On Tue, 21 Mar 2017 13:48:27 +0100, Dmitry Vyukov wrote:

On Tue, Mar 21, 2017 at 1:23 PM, Takashi Iwai tiwai@suse.de wrote:

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On Sat, 04 Mar 2017 17:31:21 +0100, Dmitry Vyukov wrote:

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Hello,

The following program creates processes deadlocked in snd_seq_pool_done:

https://gist.githubusercontent.com/dvyukov/97efc9cb6d63b1b2c7b737b82cc8b0d6/...

After few seconds I get:

# ps afxu | grep a.out root 8660 2.0 0.0 0 0 pts/0 Zl 16:27 0:00 [a.out] <defunct>

# kill -9 8660

# cat /proc/8660/status Name: a.out State: Z (zombie) Tgid: 8660 Ngid: 0 Pid: 8660 PPid: 1 TracerPid: 0 Uid: 0 0 0 0 Gid: 0 0 0 0 FDSize: 0 Groups: 0 NStgid: 8660 NSpid: 8660 NSpgid: 8660 NSsid: 2971 Threads: 2 SigQ: 1/3304 SigPnd: 0000000000000000 ShdPnd: 0000000000000100 SigBlk: 0000000000000000 SigIgn: 0000000180000000 SigCgt: 0000000000000440 CapInh: 0000000000000000 CapPrm: 0000003fffffffff CapEff: 0000003fffffffff CapBnd: 0000003fffffffff CapAmb: 0000000000000000 NoNewPrivs: 0 Seccomp: 0 Cpus_allowed: f Cpus_allowed_list: 0-3 Mems_allowed: 00000000,00000001 Mems_allowed_list: 0 voluntary_ctxt_switches: 12 nonvoluntary_ctxt_switches: 0

# cat /proc/8660/task/*/stack [<ffffffff835406db>] snd_seq_pool_done+0x31b/0x620 sound/core/seq/seq_memory.c:436 [<ffffffff8353a11e>] snd_seq_ioctl_set_client_pool+0x1ae/0x600 sound/core/seq/seq_clientmgr.c:1836 [<ffffffff835382ba>] snd_seq_ioctl+0x2da/0x4d0 sound/core/seq/seq_clientmgr.c:2130 [<ffffffff81aced2f>] vfs_ioctl fs/ioctl.c:45 [inline] [<ffffffff81aced2f>] do_vfs_ioctl+0x1bf/0x1790 fs/ioctl.c:685 [<ffffffff81ad038f>] SYSC_ioctl fs/ioctl.c:700 [inline] [<ffffffff81ad038f>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:691 [<ffffffff8457dc41>] entry_SYSCALL_64_fastpath+0x1f/0xc2

Now I've been trying to reproduce the hang, alas it didn't happen on my VM by some reason.

In anyway, below is an untested fix for possible races against snd_seq_pool_done() and cell insertions. Could you check whether it cures your issue?

I reproduced it again within 10 second. Then restarted kernel with this patch and now it runs for 10 minutes without any hanged processes.

Tested-by: Dmitry Vyukov dvyukov@google.com

Thanks!

Good to hear! FWIW, below is the formal patch I'm going to queue.

thanks,

Takashi

-- 8< -- From: Takashi Iwai tiwai@suse.de Subject: [PATCH] ALSA: seq: Fix racy cell insertions during snd_seq_pool_done()

When snd_seq_pool_done() is called, it marks the closing flag to refuse the further cell insertions. But snd_seq_pool_done() itself doesn't clear the cells but just waits until all cells are cleared by the caller side. That is, it's racy, and this leads to the endless stall as syzkaller spotted.

This patch addresses the racy by splitting the setup of pool->closing flag out of snd_seq_pool_done(), and calling it properly before snd_seq_pool_done().

BugLink: http://lkml.kernel.org/r/CACT4Y+aqqy8bZA1fFieifNxR2fAfFQQABcBHj801+u5ePV0URw... Reported-and-tested-by: Dmitry Vyukov dvyukov@google.com Cc: stable@vger.kernel.org Signed-off-by: Takashi Iwai tiwai@suse.de --- sound/core/seq/seq_clientmgr.c | 1 + sound/core/seq/seq_fifo.c | 3 +++ sound/core/seq/seq_memory.c | 17 +++++++++++++---- sound/core/seq/seq_memory.h | 1 + 4 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/sound/core/seq/seq_clientmgr.c b/sound/core/seq/seq_clientmgr.c index 4c935202ce23..f3b1d7f50b81 100644 --- a/sound/core/seq/seq_clientmgr.c +++ b/sound/core/seq/seq_clientmgr.c @@ -1832,6 +1832,7 @@ static int snd_seq_ioctl_set_client_pool(struct snd_seq_client *client, info->output_pool != client->pool->size)) { if (snd_seq_write_pool_allocated(client)) { /* remove all existing cells */ + snd_seq_pool_mark_closing(client->pool); snd_seq_queue_client_leave_cells(client->number); snd_seq_pool_done(client->pool); } diff --git a/sound/core/seq/seq_fifo.c b/sound/core/seq/seq_fifo.c index 448efd4e980e..33980d1c8037 100644 --- a/sound/core/seq/seq_fifo.c +++ b/sound/core/seq/seq_fifo.c @@ -72,6 +72,9 @@ void snd_seq_fifo_delete(struct snd_seq_fifo **fifo) return; *fifo = NULL;

+ if (f->pool) + snd_seq_pool_mark_closing(f->pool); + snd_seq_fifo_clear(f);

/* wake up clients if any */ diff --git a/sound/core/seq/seq_memory.c b/sound/core/seq/seq_memory.c index 1a1acf3ddda4..d4c61ec9be13 100644 --- a/sound/core/seq/seq_memory.c +++ b/sound/core/seq/seq_memory.c @@ -415,6 +415,18 @@ int snd_seq_pool_init(struct snd_seq_pool *pool) return 0; }

+/* refuse the further insertion to the pool */ +void snd_seq_pool_mark_closing(struct snd_seq_pool *pool) +{ + unsigned long flags; + + if (snd_BUG_ON(!pool)) + return; + spin_lock_irqsave(&pool->lock, flags); + pool->closing = 1; + spin_unlock_irqrestore(&pool->lock, flags); +} + /* remove events */ int snd_seq_pool_done(struct snd_seq_pool *pool) { @@ -425,10 +437,6 @@ int snd_seq_pool_done(struct snd_seq_pool *pool) return -EINVAL;

/* wait for closing all threads */ - spin_lock_irqsave(&pool->lock, flags); - pool->closing = 1; - spin_unlock_irqrestore(&pool->lock, flags); - if (waitqueue_active(&pool->output_sleep)) wake_up(&pool->output_sleep);

@@ -485,6 +493,7 @@ int snd_seq_pool_delete(struct snd_seq_pool **ppool) *ppool = NULL; if (pool == NULL) return 0; + snd_seq_pool_mark_closing(pool); snd_seq_pool_done(pool); kfree(pool); return 0; diff --git a/sound/core/seq/seq_memory.h b/sound/core/seq/seq_memory.h index 4a2ec779b8a7..32f959c17786 100644 --- a/sound/core/seq/seq_memory.h +++ b/sound/core/seq/seq_memory.h @@ -84,6 +84,7 @@ static inline int snd_seq_total_cells(struct snd_seq_pool *pool) int snd_seq_pool_init(struct snd_seq_pool *pool);

/* done pool - free events */ +void snd_seq_pool_mark_closing(struct snd_seq_pool *pool); int snd_seq_pool_done(struct snd_seq_pool *pool);

/* create pool */