2020-06-08 10:09:08

by Michał Mirosław

Subject: [PATCH 0/2] ALSA: pcm: stream linking locking fixes

Two patches fixing locking issues for SNDRV_PCM_IOCTL_LINK handling:
first adds a check preventing linking a stream to itself, second quiets
lockdep warning about nested locks.

Michał Mirosław (2):
ALSA: pcm: disallow linking stream to itself
ALSA: pcm: fix snd_pcm_link() lockdep splat

sound/core/pcm_native.c | 20 ++++++++++++++++++--
1 file changed, 18 insertions(+), 2 deletions(-)

--
2.20.1

2020-06-08 10:10:52

by Michał Mirosław

Subject: [PATCH 2/2] ALSA: pcm: fix snd_pcm_link() lockdep splat

Add and use snd_pcm_stream_lock_nested() in snd_pcm_link/unlink
implementation. The code is fine, but generates a lockdep complaint:

============================================
WARNING: possible recursive locking detected
5.7.1mq+ #381 Tainted: G O
--------------------------------------------
pulseaudio/4180 is trying to acquire lock:
ffff888402d6f508 (&group->lock){-...}-{2:2}, at: snd_pcm_common_ioctl+0xda8/0xee0 [snd_pcm]

but task is already holding lock:
ffff8883f7a8cf18 (&group->lock){-...}-{2:2}, at: snd_pcm_common_ioctl+0xe4e/0xee0 [snd_pcm]

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock(&group->lock);
lock(&group->lock);

*** DEADLOCK ***

May be due to missing lock nesting notation

2 locks held by pulseaudio/4180:
#0: ffffffffa1a05190 (snd_pcm_link_rwsem){++++}-{3:3}, at: snd_pcm_common_ioctl+0xca0/0xee0 [snd_pcm]
#1: ffff8883f7a8cf18 (&group->lock){-...}-{2:2}, at: snd_pcm_common_ioctl+0xe4e/0xee0 [snd_pcm]
[...]

Cc: [email protected]
Fixes: f57f3df03a8e ("ALSA: pcm: More fine-grained PCM link locking")
Signed-off-by: Michał Mirosław <[email protected]>
---
sound/core/pcm_native.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index 3ad399cb6f30..5b36881f7095 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -138,6 +138,16 @@ void snd_pcm_stream_lock_irq(struct snd_pcm_substream *substream)
}
EXPORT_SYMBOL_GPL(snd_pcm_stream_lock_irq);

+static void snd_pcm_stream_lock_nested(struct snd_pcm_substream *substream)
+{
+ struct snd_pcm_group *group = &substream->self_group;
+
+ if (substream->pcm->nonatomic)
+ mutex_lock_nested(&group->mutex, SINGLE_DEPTH_NESTING);
+ else
+ spin_lock_nested(&group->lock, SINGLE_DEPTH_NESTING);
+}
+
/**
* snd_pcm_stream_unlock_irq - Unlock the PCM stream
* @substream: PCM substream
@@ -2200,7 +2210,7 @@ static int snd_pcm_link(struct snd_pcm_substream *substream, int fd)
snd_pcm_stream_unlock_irq(substream);

snd_pcm_group_lock_irq(target_group, nonatomic);
- snd_pcm_stream_lock(substream1);
+ snd_pcm_stream_lock_nested(substream1);
snd_pcm_group_assign(substream1, target_group);
refcount_inc(&target_group->refs);
snd_pcm_stream_unlock(substream1);
@@ -2216,7 +2226,7 @@ static int snd_pcm_link(struct snd_pcm_substream *substream, int fd)

static void relink_to_local(struct snd_pcm_substream *substream)
{
- snd_pcm_stream_lock(substream);
+ snd_pcm_stream_lock_nested(substream);
snd_pcm_group_assign(substream, &substream->self_group);
snd_pcm_stream_unlock(substream);
}
--
2.20.1

2020-06-08 13:07:00

by Takashi Iwai

Subject: Re: [PATCH 2/2] ALSA: pcm: fix snd_pcm_link() lockdep splat

On Mon, 08 Jun 2020 12:06:32 +0200,
Michał Mirosław wrote:
>
> Add and use snd_pcm_stream_lock_nested() in snd_pcm_link/unlink
> implementation. The code is fine, but generates a lockdep complaint:
>
> ============================================
> WARNING: possible recursive locking detected
> 5.7.1mq+ #381 Tainted: G O
> --------------------------------------------
> pulseaudio/4180 is trying to acquire lock:
> ffff888402d6f508 (&group->lock){-...}-{2:2}, at: snd_pcm_common_ioctl+0xda8/0xee0 [snd_pcm]
>
> but task is already holding lock:
> ffff8883f7a8cf18 (&group->lock){-...}-{2:2}, at: snd_pcm_common_ioctl+0xe4e/0xee0 [snd_pcm]
>
> other info that might help us debug this:
> Possible unsafe locking scenario:
>
> CPU0
> ----
> lock(&group->lock);
> lock(&group->lock);
>
> *** DEADLOCK ***
>
> May be due to missing lock nesting notation
>
> 2 locks held by pulseaudio/4180:
> #0: ffffffffa1a05190 (snd_pcm_link_rwsem){++++}-{3:3}, at: snd_pcm_common_ioctl+0xca0/0xee0 [snd_pcm]
> #1: ffff8883f7a8cf18 (&group->lock){-...}-{2:2}, at: snd_pcm_common_ioctl+0xe4e/0xee0 [snd_pcm]
> [...]
>
> Cc: [email protected]
> Fixes: f57f3df03a8e ("ALSA: pcm: More fine-grained PCM link locking")
> Signed-off-by: Michał Mirosław <[email protected]>

Applied now. Thanks.


Takashi