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Date: Mon, 6 Nov 2017 17:01:07 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Al Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
syzbot
<bot+f99f3a0db9007f4f4e32db54229a240c4fe57c15@...kaller.appspotmail.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Johannes Weiner <hannes@...xchg.org>, jlayton@...hat.com,
LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
npiggin@...il.com, rgoldwyn@...e.com, ross.zwisler@...ux.intel.com,
syzkaller-bugs@...glegroups.com, Ingo Molnar <mingo@...hat.com>
Subject: Re: possible deadlock in generic_file_write_iter
On Mon, Nov 06, 2017 at 02:35:44PM +0100, Dmitry Vyukov wrote:
> On Mon, Nov 6, 2017 at 2:33 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> > On Mon, Nov 06, 2017 at 02:15:44PM +0100, Jan Kara wrote:
> >
> >> > Should we annotate these inodes with different lock types? Or use
> >> > nesting annotations?
> >>
> >> Well, you'd need to have a completely separate set of locking classes for
> >> each filesystem to avoid false positives like these. And that would
> >> increase number of classes lockdep has to handle significantly. So I'm not
> >> sure it's really worth it...
> >
> > Especially when you consider that backing file might be on a filesystem
> > that lives on another loop device. *All* per-{device,fs} locks involved
> > would need classes split that way...
>
>
> This crashes our test machines left and right. We've seen 100000+ of
> these crashes. We need to do at least something. Can we disable all
> checking of these mutexes if they inherently have positives?
Its not the mutexes that's the problem.. Its the completion that crosses
the filesystem layers.
> +Ingo, Peter, maybe you have some suggestions of how to fight this
> lockdep false positives. Full thread is here:
> https://groups.google.com/forum/#!msg/syzkaller-bugs/NJ_4llH84XI/c7M9jNLTAgAJ
The best I could come up with is something like the below; its not
at all pretty and I could see people objecting; least of all myself for
the __complete() thing, but I ran out of creative naming juice.
---
block/bio.c | 2 +-
block/blk-core.c | 8 ++++++--
drivers/block/loop.c | 9 +++++++++
include/linux/blk_types.h | 1 +
include/linux/blkdev.h | 1 +
include/linux/completion.h | 8 +++++++-
kernel/sched/completion.c | 11 ++++-------
7 files changed, 29 insertions(+), 11 deletions(-)
diff --git a/block/bio.c b/block/bio.c
index cc60213e56d8..22bedceb7bae 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -919,7 +919,7 @@ EXPORT_SYMBOL_GPL(bio_iov_iter_get_pages);
static void submit_bio_wait_endio(struct bio *bio)
{
- complete(bio->bi_private);
+ __complete(bio->bi_private, !bio_flagged(bio, BIO_STACKED));
}
/**
diff --git a/block/blk-core.c b/block/blk-core.c
index 048be4aa6024..bb4092d716c3 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -194,8 +194,12 @@ static void req_bio_endio(struct request *rq, struct bio *bio,
if (error)
bio->bi_status = error;
- if (unlikely(rq->rq_flags & RQF_QUIET))
- bio_set_flag(bio, BIO_QUIET);
+ if (unlikely(rq->rq_flags & (RQF_QUIET|RQF_STACKED))) {
+ if (rq->rq_flags & RQF_QUIET)
+ bio_set_flag(bio, BIO_QUIET);
+ if (rq->rq_flags & RQF_STACKED)
+ bio_set_flag(bio, BIO_STACKED);
+ }
bio_advance(bio, nbytes);
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 85de67334695..7d702d2c4ade 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -452,6 +452,15 @@ static void lo_complete_rq(struct request *rq)
{
struct loop_cmd *cmd = blk_mq_rq_to_pdu(rq);
+ /*
+ * Assuming loop ensures the associated filesystems form a DAG, this
+ * cross-filesystem release can never form a deadlock.
+ *
+ * Inform the request that the corresponding BIO is of a stacked
+ * device and thereby forgo dependency checking.
+ */
+ rq->rq_flags |= RQF_STACKED;
+
if (unlikely(req_op(cmd->rq) == REQ_OP_READ && cmd->use_aio &&
cmd->ret >= 0 && cmd->ret < blk_rq_bytes(cmd->rq))) {
struct bio *bio = cmd->rq->bio;
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index 96ac3815542c..bf9b37de7975 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -136,6 +136,7 @@ struct bio {
* throttling rules. Don't do it again. */
#define BIO_TRACE_COMPLETION 10 /* bio_endio() should trace the final completion
* of this bio. */
+#define BIO_STACKED 11
/* See BVEC_POOL_OFFSET below before adding new flags */
/*
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 8da66379f7ea..dcf4b1a70f77 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -121,6 +121,7 @@ typedef __u32 __bitwise req_flags_t;
/* Look at ->special_vec for the actual data payload instead of the
bio chain. */
#define RQF_SPECIAL_PAYLOAD ((__force req_flags_t)(1 << 18))
+#define RQF_STACKED ((__force req_flags_t)(1 << 19))
/* flags that prevent us from merging requests: */
#define RQF_NOMERGE_FLAGS \
diff --git a/include/linux/completion.h b/include/linux/completion.h
index 0662a417febe..a6680197f2af 100644
--- a/include/linux/completion.h
+++ b/include/linux/completion.h
@@ -161,7 +161,13 @@ extern long wait_for_completion_killable_timeout(
extern bool try_wait_for_completion(struct completion *x);
extern bool completion_done(struct completion *x);
-extern void complete(struct completion *);
+extern void __complete(struct completion *, bool);
+
+static inline void complete(struct completion *x)
+{
+ __complete(x, true);
+}
+
extern void complete_all(struct completion *);
#endif
diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
index 2ddaec40956f..a2071513decf 100644
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -28,23 +28,20 @@
* It may be assumed that this function implies a write memory barrier before
* changing the task state if and only if any tasks are woken up.
*/
-void complete(struct completion *x)
+void __complete(struct completion *x, bool link)
{
unsigned long flags;
spin_lock_irqsave(&x->wait.lock, flags);
-
- /*
- * Perform commit of crossrelease here.
- */
- complete_release_commit(x);
+ if (link)
+ complete_release_commit(x);
if (x->done != UINT_MAX)
x->done++;
__wake_up_locked(&x->wait, TASK_NORMAL, 1);
spin_unlock_irqrestore(&x->wait.lock, flags);
}
-EXPORT_SYMBOL(complete);
+EXPORT_SYMBOL(__complete);
/**
* complete_all: - signals all threads waiting on this completion
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