2015-01-13 13:50 GMT+03:00 Hallvard Breien Furuseth h.b.furuseth@usit.uio.no:

On 01/12/2015 07:15 PM, leo@yuriev.ru wrote:

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Unfortunately IMDB programmed without accounting possibility of memory reordering and cache incoherency (as a proof - the 'volatile' was missed before ITS#7969). So, currently seems LMDB is durable only on x86 and may be on MIPS.

I gather "volatile" in thread code can be read as "warning: dubious code" or at best "please access this sooner rather than later".

comp.programming.threads's advise is that volatile is useless for thread sync: You need sync primitives, and then you don't need volatile since the compiler/hardware may not move the memory access past the primitive.

Excuse me, but seems you are misunderstand comp.programming.threads in case of blocking/locking multithreading and non-blocking multithreading (aka lock-free/wait-free).

Yes, `volatile` don't needed, when a locking (e.g. mutex) is used for access to any variables that shared between threads.

But LMDB don't uses locking primitives for interaction between writers and readers. LMDB properties declared as: - readers don't block writers and writers don't block readers; - read transactions are extremely cheap, and can be performed using no mallocs or any other _blocking_ calls;

THEREFORE 'volatile' and memory-ordering ISSUES MUST BE HANDLED VERY CAREFULLY AND ANXIOUSLY.

Unless the primitive's spec says so, I guess. E.g. with an "asm" statement where the compiler does not know what the assembly code does and must be told not to mess with ordering.

Please study GCC's manual about __asm __volalile (:::"memory"), this is just a "full compiler fence".

The best fix is likely a format change which reduces the need for sync primitives, for LDMBv2. Checksum the MDB_meta, and/or replace the variable part with a single word which refers to the meta. Don't know if we can get rid of unportable sync code though.

Yes it is possible, but requires C++, see http://en.cppreference.com/w/cpp/atomic/memory_order By using just C, include C99 - not possible, see http://stackoverflow.com/questions/27301371/memory-order-consistency-model-a...

Leaving the rest to Howard, I'm too busy just now to do more than blow hot air:-)

Ok ;)

I created gist for public review, please share it. https://gist.github.com/leo-yuriev/ba186a6bf5cf3a27bae7

Leonid.