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Re: Draft pthread_spin_init(2) manual page


Hi Florian,


Thanks for looking at the page!

On 10/18/2017 11:09 AM, Florian Weimer wrote:
> On 10/18/2017 10:10 AM, Michael Kerrisk (man-pages) wrote:
> 
>> DESCRIPTION
>>         The  pthread_spin_init() function allocates any resources required
>>         for the use of the spin lock referred to by lock  and  initializes
>>         the  lock  to be in the unlocked state.  The pshared argument must
>>         have one of the following values:
> 
> I think somewhere this should say that it is not possible to change the 
> address of a spinlock after initialization (you can't memcpy it 
> somewhere else and expect that it will keep working).  Other pthread 
> objects have the same problem, of course.

Yes. I added this:

       The  result of performing operations such as pthread_spin_lock(3),
       pthread_spin_unlock(3), and pthread_spin_destroy(3) on  copies  of
       the object referred to by lock is undefined.


>> NOTES
>>         Spin locks should be employed in conjunction with real-time sched‐
>>         uling policies (SCHED_FIFO, or possibly SCHED_RR).   Use  of  spin
>>         locks   with   nondeterministic   scheduling   policies   such  as
>>         SCHED_OTHER probably indicates a design mistake.  The  problem  is
>>         that  if  a  thread operating under such a policy is scheduled off
>>         the CPU while it holds a spin lock, then other threads will  waste
>>         time  spinning  on  the  lock  until  the lock holder is once more
>>         rescheduled and releases the lock.
> 
> Isn't the more important concern whether there are sufficient resources 
> to run all the threads that block on spinlocks?

See my later replies.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/


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