Re: [dpdk-dev] [PATCH 18.11] malloc: fix deadlock when using malloc stats

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 21-Dec-18 12:09 PM, Thomas Monjalon wrote:
> 04/12/2018 13:22, Anatoly Burakov:
>> Currently, malloc statistics and external heap creation code
>> use memory hotplug lock as a way to synchronize accesses to
>> heaps (as in, locking the hotplug lock to prevent list of heaps
>> from changing under our feet). At the same time, malloc
>> statistics code will also lock the heap because it needs to
>> access heap data and does not want any other thread to allocate
>> anything from that heap.
>>
>> In such scheme, it is possible to enter a deadlock with the
>> following sequence of events:
>>
>> thread 1		thread 2
>> rte_malloc()
>> 			rte_malloc_dump_stats()
>> take heap lock
>> 			take hotplug lock
>> failed to allocate,
>> attempt to take
>> hotplug lock
>> 			attempt to take heap lock
>>
>> Neither thread will be able to continue, as both of them are
>> waiting for the other one to drop the lock. Adding an
>> additional lock will require an ABI change, so instead of
>> that, make malloc statistics calls thread-unsafe with
>> respect to creating/destroying heaps.
>>
>> Fixes: 72cf92b31855 ("malloc: index heaps using heap ID rather than NUMA node")
>> Cc: stable@dpdk.org
>>
>> Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
>> ---
>>
>> Notes:
>>      IMO this is the best we can do for 18.11 without breaking ABI.
>>      For 19.02, we can introduce a new global heap lock (something
>>      i should've done in the first place...), so this patch is
>>      not applicable to 19.02. For 19.02, we can fix this properly
>>      by introducing another lock and breaking the EAL ABI.
>>      
>>      Not sure where to put docs update, feedback welcome.
> 
> This patch is also changing the API, because functions become not thread-safe.
> I think you should note it in the release notes.
> About 19.02, do we want to take this patch (with release notes updated)?
> 

Yes and yes.

Technically, they still are thread-safe when it comes to individual heap 
access - they just aren't thread-safe with regards to 
creating/destroying heaps (so, we may enter the dump function, and a 
heap may be added/removed while we're iterating over the list of heaps).

I'll send a v2 with release notes update.

-- 
Thanks,
Anatoly