On Fri, 2012-07-13 at 08:50 -0400, Chris Mason wrote:
> On Wed, Jul 11, 2012 at 11:47:40PM -0600, Mike Galbraith wrote:
> > Greetings,
>
> [ deadlocks with btrfs and the recent RT kernels ]
>
> I talked with Thomas about this and I think the problem is the
> single-reader nature of the RW rwlocks. The lockdep report below
> mentions that btrfs is calling:
>
> > [ 692.963099] [<ffffffff811fabd2>] btrfs_clear_path_blocking+0x32/0x70
>
> In this case, the task has a number of blocking read locks on the btrfs buffers,
> and we're trying to turn them back into spinning read locks. Even
> though btrfs is taking the read rwlock, it doesn't think of this as a new
> lock operation because we were blocking out new writers.
>
> If the second task has taken the spinning read lock, it is going to
> prevent that clear_path_blocking operation from progressing, even though
> it would have worked on a non-RT kernel.
>
> The solution should be to make the blocking read locks in btrfs honor the
> single-reader semantics. This means not allowing more than one blocking
> reader and not allowing a spinning reader when there is a blocking
> reader. Strictly speaking btrfs shouldn't need recursive readers on a
> single lock, so I wouldn't worry about that part.
>
> There is also a chunk of code in btrfs_clear_path_blocking that makes
> sure to strictly honor top down locking order during the conversion. It
> only does this when lockdep is enabled because in non-RT kernels we
> don't need to worry about it. For RT we'll want to enable that as well.
>
> I'll give this a shot later today.
I took a poke at it. Did I do something similar to what you had in
mind, or just hide behind performance stealing paranoid trylock loops?
Box survived 1000 x xfstests 006 and dbench [-s] massive right off the
bat, so it gets posted despite skepticism.
diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 4106264..ae47cc2 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -77,7 +77,7 @@ noinline void btrfs_clear_path_blocking(struct btrfs_path *p,
{
int i;
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
+#if (defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_PREEMPT_RT_BASE))
/* lockdep really cares that we take all of these spinlocks
* in the right order. If any of the locks in the path are not
* currently blocking, it is going to complain. So, make really
@@ -104,7 +104,7 @@ noinline void btrfs_clear_path_blocking(struct btrfs_path *p,
}
}
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
+#if (defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_PREEMPT_RT_BASE))
if (held)
btrfs_clear_lock_blocking_rw(held, held_rw);
#endif
diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index 272f911..4db7c14 100644
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -19,6 +19,7 @@
#include <linux/pagemap.h>
#include <linux/spinlock.h>
#include <linux/page-flags.h>
+#include <linux/delay.h>
#include <asm/bug.h>
#include "ctree.h"
#include "extent_io.h"
@@ -97,7 +98,18 @@ void btrfs_clear_lock_blocking_rw(struct extent_buffer *eb, int rw)
void btrfs_tree_read_lock(struct extent_buffer *eb)
{
again:
+#ifdef CONFIG_PREEMPT_RT_BASE
+ while (atomic_read(&eb->blocking_readers))
+ cpu_chill();
+ while(!read_trylock(&eb->lock))
+ cpu_chill();
+ if (atomic_read(&eb->blocking_readers)) {
+ read_unlock(&eb->lock);
+ goto again;
+ }
+#else
read_lock(&eb->lock);
+#endif
if (atomic_read(&eb->blocking_writers) &&
current->pid == eb->lock_owner) {
/*
@@ -131,11 +143,26 @@ int btrfs_try_tree_read_lock(struct extent_buffer *eb)
if (atomic_read(&eb->blocking_writers))
return 0;
+#ifdef CONFIG_PREEMPT_RT_BASE
+ if (atomic_read(&eb->blocking_readers))
+ return 0;
+ while(!read_trylock(&eb->lock))
+ cpu_chill();
+#else
read_lock(&eb->lock);
+#endif
+
if (atomic_read(&eb->blocking_writers)) {
read_unlock(&eb->lock);
return 0;
}
+
+#ifdef CONFIG_PREEMPT_RT_BASE
+ if (atomic_read(&eb->blocking_readers)) {
+ read_unlock(&eb->lock);
+ return 0;
+ }
+#endif
atomic_inc(&eb->read_locks);
atomic_inc(&eb->spinning_readers);
return 1;
--
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