[PATCH 1/5] VFS: Do (nearly) lockless generic_file_llseek

[Andi Kleen]

From: Andi Kleen <ak@xxxxxxxxxxxxxxx>

The i_mutex locking of generic _file_llseek hurts currently. It means
that independent processes who just access the same file still
synchronize. Under high utilization this can cause llseek to scale
very poorly on larger systems.

First the 64bit f_pos is not necessarily atomic without locks
on 32bit systems. This can already cause races with read() today.
This was discussed on linux-kernel in the past and deemed acceptable.

The different seek modi:

SEEK_SET: Doesn't really need any locking.
If there's a race one writer wins, the other loses.

For 32bit the non atomic update races against read()
stay the same. Without a lock they can also happen
against write() now.  The read() race was deemed
acceptable in past discussions, and I think if it's
ok for read it's not ok for write too.

Don't need a lock.

SEEK_END: This behaves like SEEK_SET plus it reads
the maximum size too. Reading the maximum size would have the
32bit atomic problem. But luckily we already have a way to read
the maximum size without locking (i_size_read), so we
can just use that instead.

Without i_mutex there is no synchronization with write() anymore,
however since the write() update is atomic on 64bit it just behaves
like another racy SEEK_SET.  On non atomic 32bit it's the same
as SEEK_SET.

Don't need a lock, but need to use i_size_read()

SEEK_CUR: This has a read-modify-write race window
on the same file. One could argue that any application
doing unsynchronized seeks on the same file is already broken.
But for the sake of not adding a regression here I'm
using the file->f_lock to synchronize this. Using this
lock is much better than the inode mutex because it doesn't
synchronize between processes.

So still need a lock, but can use a cheap local one.

This patch implements this new scheme in generic_file_llseek.
I dropped generic_file_llseek_unlocked and changed all callers.

Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
---
 fs/gfs2/file.c     |    4 ++--
 fs/nfs/file.c      |    5 +++--
 fs/read_write.c    |   33 ++++++---------------------------
 include/linux/fs.h |    2 --
 4 files changed, 11 insertions(+), 33 deletions(-)

diff --git a/fs/gfs2/file.c b/fs/gfs2/file.c
index a9f5cbe..5590c0f 100644
--- a/fs/gfs2/file.c
+++ b/fs/gfs2/file.c
@@ -63,11 +63,11 @@ static loff_t gfs2_llseek(struct file *file, loff_t offset, int origin)
 		error = gfs2_glock_nq_init(ip->i_gl, LM_ST_SHARED, LM_FLAG_ANY,
 					   &i_gh);
 		if (!error) {
-			error = generic_file_llseek_unlocked(file, offset, origin);
+			error = generic_file_llseek(file, offset, origin);
 			gfs2_glock_dq_uninit(&i_gh);
 		}
 	} else
-		error = generic_file_llseek_unlocked(file, offset, origin);
+		error = generic_file_llseek(file, offset, origin);
 
 	return error;
 }
diff --git a/fs/nfs/file.c b/fs/nfs/file.c
index 2f093ed..6b52b09 100644
--- a/fs/nfs/file.c
+++ b/fs/nfs/file.c
@@ -195,11 +195,12 @@ static loff_t nfs_file_llseek(struct file *filp, loff_t offset, int origin)
 		if (retval < 0)
 			return (loff_t)retval;
 
+		/* AK: should drop this lock. Unlikely to be needed. */
 		spin_lock(&inode->i_lock);
-		loff = generic_file_llseek_unlocked(filp, offset, origin);
+		loff = generic_file_llseek(filp, offset, origin);
 		spin_unlock(&inode->i_lock);
 	} else
-		loff = generic_file_llseek_unlocked(filp, offset, origin);
+		loff = generic_file_llseek(filp, offset, origin);
 	return loff;
 }
 
diff --git a/fs/read_write.c b/fs/read_write.c
index 5520f8a..9bd5d23 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -36,22 +36,23 @@ static inline int unsigned_offsets(struct file *file)
 }
 
 /**
- * generic_file_llseek_unlocked - lockless generic llseek implementation
+ * generic_file_llseek - generic llseek implementation for regular files
  * @file:	file structure to seek on
  * @offset:	file offset to seek to
  * @origin:	type of seek
  *
- * Updates the file offset to the value specified by @offset and @origin.
- * Locking must be provided by the caller.
+ * This is a generic implemenation of ->llseek useable for all normal local
+ * filesystems.  It just updates the file offset to the value specified by
+ * @offset and @origin under i_mutex.
  */
 loff_t
-generic_file_llseek_unlocked(struct file *file, loff_t offset, int origin)
+generic_file_llseek(struct file *file, loff_t offset, int origin)
 {
 	struct inode *inode = file->f_mapping->host;
 
 	switch (origin) {
 	case SEEK_END:
-		offset += inode->i_size;
+		offset += i_size_read(inode);
 		break;
 	case SEEK_CUR:
 		/*
@@ -79,28 +80,6 @@ generic_file_llseek_unlocked(struct file *file, loff_t offset, int origin)
 
 	return offset;
 }
-EXPORT_SYMBOL(generic_file_llseek_unlocked);
-
-/**
- * generic_file_llseek - generic llseek implementation for regular files
- * @file:	file structure to seek on
- * @offset:	file offset to seek to
- * @origin:	type of seek
- *
- * This is a generic implemenation of ->llseek useable for all normal local
- * filesystems.  It just updates the file offset to the value specified by
- * @offset and @origin under i_mutex.
- */
-loff_t generic_file_llseek(struct file *file, loff_t offset, int origin)
-{
-	loff_t rval;
-
-	mutex_lock(&file->f_dentry->d_inode->i_mutex);
-	rval = generic_file_llseek_unlocked(file, offset, origin);
-	mutex_unlock(&file->f_dentry->d_inode->i_mutex);
-
-	return rval;
-}
 EXPORT_SYMBOL(generic_file_llseek);
 
 /**
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 1c77787..deec2cd 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2335,8 +2335,6 @@ file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping);
 extern loff_t noop_llseek(struct file *file, loff_t offset, int origin);
 extern loff_t no_llseek(struct file *file, loff_t offset, int origin);
 extern loff_t generic_file_llseek(struct file *file, loff_t offset, int origin);
-extern loff_t generic_file_llseek_unlocked(struct file *file, loff_t offset,
-			int origin);
 extern int generic_file_open(struct inode * inode, struct file * filp);
 extern int nonseekable_open(struct inode * inode, struct file * filp);
 
-- 
1.7.4.4

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