lib: percpu_counter_sub

Hugh spotted that some code does:
  percpu_counter_add(&counter, -unsignedlong)

which, when the amount argument is of type s32, sort-of works thanks to
two's-complement. However when we'd change the type to s64 this breaks on 32bit
machines, because the promotion rules zero extend the unsigned number.

Provide percpu_counter_sub() to hide the s64 cast. That is:
  percpu_counter_sub(&counter, foo)
is equal to:
  percpu_counter_add(&counter, -(s64)foo);

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
index a141456cd491..8d59eec2e82b 100644
--- a/fs/ext4/balloc.c
+++ b/fs/ext4/balloc.c
@@ -1647,7 +1647,7 @@ allocated:
         gdp->bg_free_blocks_count =
                         cpu_to_le16(le16_to_cpu(gdp->bg_free_blocks_count)-num);
         spin_unlock(sb_bgl_lock(sbi, group_no));
-        percpu_counter_add(&sbi->s_freeblocks_counter, -num);
+        percpu_counter_sub(&sbi->s_freeblocks_counter, num);
 
         BUFFER_TRACE(gdp_bh, "journal_dirty_metadata for group descriptor");
         err = ext4_journal_dirty_metadata(handle, gdp_bh);