| Message ID | 20180613144606.nvbcyg2rdjpxhf7s@quack2.suse.cz (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Wed, Jun 13, 2018 at 7:46 AM Jan Kara <jack@suse.cz> wrote: > > On Wed 13-06-18 19:43:47, Tetsuo Handa wrote: > > Can't we utilize RCU grace period (like shown below) ? > > Honestly, the variant 1 looks too ugly to me. However variant 2 looks > mostly OK. The versions that don't have that conditional locking look fine to me, yes. > Also I'd avoid the addition argument to wb_writeback() and split the function instead. The > patch resulting from your and mine ideas is attached. Thoughts? Is there a reason for this model: + if (cgwb_start_shutdown(wb)) + __wb_shutdown(wb); when there is just one call site of this? Why not just make the function void, and make it do that __wb_shutdown() itself in the true case? IOW, just make it be + cgwb_shutdown(wb); instead? That's what "wb_shutdown()" does - it does the "wb_start_shutdown()" test internally, and does __wb_shutdown() all inside itself, instead of expecting the caller to do it. I dunno. Linus Linus
On 2018/06/13 23:46, Jan Kara wrote: > On Wed 13-06-18 19:43:47, Tetsuo Handa wrote: >> Can't we utilize RCU grace period (like shown below) ? > > Honestly, the variant 1 looks too ugly to me. However variant 2 looks > mostly OK. We can also avoid the schedule_timeout_uninterruptible(HZ / 10) > from your patch by careful handling of the bit waitqueues. Also I'd avoid > the addition argument to wb_writeback() and split the function instead. The > patch resulting from your and mine ideas is attached. Thoughts? > > Honza > +static bool cgwb_start_shutdown(struct bdi_writeback *wb) + __releases(cgwb_lock) +{ + if (!wb_start_shutdown(wb)) { + DEFINE_WAIT(wait); + wait_queue_head_t *wqh = bit_waitqueue(&wb->state, + WB_shutting_down); + bool sleep; + + prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE); + sleep = test_bit(WB_shutting_down, &wb->state); + spin_unlock_irq(&cgwb_lock); + if (sleep) + schedule(); + return false; + } + spin_unlock_irq(&cgwb_lock); + return true; +} Since multiple addresses share bit_wait_table[256], isn't it possible that cgwb_start_shutdown() prematurely returns false due to wake_up_bit() by hash-conflicting addresses (i.e. not limited to clear_and_wake_up_bit() from wb_shutdown())? I think that we cannot be sure without confirming that test_bit(WB_shutting_down, &wb->state) == false after returning from schedule().
On Wed, Jun 13, 2018 at 9:21 AM Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> wrote: > > Since multiple addresses share bit_wait_table[256], isn't it possible that > cgwb_start_shutdown() prematurely returns false due to wake_up_bit() by > hash-conflicting addresses (i.e. not limited to clear_and_wake_up_bit() from > wb_shutdown())? I think that we cannot be sure without confirming that > test_bit(WB_shutting_down, &wb->state) == false after returning from schedule(). Right. That's _always_ true, btw. Something else entirely could have woken you up. TASK_UNINTERRUPTIBLE does not mean "nothing else wakes me", it just means "_signals_ don't wake me". So every single sleep always needs to be in a loop. Always. Linus
On Wed 13-06-18 09:25:03, Linus Torvalds wrote: > On Wed, Jun 13, 2018 at 9:21 AM Tetsuo Handa > <penguin-kernel@i-love.sakura.ne.jp> wrote: > > > > Since multiple addresses share bit_wait_table[256], isn't it possible that > > cgwb_start_shutdown() prematurely returns false due to wake_up_bit() by > > hash-conflicting addresses (i.e. not limited to clear_and_wake_up_bit() from > > wb_shutdown())? I think that we cannot be sure without confirming that > > test_bit(WB_shutting_down, &wb->state) == false after returning from schedule(). > > Right. > > That's _always_ true, btw. Something else entirely could have woken > you up. TASK_UNINTERRUPTIBLE does not mean "nothing else wakes me", it > just means "_signals_ don't wake me". > > So every single sleep always needs to be in a loop. Always. Agreed and in my patch it actually is in a loop - the one iterating the list of active writeback structures. If we get a false wakeup, we find the same structure in the list again and wait again... Honza
On 2018/06/14 1:45, Jan Kara wrote: > On Wed 13-06-18 09:25:03, Linus Torvalds wrote: >> On Wed, Jun 13, 2018 at 9:21 AM Tetsuo Handa >> <penguin-kernel@i-love.sakura.ne.jp> wrote: >>> >>> Since multiple addresses share bit_wait_table[256], isn't it possible that >>> cgwb_start_shutdown() prematurely returns false due to wake_up_bit() by >>> hash-conflicting addresses (i.e. not limited to clear_and_wake_up_bit() from >>> wb_shutdown())? I think that we cannot be sure without confirming that >>> test_bit(WB_shutting_down, &wb->state) == false after returning from schedule(). >> >> Right. >> >> That's _always_ true, btw. Something else entirely could have woken >> you up. TASK_UNINTERRUPTIBLE does not mean "nothing else wakes me", it >> just means "_signals_ don't wake me". >> >> So every single sleep always needs to be in a loop. Always. > > Agreed and in my patch it actually is in a loop - the one iterating the > list of active writeback structures. If we get a false wakeup, we find the > same structure in the list again and wait again... Indeed. I overlooked that wb = list_first_entry() will select same wb again if cgwb_remove_from_bdi_list() is not yet called. Well, we could update "(in which case we also wait for it to finish)" part or move the body of cgwb_start_shutdown() to cgwb_bdi_unregister() so that it becomes clear that false wake-up is not a problem in this case.
On Thu 14-06-18 06:04:04, Tetsuo Handa wrote: > On 2018/06/14 1:45, Jan Kara wrote: > > On Wed 13-06-18 09:25:03, Linus Torvalds wrote: > >> On Wed, Jun 13, 2018 at 9:21 AM Tetsuo Handa > >> <penguin-kernel@i-love.sakura.ne.jp> wrote: > >>> > >>> Since multiple addresses share bit_wait_table[256], isn't it possible that > >>> cgwb_start_shutdown() prematurely returns false due to wake_up_bit() by > >>> hash-conflicting addresses (i.e. not limited to clear_and_wake_up_bit() from > >>> wb_shutdown())? I think that we cannot be sure without confirming that > >>> test_bit(WB_shutting_down, &wb->state) == false after returning from schedule(). > >> > >> Right. > >> > >> That's _always_ true, btw. Something else entirely could have woken > >> you up. TASK_UNINTERRUPTIBLE does not mean "nothing else wakes me", it > >> just means "_signals_ don't wake me". > >> > >> So every single sleep always needs to be in a loop. Always. > > > > Agreed and in my patch it actually is in a loop - the one iterating the > > list of active writeback structures. If we get a false wakeup, we find the > > same structure in the list again and wait again... > > Indeed. I overlooked that wb = list_first_entry() will select same wb again > if cgwb_remove_from_bdi_list() is not yet called. Well, we could update > "(in which case we also wait for it to finish)" part or move the body of > cgwb_start_shutdown() to cgwb_bdi_unregister() so that it becomes clear > that false wake-up is not a problem in this case. I prefer to keep the wb shutdown in a separate function but I've added some comments to explain that. Honza
From d0fbfad964ae5b0a331f0d814d527150bd0c305c Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Date: Wed, 13 Jun 2018 16:39:00 +0200
Subject: [PATCH] bdi: Fix another oops in wb_workfn()
syzbot is reporting NULL pointer dereference at wb_workfn() [1] due to
wb->bdi->dev being NULL. And Dmitry confirmed that wb->state was
WB_shutting_down after wb->bdi->dev became NULL. This indicates that
unregister_bdi() failed to call wb_shutdown() on one of wb objects.
The problem is in cgwb_bdi_unregister() which does cgwb_kill() and thus
drops bdi's reference to wb structures before going through the list of
wbs again and calling wb_shutdown() on each of them. This way the loop
iterating through all wbs can easily miss a wb if that wb has already
passed through cgwb_remove_from_bdi_list() called from wb_shutdown()
from cgwb_release_workfn() and as a result fully shutdown bdi although
wb_workfn() for this wb structure is still running. In fact there are
also other ways cgwb_bdi_unregister() can race with
cgwb_release_workfn() leading e.g. to use-after-free issues:
CPU1 CPU2
cgwb_bdi_unregister()
cgwb_kill(*slot);
cgwb_release()
queue_work(cgwb_release_wq, &wb->release_work);
cgwb_release_workfn()
wb = list_first_entry(&bdi->wb_list, ...)
spin_unlock_irq(&cgwb_lock);
wb_shutdown(wb);
...
kfree_rcu(wb, rcu);
wb_shutdown(wb); -> oops use-after-free
We solve these issues by making wb_shutdown() remove the writeback
structure from the bdi_list only after all the shutdown is done and by
making sure cgwb_bdi_unregister() properly synchronizes with concurrent
shutdowns using WB_shutting_down bit.
Signed-off-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Jan Kara <jack@suse.cz>
---
mm/backing-dev.c | 69 ++++++++++++++++++++++++++++++++++++++++++++------------
1 file changed, 55 insertions(+), 14 deletions(-)
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 347cc834c04a..433807ae364e 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -350,27 +350,21 @@ static int wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi,
static void cgwb_remove_from_bdi_list(struct bdi_writeback *wb);
-/*
- * Remove bdi from the global list and shutdown any threads we have running
- */
-static void wb_shutdown(struct bdi_writeback *wb)
+static bool wb_start_shutdown(struct bdi_writeback *wb)
{
/* Make sure nobody queues further work */
spin_lock_bh(&wb->work_lock);
if (!test_and_clear_bit(WB_registered, &wb->state)) {
spin_unlock_bh(&wb->work_lock);
- /*
- * Wait for wb shutdown to finish if someone else is just
- * running wb_shutdown(). Otherwise we could proceed to wb /
- * bdi destruction before wb_shutdown() is finished.
- */
- wait_on_bit(&wb->state, WB_shutting_down, TASK_UNINTERRUPTIBLE);
- return;
+ return false;
}
set_bit(WB_shutting_down, &wb->state);
spin_unlock_bh(&wb->work_lock);
+ return true;
+}
- cgwb_remove_from_bdi_list(wb);
+static void __wb_shutdown(struct bdi_writeback *wb)
+{
/*
* Drain work list and shutdown the delayed_work. !WB_registered
* tells wb_workfn() that @wb is dying and its work_list needs to
@@ -379,6 +373,7 @@ static void wb_shutdown(struct bdi_writeback *wb)
mod_delayed_work(bdi_wq, &wb->dwork, 0);
flush_delayed_work(&wb->dwork);
WARN_ON(!list_empty(&wb->work_list));
+ cgwb_remove_from_bdi_list(wb);
/*
* Make sure bit gets cleared after shutdown is finished. Matches with
* the barrier provided by test_and_clear_bit() above.
@@ -387,6 +382,23 @@ static void wb_shutdown(struct bdi_writeback *wb)
clear_and_wake_up_bit(WB_shutting_down, &wb->state);
}
+/*
+ * Remove bdi from the global list and shutdown any threads we have running
+ */
+static void wb_shutdown(struct bdi_writeback *wb)
+{
+ if (!wb_start_shutdown(wb)) {
+ /*
+ * Wait for wb shutdown to finish if someone else is just
+ * running wb_shutdown(). Otherwise we could proceed to wb /
+ * bdi destruction before wb_shutdown() is finished.
+ */
+ wait_on_bit(&wb->state, WB_shutting_down, TASK_UNINTERRUPTIBLE);
+ return;
+ }
+ __wb_shutdown(wb);
+}
+
static void wb_exit(struct bdi_writeback *wb)
{
int i;
@@ -706,6 +718,35 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi)
return ret;
}
+/*
+ * Mark writeback structure as shutting down. The function returns true if
+ * we successfully marked the structure, false if shutdown was already in
+ * progress (in which case we also wait for it to finish).
+ *
+ * The function requires cgwb_lock to be held and releases it. Note that the
+ * caller need not hold reference to the writeback structure and thus once
+ * cgwb_lock is released, the writeback structure can be freed.
+ */
+static bool cgwb_start_shutdown(struct bdi_writeback *wb)
+ __releases(cgwb_lock)
+{
+ if (!wb_start_shutdown(wb)) {
+ DEFINE_WAIT(wait);
+ wait_queue_head_t *wqh = bit_waitqueue(&wb->state,
+ WB_shutting_down);
+ bool sleep;
+
+ prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
+ sleep = test_bit(WB_shutting_down, &wb->state);
+ spin_unlock_irq(&cgwb_lock);
+ if (sleep)
+ schedule();
+ return false;
+ }
+ spin_unlock_irq(&cgwb_lock);
+ return true;
+}
+
static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
{
struct radix_tree_iter iter;
@@ -721,8 +762,8 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
while (!list_empty(&bdi->wb_list)) {
wb = list_first_entry(&bdi->wb_list, struct bdi_writeback,
bdi_node);
- spin_unlock_irq(&cgwb_lock);
- wb_shutdown(wb);
+ if (cgwb_start_shutdown(wb))
+ __wb_shutdown(wb);
spin_lock_irq(&cgwb_lock);
}
spin_unlock_irq(&cgwb_lock);
--
2.13.7
On Wed 13-06-18 19:43:47, Tetsuo Handa wrote: > Can't we utilize RCU grace period (like shown below) ? Honestly, the variant 1 looks too ugly to me. However variant 2 looks mostly OK. We can also avoid the schedule_timeout_uninterruptible(HZ / 10) from your patch by careful handling of the bit waitqueues. Also I'd avoid the addition argument to wb_writeback() and split the function instead. The patch resulting from your and mine ideas is attached. Thoughts? Honza