Another commentary on this by Linus (way back in 1996): http://www.uwsg.iu.edu/hypermail/linux/kernel/9605/0214.html Ryan David Dufresne wrote: > > on 11/12/02 7:03 AM, Jan Hudec at bulb@ucw.cz wrote: > > > > > There are two things to the barrier. One is to make sure CPU does not > > prefetch the old value and another is to make sure, that compiler does > > not reorder the statements! > > > The first one is a hardware memory barrier: > #define mb() __asm__ __volatile__ ("lock; addl $0,0(%esp)": : :"memory") > > The second one a software memory barrier: > #define barrier() __asm__ __volatile__ ("": : :"memory") > > Hardware memory barrier force strict CPU ordering and is required when > you're talking to devices. See: > - http://www.xml.com/ldd/chapter/book/ch08.html > - Include/asm-i386/system.h > > > It has the first string empty, so it expands to no code at all. > > But because of the memory clobber flag, gcc is forbiden to optimize > > across this point. Otherwise gcc could for example conclude that "I have > > A in register, I am not short of registers and I will need it few > > statements later. There is no need to store it in memory." ... and > > actually not store it. > > > > The mb() thing adds a "barrier" instruction to actualy stop CPU prefetch > > across the point. No such instruction is needed on IA32, but it's needed > > on some other arch with agressive prefetch (eg. sparc IIRC). > > > > (I am not absolutely sure, look it up in the code). > > > The hardware memory barrier is needed on IA32 only to prevent reads to pass > buffered writes. So the hardware memory barrier use a locking instruction > such as the LOCK prefix to insure that all buffered writes are drain to > memory before the reads are done. See: > - Intel Architecture Software Developer¹s Manual Volume 3: System > Programming > - Include/asm-i386/system.h > > -- > David Dufresne, Sys. admin. <dufresne@mediafusion.com> > Médiafusion Inc. <http://www.mediafusion.com> > 700, rue Wellington Bureau 1200 > Montreal (Quebec) H3C 1T4 > Tel.(514) 599-5721 Fax.(514) 599-5724 > > -- > Kernelnewbies: Help each other learn about the Linux kernel. > Archive: http://mail.nl.linux.org/kernelnewbies/ > FAQ: http://kernelnewbies.org/faq/ -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
