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: Eric Dumazet <edumazet@google.com>
Reported-by: Mateusz Guzik <mguzik@redhat.com>
Acked-by: Mateusz Guzik <mguzik@redhat.com>
Tested-by: Mateusz Guzik <mguzik@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 48 insertions, 19 deletions

diff --git a/fs/file.c b/fs/file.c
index 93c5f89c248b..3d2eb4c542a4 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -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 */
-                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);
-        }
+        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);
+        /* 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)