[PATCH v5] add barriers to buffer functions

[Mikulas Patocka]

On Fri, 5 Aug 2022, Matthew Wilcox wrote:

> On Mon, Aug 01, 2022 at 11:01:40AM -0400, Mikulas Patocka wrote:
> > 
> > In most cases, the buffer is set uptodate while it is locked, so that 
> > there is no race on the uptodate flag (the race exists on the locked 
> > flag). Are there any cases where the uptodate flag is modified on unlocked 
> > buffer, so that it needs special treatment too?
> 
> I think you misunderstand the purpose of locked/uptodate.  At least
> for pages, the lock flag does not order access to the data in the page.
> Indeed, the contents of the page can be changed while you hold the lock.
> But the uptodate flag does order access to the data.  At the point where
> you can observe the uptodate flag set, you know the contents of the page
> have been completely read from storage.  And you don't need to hold the
> lock to check the uptodate flag.  So this is wrong:
> 
> 	buffer_lock()
> 	*data = 0x12345678;
> 	buffer_set_uptodate_not_ordered()
> 	buffer_unlock_ordered()
> 
> because a reader can do:
> 
> 	while (!buffer_test_uptodate()) {
> 		buffer_lock();
> 		buffer_unlock();
> 	}
> 	x = *data;
> 
> and get x != 0x12345678 because the compiler can move the
> buffer_set_uptodate_not_ordered() before the store to *data.

Thanks for explanation. Would you like this patch?



From: Mikulas Patocka <mpatocks@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.

Fix this bug by adding a write memory barrier to set_buffer_uptodate and a
read memory barrier to buffer_uptodate (in the same way as
folio_test_uptodate and folio_mark_uptodate).

We also add a barrier to buffer_locked - it pairs with a barrier in
unlock_buffer.

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
@@ -117,10 +117,8 @@ static __always_inline int test_clear_bu
  * of the form "mark_buffer_foo()".  These are higher-level functions which
  * do something in addition to setting a b_state bit.
  */
-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 +133,49 @@ BUFFER_FNS(Meta, meta)
 BUFFER_FNS(Prio, prio)
 BUFFER_FNS(Defer_Completion, defer_completion)
 
+static __always_inline void set_buffer_uptodate(struct buffer_head *bh)
+{
+	/*
+	 * make it consistent with folio_mark_uptodate
+	 * pairs with smp_acquire__after_ctrl_dep in buffer_uptodate
+	 */
+	smp_wmb();
+	set_bit(BH_Uptodate, &bh->b_state);
+}
+
+static __always_inline void clear_buffer_uptodate(struct buffer_head *bh)
+{
+	clear_bit(BH_Uptodate, &bh->b_state);
+}
+
+static __always_inline int buffer_uptodate(const struct buffer_head *bh)
+{
+	bool ret = test_bit(BH_Uptodate, &bh->b_state);
+	/*
+	 * make it consistent with folio_test_uptodate
+	 * pairs with smp_wmb in set_buffer_uptodate
+	 */
+	if (ret)
+		smp_acquire__after_ctrl_dep();
+	return ret;
+}
+
+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)
+{
+	bool ret = test_bit(BH_Lock, &bh->b_state);
+	/*
+	 * pairs with smp_mb__after_atomic in unlock_buffer
+	 */
+	if (!ret)
+		smp_acquire__after_ctrl_dep();
+	return ret;
+}
+
 #define bh_offset(bh)		((unsigned long)(bh)->b_data & ~PAGE_MASK)
 
 /* If we *know* page->private refers to buffer_heads */