> The only problem is that test_bit doesn't provide any memory barriers.
> Should we add the barrier to buffer_locked() instead of wait_on_buffer()?
> Perhaps it would fix more bugs - in reiserfs, there's this piece of code:
Her I'm sending the second version of the patch that changes buffer_locked
to provide an acquire semantics.
Mikulas
From: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Let's have a look at this piece of code in __bread_slow:
get_bh(bh);
bh->b_end_io = end_buffer_read_sync;
submit_bh(REQ_OP_READ, 0, bh);
wait_on_buffer(bh);
if (buffer_uptodate(bh))
return bh;
Neither wait_on_buffer nor buffer_uptodate contain any memory barrier.
Consequently, if someone calls sb_bread and then reads the buffer data,
the read of buffer data may be executed before wait_on_buffer(bh) on
architectures with weak memory ordering and it may return invalid data.
Also, there is this pattern present several times:
wait_on_buffer(bh);
if (!buffer_uptodate(bh))
err = -EIO;
It may be possible that buffer_uptodate is executed before wait_on_buffer
and it may return spurious error.
Fix these bugs by chaning the function buffer_locked to have the acquire
semantics - so that code that follows buffer_locked cannot be moved before
it. We must also add a read barrier after wait_on_bit_io because
wait_on_bit_io doesn't provide any barrier. (perhaps, should this
smp_rmb() be moved into wait_on_bit_io?)
Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Index: linux-2.6/include/linux/buffer_head.h
===================================================================
--- linux-2.6.orig/include/linux/buffer_head.h
+++ linux-2.6/include/linux/buffer_head.h
@@ -120,7 +120,6 @@ static __always_inline int test_clear_bu
BUFFER_FNS(Uptodate, uptodate)
BUFFER_FNS(Dirty, dirty)
TAS_BUFFER_FNS(Dirty, dirty)
-BUFFER_FNS(Lock, locked)
BUFFER_FNS(Req, req)
TAS_BUFFER_FNS(Req, req)
BUFFER_FNS(Mapped, mapped)
@@ -135,6 +134,17 @@ BUFFER_FNS(Meta, meta)
BUFFER_FNS(Prio, prio)
BUFFER_FNS(Defer_Completion, defer_completion)
+static __always_inline void set_buffer_locked(struct buffer_head *bh)
+{
+ set_bit(BH_Lock, &bh->b_state);
+}
+
+static __always_inline int buffer_locked(const struct buffer_head *bh)
+{
+ unsigned long state = smp_load_acquire(&bh->b_state);
+ return test_bit(BH_Lock, &state);
+}
+
#define bh_offset(bh) ((unsigned long)(bh)->b_data & ~PAGE_MASK)
/* If we *know* page->private refers to buffer_heads */
Index: linux-2.6/fs/buffer.c
===================================================================
--- linux-2.6.orig/fs/buffer.c
+++ linux-2.6/fs/buffer.c
@@ -120,6 +120,7 @@ EXPORT_SYMBOL(buffer_check_dirty_writeba
void __wait_on_buffer(struct buffer_head * bh)
{
wait_on_bit_io(&bh->b_state, BH_Lock, TASK_UNINTERRUPTIBLE);
+ smp_rmb();
}
EXPORT_SYMBOL(__wait_on_buffer);
