Bodo Moeller wrote, > > Second you need to deal with MP cache coherency (Alpha, Itanium, > > others?). Memory models can be so weak that reads and writes to the > > same address can be reordered, and a write from a thread on one CPU > > isn't guaranteed to be immediately visible to a subsequent read > > from a thread on another. > > What do the C standards guarantee if we use 'volatile static'?
They don't say anything about semantics in multi-threaded/processor environments. > > Putting those two together, I think we end up with something like, > > [...] (nb. adapted from [1], so don't shoot me if I haven't got it > > exactly right) > > Your proposed code can't work as intended (your 'tmp' flag is a local > copy of 'init', so the second check is redundant). Hmm ... Well, looking at Doug Schmidts code (from the book) this is what he does too. OTOH, the version here, http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html#explicitMemoryBarriers (which is more recent, so probably more reliable) uses volatile differently and has an extra read before the second check, which would give us something like this, SSL_METHOD *SSLv3_client_method(void) { static volatile int init=1; /* previously not volatile */ static SSL_METHOD SSLv3_client_data; int tmp = init; /* previously volatile */ #if defined (ALPHA_MP) // CPU-specific memory barrier instruction asm("mb"); #endif if (tmp) { CRYPTO_w_lock(CRYPTO_LOCK_SSL_METHOD); tmp = init; /* added */ if (tmp) { memcpy((char *)&SSLv3_client_data,(char *)sslv3_base_method(), sizeof(SSL_METHOD)); SSLv3_client_data.ssl_connect=ssl3_connect; SSLv3_client_data.get_ssl_method=ssl3_get_client_method; #if defined (ALPHA_MP) // CPU-specific memory barrier instruction asm("mb"); #endif init=0; } CRYPTO_w_unlock(CRYPTO_LOCK_SSL_METHOD); } return(&SSLv3_client_data); } TBH, this is subtle, counter-intuitive, and hard to understand. Which is why I'd recommend avoiding this kind of lazy initialization if at all possible. Cheers, Miles ______________________________________________________________________ OpenSSL Project http://www.openssl.org Development Mailing List [EMAIL PROTECTED] Automated List Manager [EMAIL PROTECTED]
