Hi Jim, The C abstract machine definition doesn't take into account multi-processing and therefore ignores memory operation reordering as seen by external observers. Volatile means little more than "don't keep this variable in a register, it must be accessed directly from memory". It however doesn't specify anything about whether tha variable can exist in cache, or whether accesses to it can be reordered around other memory accesses. As long as volatile variables aren't kept in a register and are accessed directly from [cached] memory then cache coherency will take care of keeping the cache in sync with external memory (note that for memory-mapped devices, the device driver must have disabled caching for that region of memory). The problem of reordering being dicussed here doesn't result in incorrect values being read (cache coherency, for example, takes care of that), it's to do with assumptions being made about the order of memory operations mapping to the order of operations specified in code. DCLP is based on the assumption that the order of memory operations occurs in the same sequence as specified in the source code, and in modern MP systems that is rarely the case.
Regards, Steven -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jim Schneider Sent: Friday, 8 April 2005 7:35 AM To: [email protected] Subject: Re: OpenSSL use of DCLP may not be thread-safe on multiple processors On Thursday 07 April 2005 16:39, David Schwartz wrote: A bit off-topic, but... > If you mean 'volatile', no, that doesn't do anything. Specifically, > 'volatile' has no special semantics for multi-processors. There may be > specific compilers where it has such semantics, but the standard > doesn't provide them. According to ISO 9899-1990, section 6.5.3: An object that has volatile-qualified type may be modified in ways unknown to the implementation or have other unknown side effects. Therefore any expression referring to such an object shall be evaluated strictly according to the rules of the abstract machine, as described in 5.1.2.3. Furthermore, at every sequence point the value last stored in the object shall agree with that prescribed by the abstract machine, EXCEPT AS MODIFIED BY THE UNKNOWN FACTORS MENTIONED PREVIOUSLY [emphasis added]. Translation: The compiler can't make assumptions about the state of a variable marked "volatile", and MUST generate code that writes every result stored there as well as code that reads the variable EVERY SINGLE TIME it appears in an expression. It has nothing to do with multi-processor coherency. Any compiler that generates code that deviates from this (even a little bit) isn't compliant with the standard. ______________________________________________________________________ OpenSSL Project http://www.openssl.org Development Mailing List [email protected] Automated List Manager [EMAIL PROTECTED] ______________________________________________________________________ OpenSSL Project http://www.openssl.org Development Mailing List [email protected] Automated List Manager [EMAIL PROTECTED]
