6 files changed, 55 insertions, 18 deletions

diff --git a/fs/file_table.c b/fs/file_table.c
index 5fff9030be34..5b24008ea4f6 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -135,6 +135,7 @@ struct file *get_empty_filp(void)
         atomic_long_set(&f->f_count, 1);
         rwlock_init(&f->f_owner.lock);
         spin_lock_init(&f->f_lock);
+        mutex_init(&f->f_pos_lock);
         eventpoll_init_file(f);
         /* f->f_version: 0 */
         return f;
diff --git a/fs/namei.c b/fs/namei.c
index 385f7817bfcc..2f730ef9b4b3 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1884,7 +1884,7 @@ static int path_init(int dfd, const char *name, unsigned int flags,
 
                 nd->path = f.file->f_path;
                 if (flags & LOOKUP_RCU) {
-                        if (f.need_put)
+                        if (f.flags & FDPUT_FPUT)
                                 *fp = f.file;
                         nd->seq = __read_seqcount_begin(&nd->path.dentry->d_seq);
                         rcu_read_lock();
diff --git a/fs/open.c b/fs/open.c
index 4b3e1edf2fe4..b9ed8b25c108 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -705,6 +705,10 @@ static int do_dentry_open(struct file *f,
                 return 0;
         }
 
+        /* POSIX.1-2008/SUSv4 Section XSI 2.9.7 */
+        if (S_ISREG(inode->i_mode))
+                f->f_mode |= FMODE_ATOMIC_POS;
+
         f->f_op = fops_get(inode->i_fop);
         if (unlikely(WARN_ON(!f->f_op))) {
                 error = -ENODEV;
diff --git a/fs/read_write.c b/fs/read_write.c
index edc5746a902a..932bb3414a96 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -264,10 +264,36 @@ loff_t vfs_llseek(struct file *file, loff_t offset, int whence)
 }
 EXPORT_SYMBOL(vfs_llseek);
 
+/*
+ * We only lock f_pos if we have threads or if the file might be
+ * shared with another process. In both cases we'll have an elevated
+ * file count (done either by fdget() or by fork()).
+ */
+static inline struct fd fdget_pos(int fd)
+{
+        struct fd f = fdget(fd);
+        struct file *file = f.file;
+
+        if (file && (file->f_mode & FMODE_ATOMIC_POS)) {
+                if (file_count(file) > 1) {
+                        f.flags |= FDPUT_POS_UNLOCK;
+                        mutex_lock(&file->f_pos_lock);
+                }
+        }
+        return f;
+}
+
+static inline void fdput_pos(struct fd f)
+{
+        if (f.flags & FDPUT_POS_UNLOCK)
+                mutex_unlock(&f.file->f_pos_lock);
+        fdput(f);
+}
+
 SYSCALL_DEFINE3(lseek, unsigned int, fd, off_t, offset, unsigned int, whence)
 {
         off_t retval;
-        struct fd f = fdget(fd);
+        struct fd f = fdget_pos(fd);
         if (!f.file)
                 return -EBADF;
 
@@ -278,7 +304,7 @@ SYSCALL_DEFINE3(lseek, unsigned int, fd, off_t, offset, unsigned int, whence)
                 if (res != (loff_t)retval)
                         retval = -EOVERFLOW;    /* LFS: should only happen on 32 bit platforms */
         }
-        fdput(f);
+        fdput_pos(f);
         return retval;
 }
 
@@ -498,7 +524,7 @@ static inline void file_pos_write(struct file *file, loff_t pos)
 
 SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count)
 {
-        struct fd f = fdget(fd);
+        struct fd f = fdget_pos(fd);
         ssize_t ret = -EBADF;
 
         if (f.file) {
@@ -506,7 +532,7 @@ SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count)
                 ret = vfs_read(f.file, buf, count, &pos);
                 if (ret >= 0)
                         file_pos_write(f.file, pos);
-                fdput(f);
+                fdput_pos(f);
         }
         return ret;
 }
@@ -514,7 +540,7 @@ SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count)
 SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf,
                 size_t, count)
 {
-        struct fd f = fdget(fd);
+        struct fd f = fdget_pos(fd);
         ssize_t ret = -EBADF;
 
         if (f.file) {
@@ -522,7 +548,7 @@ SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf,
                 ret = vfs_write(f.file, buf, count, &pos);
                 if (ret >= 0)
                         file_pos_write(f.file, pos);
-                fdput(f);
+                fdput_pos(f);
         }
 
         return ret;
@@ -797,7 +823,7 @@ EXPORT_SYMBOL(vfs_writev);
 SYSCALL_DEFINE3(readv, unsigned long, fd, const struct iovec __user *, vec,
                 unsigned long, vlen)
 {
-        struct fd f = fdget(fd);
+        struct fd f = fdget_pos(fd);
         ssize_t ret = -EBADF;
 
         if (f.file) {
@@ -805,7 +831,7 @@ SYSCALL_DEFINE3(readv, unsigned long, fd, const struct iovec __user *, vec,
                 ret = vfs_readv(f.file, vec, vlen, &pos);
                 if (ret >= 0)
                         file_pos_write(f.file, pos);
-                fdput(f);
+                fdput_pos(f);
         }
 
         if (ret > 0)
@@ -817,7 +843,7 @@ SYSCALL_DEFINE3(readv, unsigned long, fd, const struct iovec __user *, vec,
 SYSCALL_DEFINE3(writev, unsigned long, fd, const struct iovec __user *, vec,
                 unsigned long, vlen)
 {
-        struct fd f = fdget(fd);
+        struct fd f = fdget_pos(fd);
         ssize_t ret = -EBADF;
 
         if (f.file) {
@@ -825,7 +851,7 @@ SYSCALL_DEFINE3(writev, unsigned long, fd, const struct iovec __user *, vec,
                 ret = vfs_writev(f.file, vec, vlen, &pos);
                 if (ret >= 0)
                         file_pos_write(f.file, pos);
-                fdput(f);
+                fdput_pos(f);
         }
 
         if (ret > 0)
@@ -968,7 +994,7 @@ COMPAT_SYSCALL_DEFINE3(readv, compat_ulong_t, fd,
                 const struct compat_iovec __user *,vec,
                 compat_ulong_t, vlen)
 {
-        struct fd f = fdget(fd);
+        struct fd f = fdget_pos(fd);
         ssize_t ret;
         loff_t pos;
 
@@ -978,7 +1004,7 @@ COMPAT_SYSCALL_DEFINE3(readv, compat_ulong_t, fd,
         ret = compat_readv(f.file, vec, vlen, &pos);
         if (ret >= 0)
                 f.file->f_pos = pos;
-        fdput(f);
+        fdput_pos(f);
         return ret;
 }
 
@@ -1035,7 +1061,7 @@ COMPAT_SYSCALL_DEFINE3(writev, compat_ulong_t, fd,
                 const struct compat_iovec __user *, vec,
                 compat_ulong_t, vlen)
 {
-        struct fd f = fdget(fd);
+        struct fd f = fdget_pos(fd);
         ssize_t ret;
         loff_t pos;
 
@@ -1045,7 +1071,7 @@ COMPAT_SYSCALL_DEFINE3(writev, compat_ulong_t, fd,
         ret = compat_writev(f.file, vec, vlen, &pos);
         if (ret >= 0)
                 f.file->f_pos = pos;
-        fdput(f);
+        fdput_pos(f);
         return ret;
 }
 
diff --git a/include/linux/file.h b/include/linux/file.h
index cbacf4faf447..f2517fa2d610 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -28,12 +28,14 @@ static inline void fput_light(struct file *file, int fput_needed)
 
 struct fd {
         struct file *file;
-        int need_put;
+        unsigned int flags;
 };
+#define FDPUT_FPUT       1
+#define FDPUT_POS_UNLOCK 2
 
 static inline void fdput(struct fd fd)
 {
-        if (fd.need_put)
+        if (fd.flags & FDPUT_FPUT)
                 fput(fd.file);
 }
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 60829565e552..ebfde04bca06 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -123,6 +123,9 @@ typedef void (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
 /* File is opened with O_PATH; almost nothing can be done with it */
 #define FMODE_PATH              ((__force fmode_t)0x4000)
 
+/* File needs atomic accesses to f_pos */
+#define FMODE_ATOMIC_POS        ((__force fmode_t)0x8000)
+
 /* File was opened by fanotify and shouldn't generate fanotify events */
 #define FMODE_NONOTIFY          ((__force fmode_t)0x1000000)
 
@@ -780,13 +783,14 @@ struct file {
         const struct file_operations    *f_op;
 
         /*
-         * Protects f_ep_links, f_flags, f_pos vs i_size in lseek SEEK_CUR.
+         * Protects f_ep_links, f_flags.
          * Must not be taken from IRQ context.
          */
         spinlock_t              f_lock;
         atomic_long_t           f_count;
         unsigned int            f_flags;
         fmode_t                 f_mode;
+        struct mutex            f_pos_lock;
         loff_t                  f_pos;
         struct fown_struct      f_owner;
         const struct cred       *f_cred;