(2014/02/27 23:57), Benjamin LaHaise wrote:
On Thu, Feb 27, 2014 at 06:40:16PM +0800, Tang Chen wrote:
When doing aio ring page migration, we migrated the page, and update
ctx->ring_pages[]. Like the following:
aio_migratepage()
|-> migrate_page_copy(new, old)
| ...... /* Need barrier here */
|-> ctx->ring_pages[idx] = new
Actually, we need a memory barrier between these two operations.
Otherwise, if ctx->ring_pages[] is updated before memory copy due to
the compiler optimization, other processes may have an opportunity
to access to the not fully initialized new ring page.
So add a wmb to synchronize them.
The smp_wmb() is not needed after you added the taking of ctx->completion_lock
lock since all accesses to ring_pages is then protected by the spinlock.
Why are you adding this then? Or have you missed adding the lock somewhere?
Pleaes see following thread. It's a updated patch.
https://lkml.org/lkml/2014/2/27/237
aio_read_events_ring() accesses ring_pages without locking ctx-completion_lock.
If copying old page'date to new page runs after new page was set to ctx->ring_pages
by changing order, aio_read_events_ring() may not work correctly.
So smp_wmb() and smp_rmb() is needed.
Thanks,
Yasuaki Ishimatsu
Also, if you've changed the patch, it is customary to add a "v2" somewhere in
the patch title so that I have some idea what version of the patch should be
applied.
-ben
Reported-by: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>
Signed-off-by: Tang Chen <tangchen@xxxxxxxxxxxxxx>
---
fs/aio.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/fs/aio.c b/fs/aio.c
index 50c089c..f0ed838 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -327,6 +327,14 @@ static int aio_migratepage(struct address_space *mapping, struct page *new,
pgoff_t idx;
spin_lock_irqsave(&ctx->completion_lock, flags);
migrate_page_copy(new, old);
+
+ /*
+ * Ensure memory copy is finished before updating
+ * ctx->ring_pages[]. Otherwise other processes may access to
+ * new ring pages which are not fully initialized.
+ */
+ smp_wmb();
+
idx = old->index;
if (idx < (pgoff_t)ctx->nr_pages) {
/* And only do the move if things haven't changed */
--
1.8.3.1
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