fs: kill i_alloc_sem

i_alloc_sem is a rather special rw_semaphore.  It's the last one that may
be released by a non-owner, and it's write side is always mirrored by
real exclusion.  It's intended use it to wait for all pending direct I/O
requests to finish before starting a truncate.

Replace it with a hand-grown construct:

 - exclusion for truncates is already guaranteed by i_mutex, so it can
   simply fall way
 - the reader side is replaced by an i_dio_count member in struct inode
   that counts the number of pending direct I/O requests.  Truncate can't
   proceed as long as it's non-zero
 - when i_dio_count reaches non-zero we wake up a pending truncate using
   wake_up_bit on a new bit in i_flags
 - new references to i_dio_count can't appear while we are waiting for
   it to read zero because the direct I/O count always needs i_mutex
   (or an equivalent like XFS's i_iolock) for starting a new operation.

This scheme is much simpler, and saves the space of a spinlock_t and a
struct list_head in struct inode (typically 160 bits on a non-debug 64-bit
system).

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

4 files changed, 2 insertions, 7 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index a8251a8d345..f820e600f1a 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -78,9 +78,6 @@
  *  ->i_mutex                   (generic_file_buffered_write)
  *    ->mmap_sem                (fault_in_pages_readable->do_page_fault)
  *
- *  ->i_mutex
- *    ->i_alloc_sem             (various)
- *
  *  inode_wb_list_lock
  *    sb_lock                   (fs/fs-writeback.c)
  *    ->mapping->tree_lock      (__sync_single_inode)
diff --git a/mm/madvise.c b/mm/madvise.c
index 2221491ed50..74bf193eff0 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -218,7 +218,7 @@ static long madvise_remove(struct vm_area_struct *vma,
         endoff = (loff_t)(end - vma->vm_start - 1)
                         + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
 
-        /* vmtruncate_range needs to take i_mutex and i_alloc_sem */
+        /* vmtruncate_range needs to take i_mutex */
         up_read(&current->mm->mmap_sem);
         error = vmtruncate_range(mapping->host, offset, endoff);
         down_read(&current->mm->mmap_sem);
diff --git a/mm/rmap.c b/mm/rmap.c
index 23295f65ae4..2540a39eea4 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -21,7 +21,6 @@
  * Lock ordering in mm:
  *
  * inode->i_mutex       (while writing or truncating, not reading or faulting)
- *   inode->i_alloc_sem (vmtruncate_range)
  *   mm->mmap_sem
  *     page->flags PG_locked (lock_page)
  *       mapping->i_mmap_mutex
diff --git a/mm/truncate.c b/mm/truncate.c
index e13f22efaad..003c6c685fc 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -622,12 +622,11 @@ int vmtruncate_range(struct inode *inode, loff_t offset, loff_t end)
                 return -ENOSYS;
 
         mutex_lock(&inode->i_mutex);
-        down_write(&inode->i_alloc_sem);
+        inode_dio_wait(inode);
         unmap_mapping_range(mapping, offset, (end - offset), 1);
         inode->i_op->truncate_range(inode, offset, end);
         /* unmap again to remove racily COWed private pages */
         unmap_mapping_range(mapping, offset, (end - offset), 1);
-        up_write(&inode->i_alloc_sem);
         mutex_unlock(&inode->i_mutex);
 
         return 0;