #ifndef _LINUX_FS_H #define _LINUX_FS_H - #include #include #include @@ -31,6 +30,7 @@ #include #include #include +#include #include #include @@ -1275,16 +1275,9 @@ enum { #define SB_FREEZE_LEVELS (SB_FREEZE_COMPLETE - 1) struct sb_writers { - /* Counters for counting writers at each level */ - struct percpu_counter counter[SB_FREEZE_LEVELS]; - wait_queue_head_t wait; /* queue for waiting for - writers / faults to finish */ - int frozen; /* Is sb frozen? */ - wait_queue_head_t wait_unfrozen; /* queue for waiting for - sb to be thawed */ -#ifdef CONFIG_DEBUG_LOCK_ALLOC - struct lockdep_map lock_map[SB_FREEZE_LEVELS]; -#endif + int frozen; /* Is sb frozen? */ + wait_queue_head_t wait_unfrozen; /* for get_super_thawed() */ + struct percpu_rw_semaphore rw_sem[SB_FREEZE_LEVELS]; }; struct super_block { @@ -1393,9 +1386,9 @@ void __sb_end_write(struct super_block *sb, int level); int __sb_start_write(struct super_block *sb, int level, bool wait); #define __sb_writers_acquired(sb, lev) \ - rwsem_acquire_read(&(sb)->s_writers.lock_map[(lev)-1], 0, 1, _THIS_IP_) + percpu_rwsem_acquire(&(sb)->s_writers.rw_sem[(lev)-1], 1, _THIS_IP_) #define __sb_writers_release(sb, lev) \ - rwsem_release(&(sb)->s_writers.lock_map[(lev)-1], 1, _THIS_IP_) + percpu_rwsem_release(&(sb)->s_writers.rw_sem[(lev)-1], 1, _THIS_IP_) /** * sb_end_write - drop write access to a superblock -- cgit v1.2.3