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Date: Tue, 26 May 2009 23:11:19 +0200
From: Jens Axboe <jens.axboe@...cle.com>
To: Damien Wyart <damien.wyart@...e.fr>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
chris.mason@...cle.com, david@...morbit.com, hch@...radead.org,
akpm@...ux-foundation.org, jack@...e.cz,
yanmin_zhang@...ux.intel.com, richard@....demon.co.uk
Subject: Re: [PATCH 0/12] Per-bdi writeback flusher threads v7
On Tue, May 26 2009, Jens Axboe wrote:
> On Tue, May 26 2009, Damien Wyart wrote:
> > > > I have been playing with v7 since your sending and after a while
> > > > (short on laptop, longer on desktop, a few hours), writeback doesn't
> > > > seem to work anymore. Manual call to sync hangs (process in D state)
> > > > and Dirty value in meminfo gets growing. As previous versions had
> > > > been heavily tested, I guess there is some regression in v7.
> >
> > > Not good, the prime suspect is the sync notification stuff. I'll take
> > > a look and get that fixed. You didn't happen to catch any sysrq-t back
> > > traces or anything like that? Would be interesting to see where
> > > bdi-default and the bdi-* threads are stuck.
> >
> > No, as I was doing many things at the same time and not exclusively
> > debugging, I just rebooted hard and went back to an upatched kernel when
> > the problems occured. But I noticed only bdi-default was alive, the
> > other bdi-* threads had disappeared and the sync commands I had tried
> > were all in D state. Also I tried to reinstall a kernel .deb (these
> > systems are Debian) and this got stuck guring installation, when probing
> > grub config (do not know if there is some sync syscall inthere).
> >
> > Can try to go further tomorrow but will not have a lot of time...
>
> OK, I spotted the problem. If we fallback to the on-stack allocation in
> bdi_writeback_all(), then we do the wait for the work completion with
> the bdi_lock mutex held. This can deadlock with bdi_forker_task(), so if
> we require that to be invoked to make progress (happens if a thread
> needs to be restarted), then we have a deadlock on that mutex.
>
> I'll cook up a fix for this, but probably not before the morning.
Untested fix. I think it should work, but I haven't run it here yet.
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index a185a16..1662ede 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -122,12 +122,11 @@ static void bdi_work_free(struct rcu_head *head)
static void wb_work_complete(struct bdi_work *work)
{
- if (!bdi_work_on_stack(work)) {
- bdi_work_clear(work);
+ const enum writeback_sync_modes sync_mode = work->sync_mode;
- if (work->sync_mode == WB_SYNC_NONE)
- call_rcu(&work->rcu_head, bdi_work_free);
- } else
+ if (!bdi_work_on_stack(work))
+ bdi_work_clear(work);
+ if (sync_mode == WB_SYNC_NONE || bdi_work_on_stack(work))
call_rcu(&work->rcu_head, bdi_work_free);
}
@@ -272,7 +271,7 @@ void bdi_start_writeback(struct backing_dev_info *bdi, struct super_block *sb,
*/
if (work == &work_stack || must_wait) {
bdi_wait_on_work_clear(work);
- if (must_wait)
+ if (must_wait && work != &work_stack)
call_rcu(&work->rcu_head, bdi_work_free);
}
}
@@ -511,10 +510,9 @@ int bdi_writeback_task(struct bdi_writeback *wb)
* we are simply called for WB_SYNC_NONE, then writeback will merely be
* scheduled to run.
*/
-void bdi_writeback_all(struct super_block *sb, long nr_pages,
- enum writeback_sync_modes sync_mode)
+void bdi_writeback_all(struct super_block *sb, struct writeback_control *wbc)
{
- const bool must_wait = sync_mode == WB_SYNC_ALL;
+ const bool must_wait = wbc->sync_mode == WB_SYNC_ALL;
struct backing_dev_info *bdi, *tmp;
struct bdi_work *work;
LIST_HEAD(list);
@@ -522,31 +520,21 @@ void bdi_writeback_all(struct super_block *sb, long nr_pages,
mutex_lock(&bdi_lock);
list_for_each_entry_safe(bdi, tmp, &bdi_list, bdi_list) {
- struct bdi_work *work, work_stack;
+ struct bdi_work *work;
if (!bdi_has_dirty_io(bdi))
continue;
- work = bdi_alloc_work(sb, nr_pages, sync_mode);
+ work = bdi_alloc_work(sb, wbc->nr_to_write, wbc->sync_mode);
if (!work) {
- work = &work_stack;
- bdi_work_init_on_stack(work, sb, nr_pages, sync_mode);
- } else if (must_wait)
+ generic_sync_bdi_inodes(sb, wbc);
+ continue;
+ }
+ if (must_wait)
list_add_tail(&work->wait_list, &list);
bdi_queue_work(bdi, work);
__bdi_start_work(bdi, work);
-
- /*
- * Do the wait inline if this came from the stack. This
- * only happens if we ran out of memory, so should very
- * rarely trigger.
- */
- if (work == &work_stack) {
- bdi_wait_on_work_clear(work);
- if (must_wait)
- call_rcu(&work->rcu_head, bdi_work_free);
- }
}
mutex_unlock(&bdi_lock);
@@ -1082,7 +1070,7 @@ void generic_sync_sb_inodes(struct super_block *sb,
if (wbc->bdi)
bdi_start_writeback(wbc->bdi, sb, wbc->nr_to_write, wbc->sync_mode);
else
- bdi_writeback_all(sb, wbc->nr_to_write, wbc->sync_mode);
+ bdi_writeback_all(sb, wbc);
if (wbc->sync_mode == WB_SYNC_ALL) {
struct inode *inode, *old_inode = NULL;
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index dee38ec..679cfb8 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -107,8 +107,7 @@ void bdi_unregister(struct backing_dev_info *bdi);
void bdi_start_writeback(struct backing_dev_info *bdi, struct super_block *sb,
long nr_pages, enum writeback_sync_modes sync_mode);
int bdi_writeback_task(struct bdi_writeback *wb);
-void bdi_writeback_all(struct super_block *sb, long nr_pages,
- enum writeback_sync_modes sync_mode);
+void bdi_writeback_all(struct super_block *sb, struct writeback_control *wbc);
void bdi_add_default_flusher_task(struct backing_dev_info *bdi);
void bdi_add_flusher_task(struct backing_dev_info *bdi);
int bdi_has_dirty_io(struct backing_dev_info *bdi);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 7dd7de7..dd403cf 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -669,10 +669,18 @@ void throttle_vm_writeout(gfp_t gfp_mask)
*/
void wakeup_flusher_threads(long nr_pages)
{
+ struct writeback_control wbc = {
+ .sync_mode = WB_SYNC_NONE,
+ .older_than_this = NULL,
+ .range_cyclic = 1,
+ };
+
if (nr_pages == 0)
nr_pages = global_page_state(NR_FILE_DIRTY) +
global_page_state(NR_UNSTABLE_NFS);
- bdi_writeback_all(NULL, nr_pages, WB_SYNC_NONE);
+
+ wbc.nr_to_write = nr_pages;
+ bdi_writeback_all(NULL, &wbc);
}
static void laptop_timer_fn(unsigned long unused);
--
Jens Axboe
--
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