| Message ID | 20150625001606.E834E08B@viggo.jf.intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Wed 24-06-15 17:16:06, Dave Hansen wrote: > > From: Dave Hansen <dave.hansen@linux.intel.com> This has nothing to do with fsnotify so just remove it from the subject line please. Thanks! > If I sit in a loop and do write()s to small tmpfs files, > __sb_end_write() is third-hottest kernel function due to its > smp_mb(). > > __sb_end_write() uses the barrier to avoid races with freeze_super() > and its calls to sb_wait_write(). But, now that freeze_super() is > calling synchronize_rcu() before each sb_wait_write() call, we can > use that to our advantage. > > The synchronize_rcu() ensures that all __sb_end_write() will see > freeze_super()'s updates to s_writers.counter. That, in turn, > guarantees that __sb_end_write() will try to wake up any subsequent > call by freeze_super() to sb_wait_write(). What gains does this patch bring? Otherwise the patch looks good to me. You can add: Reviewed-by: Jan Kara <jack@suse.cz> Honza > Cc: Jan Kara <jack@suse.cz> > Cc: Alexander Viro <viro@zeniv.linux.org.uk> > Cc: linux-fsdevel@vger.kernel.org > Cc: linux-kernel@vger.kernel.org > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > Cc: Tim Chen <tim.c.chen@linux.intel.com> > Cc: Andi Kleen <ak@linux.intel.com> > Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> > --- > > b/fs/super.c | 17 +++++++++++++---- > 1 file changed, 13 insertions(+), 4 deletions(-) > > diff -puN fs/super.c~selectively-do-barriers-in-__sb_end_write fs/super.c > --- a/fs/super.c~selectively-do-barriers-in-__sb_end_write 2015-06-24 17:14:35.315142611 -0700 > +++ b/fs/super.c 2015-06-24 17:14:35.318142745 -0700 > @@ -1146,14 +1146,23 @@ out: > */ > void __sb_end_write(struct super_block *sb, int level) > { > + rcu_read_lock(); > percpu_counter_dec(&sb->s_writers.counter[level-1]); > /* > - * Make sure s_writers are updated before we wake up waiters in > - * freeze_super(). > + * We are racing here with freeze_super()'s calls to > + * sb_wait_write(). We want to ensure that we call > + * wake_up() whenever one of those calls _might_ be > + * in sb_wait_write(). > + * > + * Since freeze_super() does a synchronize_rcu() before > + * each of its sb_wait_write() calls, it can guarantee > + * that it sees our update to s_writers.counter as well > + * as that we see its update to s_writers.frozen. > */ > - smp_mb(); > - if (waitqueue_active(&sb->s_writers.wait)) > + if (unlikely(sb->s_writers.frozen >= level)) > wake_up(&sb->s_writers.wait); > + rcu_read_unlock(); > + > rwsem_release(&sb->s_writers.lock_map[level-1], 1, _RET_IP_); > } > EXPORT_SYMBOL(__sb_end_write); > _
diff -puN fs/super.c~selectively-do-barriers-in-__sb_end_write fs/super.c --- a/fs/super.c~selectively-do-barriers-in-__sb_end_write 2015-06-24 17:14:35.315142611 -0700 +++ b/fs/super.c 2015-06-24 17:14:35.318142745 -0700 @@ -1146,14 +1146,23 @@ out: */ void __sb_end_write(struct super_block *sb, int level) { + rcu_read_lock(); percpu_counter_dec(&sb->s_writers.counter[level-1]); /* - * Make sure s_writers are updated before we wake up waiters in - * freeze_super(). + * We are racing here with freeze_super()'s calls to + * sb_wait_write(). We want to ensure that we call + * wake_up() whenever one of those calls _might_ be + * in sb_wait_write(). + * + * Since freeze_super() does a synchronize_rcu() before + * each of its sb_wait_write() calls, it can guarantee + * that it sees our update to s_writers.counter as well + * as that we see its update to s_writers.frozen. */ - smp_mb(); - if (waitqueue_active(&sb->s_writers.wait)) + if (unlikely(sb->s_writers.frozen >= level)) wake_up(&sb->s_writers.wait); + rcu_read_unlock(); + rwsem_release(&sb->s_writers.lock_map[level-1], 1, _RET_IP_); } EXPORT_SYMBOL(__sb_end_write);