[PATCH v6] add barriers to buffer_uptodate and set_buffer_uptodate

[Mikulas Patocka]

On Mon, 8 Aug 2022, Matthew Wilcox wrote:

> On Mon, Aug 08, 2022 at 10:57:45AM -0400, Mikulas Patocka wrote:
> > On Mon, 8 Aug 2022, Matthew Wilcox wrote:
> > 
> > > On Mon, Aug 08, 2022 at 10:26:10AM -0400, Mikulas Patocka wrote:
> > > > On Sun, 7 Aug 2022, Matthew Wilcox wrote:
> > > > > > +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;
> > > > > > +}
> > > > > 
> > > > > Are there places that think that lock/unlock buffer implies a memory
> > > > > barrier?
> > > > 
> > > > There's this in fs/reiserfs:
> > > > 
> > > > if (!buffer_dirty(bh) && !buffer_locked(bh)) {
> > > > 	reiserfs_free_jh(bh); <--- this could be moved before buffer_locked
> > > 
> > > It might be better to think of buffer_locked() as
> > > buffer_someone_has_exclusive_access().  I can't see the problem with
> > > moving the reads in reiserfs_free_jh() before the read of buffer_locked.
> > > 
> > > > if (buffer_locked((journal->j_header_bh))) {
> > > > 	...
> > > > }
> > > > journal->j_last_flush_trans_id = trans_id;
> > > > journal->j_first_unflushed_offset = offset;
> > > > jh = (struct reiserfs_journal_header *)(journal->j_header_bh->b_data); <--- this could be moved before buffer_locked
> > > 
> > > I don't think b_data is going to be changed while someone else holds
> > > the buffer locked.  That's initialised by set_bh_page(), which is an
> > > initialisation-time thing, before the BH is visible to any other thread.
> > 
> > So, do you think that we don't need a barrier in buffer_locked()?
> 
> That's my feeling.  Of course, you might not be the only one confused,
> and if fs authors in general have made the mistake of thinking that
> buffer_locked is serialising, then it might be better to live up to
> that expectation.

In my spadfs filesystem, I used lock_buffer/unlock_buffer to prevent the 
system from seeing or writing back incomplete data. The patterns is
	lock_buffer(bh);
	... do several changes to the buffer that should appear atomically
	unlock_buffer(bh);
	mark_buffer_dirty(bh);
but it seems to be ok, because both lock_buffer and unlock_buffer have 
acquire/release semantics. I'm not sure about buffer_locked - perhaps it 
really doesn't need the barriers - spin_is_locked, mutex_is_locked and 
rwsem_is_locked also don't have any barriers.

Here I'm sending the patch without the change to buffer_locked.

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.

Fix this bug by adding a memory barrier to set_buffer_uptodate and an
acquire barrier to buffer_uptodate (in a similar way as
folio_test_uptodate and folio_mark_uptodate).

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
@@ -117,7 +117,6 @@ 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)
@@ -135,6 +134,30 @@ 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_load_acquire in buffer_uptodate
+	 */
+	smp_mb__before_atomic();
+	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)
+{
+	/*
+	 * make it consistent with folio_test_uptodate
+	 * pairs with smp_mb__before_atomic in set_buffer_uptodate
+	 */
+	return (smp_load_acquire(&bh->b_state) & (1UL << BH_Uptodate)) != 0;
+}
+
 #define bh_offset(bh)		((unsigned long)(bh)->b_data & ~PAGE_MASK)
 
 /* If we *know* page->private refers to buffer_heads */