Re: [PATCH 3/3] xen-blkback: rm BUG_ON() in purge_persistent_gnt()

[Bob Liu <bob.liu@oracle.com>]

On 07/21/2015 05:13 PM, Roger Pau Monné wrote:
> El 21/07/15 a les 5.30, Bob Liu ha escrit:
>> This BUG_ON() will be triggered when previous purge work haven't finished.
>> It's reasonable under pretty extreme load and should not panic the system.
>>
>> Signed-off-by: Bob Liu <bob.liu@oracle.com>
>> ---
>>  drivers/block/xen-blkback/blkback.c |    4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
>> index ced9677..b90ac8e 100644
>> --- a/drivers/block/xen-blkback/blkback.c
>> +++ b/drivers/block/xen-blkback/blkback.c
>> @@ -394,7 +394,9 @@ static void purge_persistent_gnt(struct xen_blkif *blkif)
>>  
>>  	pr_debug("Going to purge %u persistent grants\n", num_clean);
>>  
>> -	BUG_ON(!list_empty(&blkif->persistent_purge_list));
>> +	if (!list_empty(&blkif->persistent_purge_list))
>> +		return;
>> +
> 
> I see the problem with this, there's a check for work_pending before
> this BUG_ON, but it doesn't account if the work is currently running. I

Exactly.

> would rather prefer to replace the work_pending check with work_busy
> instead, which will also take into account if the work is still running.
> The comment on work_busy however makes me nervous:
> 
> * Test whether @work is currently pending or running.  There is no
> * synchronization around this function and the test result is
> * unreliable and only useful as advisory hints or for debugging.
> 
> AFAICT I think it should be safe because we don't have concurrent
> purge_persistent_gnt calls, but I'm no expert on Linux workqueues. It

Me neither, that's why I just replace this BUG_ON() with a simple return.

> also makes me wonder why we have such a half-baked function in the Linux
> kernel.
>