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digitalmars.D.learn - About spinlock implementation

reply mogu <mogucpp 163.com> writes:
I found an implementation of spinlock in concurrency.d.
```
static shared struct SpinLock
{
     void lock() { while (!cas(&locked, false, true)) { 
Thread.yield(); } }
     void unlock() { atomicStore!(MemoryOrder.rel)(locked, false); 
}
     bool locked;
}
```
Why atomicStore use MemoryOrder.rel instead of MemoryOrder.raw?
Aug 31 2016
parent reply qznc <qznc web.de> writes:
On Thursday, 1 September 2016 at 06:44:13 UTC, mogu wrote:

 I found an implementation of spinlock in concurrency.d.
 ```
 static shared struct SpinLock
 {
     void lock() { while (!cas(&locked, false, true)) { 
 Thread.yield(); } }
     void unlock() { atomicStore!(MemoryOrder.rel)(locked, 
 false); }
     bool locked;
 }
 ```
 Why atomicStore use MemoryOrder.rel instead of MemoryOrder.raw?


I'm not sure I understand rel [0], but raw is too weak. Raw means 
no sequencing barrier, so

   local_var = protected_value;
   spinlock.unlock();

could be transformed (by compiler or CPU) to

   spinlock.unlock();
   local_var = protected_value;

This effectively makes the access to the protected value 
unprotected and nullifies the effect of the spinlock.

I find the documentation on MemoryOrder lacking about the 
semantics of rel. :(

[0] https://dlang.org/library/core/atomic/memory_order.html
Sep 01 2016
next sibling parent mogu <mogucpp 163.com> writes:
On Thursday, 1 September 2016 at 07:46:04 UTC, qznc wrote:

 I'm not sure I understand rel [0], but raw is too weak. Raw 
 means no sequencing barrier, so

   local_var = protected_value;
   spinlock.unlock();

 could be transformed (by compiler or CPU) to

   spinlock.unlock();
   local_var = protected_value;

 This effectively makes the access to the protected value 
 unprotected and nullifies the effect of the spinlock.

 I find the documentation on MemoryOrder lacking about the 
 semantics of rel. :(

 [0] https://dlang.org/library/core/atomic/memory_order.html


Thanks very much. I finally got it. :)
Sep 01 2016
prev sibling next sibling parent mogu <mogucpp 163.com> writes:
On Thursday, 1 September 2016 at 07:46:04 UTC, qznc wrote:

 This effectively makes the access to the protected value 
 unprotected and nullifies the effect of the spinlock.


So the cas operation implicit an MemoryOrder.acq? Does it make 
any other MemoryOrder guarantee?
Sep 01 2016
prev sibling parent reply Guillaume Piolat <first.last gmail.com> writes:
On Thursday, 1 September 2016 at 07:46:04 UTC, qznc wrote:

 I find the documentation on MemoryOrder lacking about the 
 semantics of rel. :(

 [0] https://dlang.org/library/core/atomic/memory_order.html


What helped me was to read std::memory_order documentation
http://en.cppreference.com/w/cpp/atomic/memory_order
Sep 01 2016
parent reply qznc <qznc web.de> writes:
On Thursday, 1 September 2016 at 10:30:12 UTC, Guillaume Piolat 
wrote:

 On Thursday, 1 September 2016 at 07:46:04 UTC, qznc wrote:

 I find the documentation on MemoryOrder lacking about the 
 semantics of rel. :(

 [0] https://dlang.org/library/core/atomic/memory_order.html


 What helped me was to read std::memory_order documentation
 http://en.cppreference.com/w/cpp/atomic/memory_order


Yes, but how do they map? Is D's rel = relaxed or release or 
acq_rel?

Also, reading C++ documentation should not be required of course. 
;)
Sep 01 2016
parent Guillaume Piolat <first.last gmail.com> writes:
On Thursday, 1 September 2016 at 10:38:07 UTC, qznc wrote:

 On Thursday, 1 September 2016 at 10:30:12 UTC, Guillaume Piolat 
 wrote:

 On Thursday, 1 September 2016 at 07:46:04 UTC, qznc wrote:

 I find the documentation on MemoryOrder lacking about the 
 semantics of rel. :(

 [0] https://dlang.org/library/core/atomic/memory_order.html


 What helped me was to read std::memory_order documentation
 http://en.cppreference.com/w/cpp/atomic/memory_order


 Yes, but how do they map? Is D's rel = relaxed or release or 
 acq_rel?

 Also, reading C++ documentation should not be required of 
 course. ;)


MemoryOrder.rel must be std::memory_order::release (70% 
confidence)
And std::memory_order::relaxed is MemoryOrder.raw of course (90% 
confidence).
Sep 01 2016