On Wed, 8 Oct 2003, Pawel Jakub Dawidek wrote: PJD>On Wed, Oct 08, 2003 at 11:51:06AM +0200, Harti Brandt wrote: PJD>+> You need to lock when reading if you insist on consistent data. Even a PJD>+> simple read may be non-atomic (this should be the case for 64bit PJD>+> operations on all our platforms). So you need to do PJD>+> PJD>+> mtx_lock(&foo_mtx); PJD>+> bar = foo; PJD>+> mtx_unlock(&foo_mtx); PJD>+> PJD>+> if foo is a datatype that is not guaranteed to be red atomically. For PJD>+> 8-bit data you should be safe without the lock on any architecture. I'm PJD>+> not sure for 16 and 32 bit, but for 64-bit you need the look for all PJD>+> our architectures, I think. PJD> PJD>But I'm not talking about non-atomic reads. What I'm want to show is that PJD>even atomic read (without lock) is dangerous in some cases. PJD> PJD>+> If you don't care about occasionally reading false data (for statistics or PJD>+> such stuff) you can go without the lock. PJD> PJD>I'm afraid that many developers thinks that atomic reads are always safe PJD>without locks (there are many such reads in sources). I hope I'm wrong.
Well, I see your point. If the writer does a non-atomic write by doing: foo = data; foo &= mask; then nothing helps. If he would do foo = data & mask; on an atomic object things may work (well, one has to read the C-standard to find out wether the compiler is allowed to convert the 2nd form to the first one.). harti -- harti brandt, http://www.fokus.fraunhofer.de/research/cc/cats/employees/hartmut.brandt/private [EMAIL PROTECTED], [EMAIL PROTECTED] _______________________________________________ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "[EMAIL PROTECTED]"
