Re: [PATCH -next] ipc: use GFP_ATOMIC under spin lock

[Manfred Spraul <manfred@colorfullife.com>]

Hello together,

On 4/28/20 1:14 PM, Matthew Wilcox wrote:
> On Tue, Apr 28, 2020 at 03:47:36AM +0000, Wei Yongjun wrote:
>> The function ipc_id_alloc() is called from ipc_addid(), in which
>> a spin lock is held, so we should use GFP_ATOMIC instead.
>>
>> Fixes: de5738d1c364 ("ipc: convert ipcs_idr to XArray")
>> Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
> I see why you think that, but it's not true.  Yes, we hold a spinlock, but
> the spinlock is in an object which is not reachable from any other CPU.

Is it really allowed that spin_lock()/spin_unlock may happen on 
different cpus?

CPU1: spin_lock()

CPU1: schedule() -> sleeps

CPU2: -> schedule() returns

CPU2: spin_unlock().


> Converting to GFP_ATOMIC is completely wrong.

What is your solution proposal?

xa_store() also gets a gfp_ flag. Thus even splitting _alloc() and 
_store() won't help

     xa_alloc(,entry=NULL,)
     new->seq = ...
     spin_lock();
     xa_store(,entry=new,GFP_KERNEL);

--

     Manfred