[RFC,2/5] ms/vfs: grab the lock instead of blocking in __fd_install during resizing

Submitted by Kirill Tkhai on Jan. 12, 2018, 3:46 p.m.

Details

Message ID 151577197769.11063.2798949203961069801.stgit@localhost.localdomain
State New
Series "Shrink big fdtable on criu restore"
Headers show

Commit Message

Kirill Tkhai Jan. 12, 2018, 3:46 p.m.
From: Mateusz Guzik <mguzik@redhat.com>

ms commit: c02b1a9b41c2

Explicit locking in the fallback case provides a safe state of the
table. Getting rid of blocking semantics makes __fd_install usable
again in non-sleepable contexts, which easies backporting efforts.

There is a side effect of slightly nicer assembly for the common case
as might_sleep can now be removed.

Signed-off-by: Mateusz Guzik <mguzik@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
 fs/file.c |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

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diff --git a/fs/file.c b/fs/file.c
index adc42eccfd67..05ff3c5a6ec8 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -586,13 +586,16 @@  void __fd_install(struct files_struct *files, unsigned int fd,
 {
 	struct fdtable *fdt;
 
-	might_sleep();
 	rcu_read_lock_sched();
 
-	while (unlikely(files->resize_in_progress)) {
+	if (unlikely(files->resize_in_progress)) {
 		rcu_read_unlock_sched();
-		wait_event(files->resize_wait, !files->resize_in_progress);
-		rcu_read_lock_sched();
+		spin_lock(&files->file_lock);
+		fdt = files_fdtable(files);
+		BUG_ON(fdt->fd[fd] != NULL);
+		rcu_assign_pointer(fdt->fd[fd], file);
+		spin_unlock(&files->file_lock);
+		return;
 	}
 	/* coupled with smp_wmb() in expand_fdtable() */
 	smp_rmb();