linux-kernel - Re: spinlock contention of files->file

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Message-ID: <1380666060.19002.53.camel@edumazet-glaptop.roam.corp.google.com>
Date:	Tue, 01 Oct 2013 15:21:00 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <Waiman.Long@...com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: spinlock contention of files->file_lock

On Tue, 2013-10-01 at 23:04 +0100, Al Viro wrote:
> On Tue, Oct 01, 2013 at 02:41:58PM -0700, Eric Dumazet wrote:
> > Maybe I am missing something obvious ?
> 
> Yes.  do_execve_common() starts with unshare_files(); there can be
> no other thread capable of modifying that descriptor table.

Hmm, then what's the point of using spin_lock() here ?

This gives wrong hints ;)

diff --git a/fs/file.c b/fs/file.c
index 4a78f98..cdbca0d 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -615,11 +615,10 @@ void do_close_on_exec(struct files_struct *files)
 	struct fdtable *fdt;
 
 	/* exec unshares first */
-	spin_lock(&files->file_lock);
+	fdt = files_fdtable(files);
 	for (i = 0; ; i++) {
 		unsigned long set;
 		unsigned fd = i * BITS_PER_LONG;
-		fdt = files_fdtable(files);
 		if (fd >= fdt->max_fds)
 			break;
 		set = fdt->close_on_exec[i];
@@ -635,14 +634,11 @@ void do_close_on_exec(struct files_struct *files)
 				continue;
 			rcu_assign_pointer(fdt->fd[fd], NULL);
 			__put_unused_fd(files, fd);
-			spin_unlock(&files->file_lock);
 			filp_close(file, files);
 			cond_resched();
-			spin_lock(&files->file_lock);
 		}
 
 	}
-	spin_unlock(&files->file_lock);
 }
 
 struct file *fget(unsigned int fd)


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