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Date:	Tue, 11 Mar 2014 14:51:38 +0100
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Bart Van Assche <bvanassche@....org>
Cc:	Jens Axboe <axboe@...nel.dk>, Kent Overstreet <kmo@...erainc.com>,
	Shaohua Li <shli@...nel.org>, Christoph Hellwig <hch@....de>,
	Mike Christie <michaelc@...wisc.edu>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] percpu_ida: Handle out-of-tags gracefully

On Mon, Mar 10, 2014 at 03:12:33PM +0100, Bart Van Assche wrote:
> Avoid that percpu_ida_alloc() hangs or crashes if there are still
> tags are available. Wait until a tag becomes available instead of
> giving up when running out of tags temporarily. This patch fixes
> the following kernel bug:

Hi Bart,

Few comments below, but the changelog does not correspond to the
actual change in 'Wait until a tag becomes available'.

> ------------[ cut here ]------------
> kernel BUG at lib/percpu_ida.c:81!
> invalid opcode: 0000 [#1] SMP
> RIP: 0010:[<ffffffff8120f00e>]  [<ffffffff8120f00e>] percpu_ida_alloc+0x33e/0x370
> Call Trace:
>  [<ffffffff811ef95f>] blk_mq_get_tag+0x2f/0x50
>  [<ffffffff811ed79c>] blk_mq_alloc_rq.isra.17+0x1c/0x90
>  [<ffffffff811eeb9b>] blk_mq_alloc_request_pinned+0x9b/0x110
>  [<ffffffff811ef4c6>] blk_mq_make_request+0x426/0x480
>  [<ffffffff811e28f0>] generic_make_request+0xc0/0x110
>  [<ffffffff811e29ab>] submit_bio+0x6b/0x140
>  [<ffffffff8117aabb>] _submit_bh+0x13b/0x220
>  [<ffffffff8117d70f>] block_read_full_page+0x1ff/0x300
>  [<ffffffff81181128>] blkdev_readpage+0x18/0x20
>  [<ffffffff811067b7>] __do_page_cache_readahead+0x277/0x280
>  [<ffffffff81106d1d>] force_page_cache_readahead+0x8d/0xc0
>  [<ffffffff81106d9b>] page_cache_sync_readahead+0x4b/0x50
>  [<ffffffff810fdf05>] generic_file_aio_read+0x4c5/0x700
>  [<ffffffff8118147b>] blkdev_aio_read+0x4b/0x70
>  [<ffffffff8114a28a>] do_sync_read+0x5a/0x90
>  [<ffffffff8114a8cb>] vfs_read+0x9b/0x160
>  [<ffffffff8114b389>] SyS_read+0x49/0xa0
>  [<ffffffff81416049>] tracesys+0xd0/0xd5
> ---[ end trace cdd1a8a7968266cf ]---
> 
> Signed-off-by: Bart Van Assche <bvanassche@....org>
> Cc: Kent Overstreet <kmo@...erainc.com>
> Cc: Shaohua Li <shli@...nel.org>
> Cc: Christoph Hellwig <hch@....de>
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: Alexander Gordeev <agordeev@...hat.com>
> Cc: Mike Christie <michaelc@...wisc.edu>
> ---
>  lib/percpu_ida.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/percpu_ida.c b/lib/percpu_ida.c
> index 93d145e..170d27c 100644
> --- a/lib/percpu_ida.c
> +++ b/lib/percpu_ida.c
> @@ -73,7 +73,7 @@ static inline void steal_tags(struct percpu_ida *pool,
>  		if (cpu >= nr_cpu_ids) {
>  			cpu = cpumask_first(&pool->cpus_have_tags);
>  			if (cpu >= nr_cpu_ids)
> -				BUG();
> +				break;

I assume the BUG() above hits? If so, I am failing to understand how
the code gets here. Mind elaborate?

>  		}
>  
>  		pool->cpu_last_stolen = cpu;
> @@ -189,6 +189,9 @@ int percpu_ida_alloc(struct percpu_ida *pool, int state)
>  		spin_unlock(&pool->lock);
>  		local_irq_restore(flags);
>  
> +		if (tags->nr_free)
> +			wake_up(&pool->wait);
> +

How 'tags->nr_free' could be checked out of locks?
Why waking up another thread instead of returning the tag on this CPU?
Why 'percpu_max_size' threshold is ignored?

Anyway, IMHO the above BUG() indicates a problem elsewhere.

>  		if (tag >= 0 || state == TASK_RUNNING)
>  			break;
>  
> -- 
> 1.8.4.5
> 

-- 
Regards,
Alexander Gordeev
agordeev@...hat.com
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