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Commit 8a81252b authored by Eric Dumazet's avatar Eric Dumazet Committed by Al Viro
Browse files

fs/file.c: don't acquire files->file_lock in fd_install() 

Mateusz Guzik reported :

 Currently obtaining a new file descriptor results in locking fdtable
 twice - once in order to reserve a slot and second time to fill it.

Holding the spinlock in __fd_install() is needed in case a resize is
done, or to prevent a resize.

Mateusz provided an RFC patch and a micro benchmark :
  http://people.redhat.com/~mguzik/pipebench.c



A resize is an unlikely operation in a process lifetime,
as table size is at least doubled at every resize.

We can use RCU instead of the spinlock.

__fd_install() must wait if a resize is in progress.

The resize must block new __fd_install() callers from starting,
and wait that ongoing install are finished (synchronize_sched())

resize should be attempted by a single thread to not waste resources.

rcu_sched variant is used, as __fd_install() and expand_fdtable() run
from process context.

It gives us a ~30% speedup using pipebench on a dual Intel(R) Xeon(R)
CPU E5-2696 v2 @ 2.50GHz

Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Reported-by: default avatarMateusz Guzik <mguzik@redhat.com>
Acked-by: default avatarMateusz Guzik <mguzik@redhat.com>
Tested-by: default avatarMateusz Guzik <mguzik@redhat.com>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 1af95de6

Documentation/filesystems/porting

+4 −0
Original line number Diff line number Diff line
@@ -500,3 +500,7 @@ in your dentry operations instead. 
	dentry,  it does not get nameidata at all and it gets called only when cookie

	is non-NULL.  Note that link body isn't available anymore, so if you need it,

	store it as cookie.

--

[mandatory]

	__fd_install() & fd_install() can now sleep. Callers should not

	hold a spinlock	or other resources that do not allow a schedule.

fs/file.c

+48 −19
Original line number Diff line number Diff line
@@ -147,6 +147,13 @@ static int expand_fdtable(struct files_struct *files, int nr) 


	spin_unlock(&files->file_lock);

	new_fdt = alloc_fdtable(nr);



	/* make sure all __fd_install() have seen resize_in_progress

	 * or have finished their rcu_read_lock_sched() section.

	 */

	if (atomic_read(&files->count) > 1)

		synchronize_sched();



	spin_lock(&files->file_lock);

	if (!new_fdt)

		return -ENOMEM;
@@ -158,21 +165,14 @@ static int expand_fdtable(struct files_struct *files, int nr) 
		__free_fdtable(new_fdt);

		return -EMFILE;

	}

	/*

	 * Check again since another task may have expanded the fd table while

	 * we dropped the lock

	 */

	cur_fdt = files_fdtable(files);

	if (nr >= cur_fdt->max_fds) {

		/* Continue as planned */

	BUG_ON(nr < cur_fdt->max_fds);

	copy_fdtable(new_fdt, cur_fdt);

	rcu_assign_pointer(files->fdt, new_fdt);

	if (cur_fdt != &files->fdtab)

		call_rcu(&cur_fdt->rcu, free_fdtable_rcu);

	} else {

		/* Somebody else expanded, so undo our attempt */

		__free_fdtable(new_fdt);

	}

	/* coupled with smp_rmb() in __fd_install() */

	smp_wmb();

	return 1;

}
@@ -185,21 +185,38 @@ static int expand_fdtable(struct files_struct *files, int nr) 
 * The files->file_lock should be held on entry, and will be held on exit.

 */

static int expand_files(struct files_struct *files, int nr)

	__releases(files->file_lock)

	__acquires(files->file_lock)

{

	struct fdtable *fdt;

	int expanded = 0;



repeat:

	fdt = files_fdtable(files);



	/* Do we need to expand? */

	if (nr < fdt->max_fds)

		return 0;

		return expanded;



	/* Can we expand? */

	if (nr >= sysctl_nr_open)

		return -EMFILE;



	if (unlikely(files->resize_in_progress)) {

		spin_unlock(&files->file_lock);

		expanded = 1;

		wait_event(files->resize_wait, !files->resize_in_progress);

		spin_lock(&files->file_lock);

		goto repeat;

	}



	/* All good, so we try */

	return expand_fdtable(files, nr);

	files->resize_in_progress = true;

	expanded = expand_fdtable(files, nr);

	files->resize_in_progress = false;



	wake_up_all(&files->resize_wait);

	return expanded;

}



static inline void __set_close_on_exec(int fd, struct fdtable *fdt)
@@ -256,6 +273,8 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp) 
	atomic_set(&newf->count, 1);



	spin_lock_init(&newf->file_lock);

	newf->resize_in_progress = false;

	init_waitqueue_head(&newf->resize_wait);

	newf->next_fd = 0;

	new_fdt = &newf->fdtab;

	new_fdt->max_fds = NR_OPEN_DEFAULT;
@@ -553,11 +572,21 @@ void __fd_install(struct files_struct *files, unsigned int fd, 
		struct file *file)

{

	struct fdtable *fdt;

	spin_lock(&files->file_lock);

	fdt = files_fdtable(files);



	might_sleep();

	rcu_read_lock_sched();



	while (unlikely(files->resize_in_progress)) {

		rcu_read_unlock_sched();

		wait_event(files->resize_wait, !files->resize_in_progress);

		rcu_read_lock_sched();

	}

	/* coupled with smp_wmb() in expand_fdtable() */

	smp_rmb();

	fdt = rcu_dereference_sched(files->fdt);

	BUG_ON(fdt->fd[fd] != NULL);

	rcu_assign_pointer(fdt->fd[fd], file);

	spin_unlock(&files->file_lock);

	rcu_read_unlock_sched();

}



void fd_install(unsigned int fd, struct file *file)

include/linux/fdtable.h

+3 −0
Original line number Diff line number Diff line
@@ -47,6 +47,9 @@ struct files_struct { 
   * read mostly part

   */

	atomic_t count;

	bool resize_in_progress;

	wait_queue_head_t resize_wait;



	struct fdtable __rcu *fdt;

	struct fdtable fdtab;

  /*