Re: [PATCH 2/3] smp/ipi:Clarify ambiguous comments around deadlock scenarios in smp_call_function variants.

[Preeti U Murthy <preeti@linux.vnet.ibm.com>]

Thanks for the pointer Thomas :)

Regards
Preeti U murthy
On 07/07/2013 01:18 AM, Thomas Gleixner wrote:
> On Sat, 6 Jul 2013, Preeti U Murthy wrote:
> 
>> Hi Wang,
>>
>> On 07/06/2013 11:42 AM, Wang YanQing wrote:
>>> On Fri, Jul 05, 2013 at 09:57:11PM +0530, Preeti U Murthy wrote:
>>>> Elaborate on when deadlocks can occur when a call is made to
>>>> smp_call_function_single() and its friends. This avoids ambiguity about
>>>> when to use these calls.
>>>>
>>>> +	 * 2. wait = 0: This function could be called from an interrupt
>>>> +	 * context, and can get blocked on the csd_lock(csd) below in
>>>> +	 * "non wait cases".
>>>> +	 * This is because the percpu copy of csd of this_cpu is used
>>>> +	 * in non wait cases. Under such circumstances, if the previous caller
>>>> +	 * of this function who got preempted by this interrupt has already taken
>>>> +	 * the lock under non wait condition, it will result in deadlock.
>>>> +	 *
>>>
>>> No, it will not cause deadlock, it is not mutex lock,  it is busy wait, so
>>> when the CSD_FLAG_LOCK be cleared, the code will go on running.
>>
>> A deadlock might not result, but a potential long wait in an interrupt
>> context could result if the source cpu got preempted by an interrupt
>> between  csd_lock(csd) and generic_exec_single(), where it actually
>> sends an ipi to the target cpu.
> 
> See https://lkml.org/lkml/2013/7/5/183 and the related thread for real
> deadlock scenarios.
> 
> Thanks,
> 
> 	tglx
>