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Subject: Re: [PATCH v3] bcache: fix deadlock in bcache_allocator
To:     Andrea Righi <andrea.righi@canonical.com>,
        Kent Overstreet <kent.overstreet@gmail.com>
Cc:     linux-bcache@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20190807103806.GA15450@xps-13>
From:   Coly Li <colyli@suse.de>
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Date:   Wed, 7 Aug 2019 21:53:46 +0800
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On 2019/8/7 6:38 下午, Andrea Righi wrote:
> bcache_allocator can call the following:
> 
>  bch_allocator_thread()
>   -> bch_prio_write()
>      -> bch_bucket_alloc()
>         -> wait on &ca->set->bucket_wait
> 
> But the wake up event on bucket_wait is supposed to come from
> bch_allocator_thread() itself => deadlock:
> 
> [ 1158.490744] INFO: task bcache_allocato:15861 blocked for more than 10 seconds.
> [ 1158.495929]       Not tainted 5.3.0-050300rc3-generic #201908042232
> [ 1158.500653] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [ 1158.504413] bcache_allocato D    0 15861      2 0x80004000
> [ 1158.504419] Call Trace:
> [ 1158.504429]  __schedule+0x2a8/0x670
> [ 1158.504432]  schedule+0x2d/0x90
> [ 1158.504448]  bch_bucket_alloc+0xe5/0x370 [bcache]
> [ 1158.504453]  ? wait_woken+0x80/0x80
> [ 1158.504466]  bch_prio_write+0x1dc/0x390 [bcache]
> [ 1158.504476]  bch_allocator_thread+0x233/0x490 [bcache]
> [ 1158.504491]  kthread+0x121/0x140
> [ 1158.504503]  ? invalidate_buckets+0x890/0x890 [bcache]
> [ 1158.504506]  ? kthread_park+0xb0/0xb0
> [ 1158.504510]  ret_from_fork+0x35/0x40
> 
> Fix by making the call to bch_prio_write() non-blocking, so that
> bch_allocator_thread() never waits on itself.
> 
> Moreover, make sure to wake up the garbage collector thread when
> bch_prio_write() is failing to allocate buckets.
> 
> BugLink: https://bugs.launchpad.net/bugs/1784665
> BugLink: https://bugs.launchpad.net/bugs/1796292
> Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

OK, I add this version into my for-test directory. Once you have a new
version, I will update it. Thanks.

Coly Li

> ---
> Changes in v3:
>  - prevent buckets leak in bch_prio_write()
> 
>  drivers/md/bcache/alloc.c  |  5 ++++-
>  drivers/md/bcache/bcache.h |  2 +-
>  drivers/md/bcache/super.c  | 27 +++++++++++++++++++++------
>  3 files changed, 26 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/md/bcache/alloc.c b/drivers/md/bcache/alloc.c
> index 6f776823b9ba..a1df0d95151c 100644
> --- a/drivers/md/bcache/alloc.c
> +++ b/drivers/md/bcache/alloc.c
> @@ -377,7 +377,10 @@ static int bch_allocator_thread(void *arg)
>  			if (!fifo_full(&ca->free_inc))
>  				goto retry_invalidate;
>  
> -			bch_prio_write(ca);
> +			if (bch_prio_write(ca, false) < 0) {
> +				ca->invalidate_needs_gc = 1;
> +				wake_up_gc(ca->set);
> +			}
>  		}
>  	}
>  out:
> diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
> index 013e35a9e317..deb924e1d790 100644
> --- a/drivers/md/bcache/bcache.h
> +++ b/drivers/md/bcache/bcache.h
> @@ -977,7 +977,7 @@ bool bch_cached_dev_error(struct cached_dev *dc);
>  __printf(2, 3)
>  bool bch_cache_set_error(struct cache_set *c, const char *fmt, ...);
>  
> -void bch_prio_write(struct cache *ca);
> +int bch_prio_write(struct cache *ca, bool wait);
>  void bch_write_bdev_super(struct cached_dev *dc, struct closure *parent);
>  
>  extern struct workqueue_struct *bcache_wq;
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index 20ed838e9413..bd153234290d 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -529,12 +529,29 @@ static void prio_io(struct cache *ca, uint64_t bucket, int op,
>  	closure_sync(cl);
>  }
>  
> -void bch_prio_write(struct cache *ca)
> +int bch_prio_write(struct cache *ca, bool wait)
>  {
>  	int i;
>  	struct bucket *b;
>  	struct closure cl;
>  
> +	pr_debug("free_prio=%zu, free_none=%zu, free_inc=%zu",
> +		 fifo_used(&ca->free[RESERVE_PRIO]),
> +		 fifo_used(&ca->free[RESERVE_NONE]),
> +		 fifo_used(&ca->free_inc));
> +
> +	/*
> +	 * Pre-check if there are enough free buckets. In the non-blocking
> +	 * scenario it's better to fail early rather than starting to allocate
> +	 * buckets and do a cleanup later in case of failure.
> +	 */
> +	if (!wait) {
> +		size_t avail = fifo_used(&ca->free[RESERVE_PRIO]) +
> +			       fifo_used(&ca->free[RESERVE_NONE]);
> +		if (prio_buckets(ca) > avail)
> +			return -ENOMEM;
> +	}
> +
>  	closure_init_stack(&cl);
>  
>  	lockdep_assert_held(&ca->set->bucket_lock);
> @@ -544,9 +561,6 @@ void bch_prio_write(struct cache *ca)
>  	atomic_long_add(ca->sb.bucket_size * prio_buckets(ca),
>  			&ca->meta_sectors_written);
>  
> -	//pr_debug("free %zu, free_inc %zu, unused %zu", fifo_used(&ca->free),
> -	//	 fifo_used(&ca->free_inc), fifo_used(&ca->unused));
> -
>  	for (i = prio_buckets(ca) - 1; i >= 0; --i) {
>  		long bucket;
>  		struct prio_set *p = ca->disk_buckets;
> @@ -564,7 +578,7 @@ void bch_prio_write(struct cache *ca)
>  		p->magic	= pset_magic(&ca->sb);
>  		p->csum		= bch_crc64(&p->magic, bucket_bytes(ca) - 8);
>  
> -		bucket = bch_bucket_alloc(ca, RESERVE_PRIO, true);
> +		bucket = bch_bucket_alloc(ca, RESERVE_PRIO, wait);
>  		BUG_ON(bucket == -1);
>  
>  		mutex_unlock(&ca->set->bucket_lock);
> @@ -593,6 +607,7 @@ void bch_prio_write(struct cache *ca)
>  
>  		ca->prio_last_buckets[i] = ca->prio_buckets[i];
>  	}
> +	return 0;
>  }
>  
>  static void prio_read(struct cache *ca, uint64_t bucket)
> @@ -1954,7 +1969,7 @@ static int run_cache_set(struct cache_set *c)
>  
>  		mutex_lock(&c->bucket_lock);
>  		for_each_cache(ca, c, i)
> -			bch_prio_write(ca);
> +			bch_prio_write(ca, true);
>  		mutex_unlock(&c->bucket_lock);
>  
>  		err = "cannot allocate new UUID bucket";
> 


-- 

Coly Li