On Fri, Feb 28, 2003 at 12:29:54AM +0100, Rok Papež wrote: > Hello! > > Dne četrtek 27. februar 2003 20:47 je Frank A. Uepping napisal(a): > > > Is that true that a spinlock gets undefined when compiled > > on a uniprocessor system? > > No. YES, IT DOES. Except _irqsave/_irqrestore variants (they reduce to cli/sti). And for 2.4, that is. With preempt patched (IIRC they are included in late 2.5) they become magic. > > Thus spinlocks can't be used to provide mutual exclusion between > > concurrently executing processes on a uniprocessor system, isn't is? > > No. There is a limitation. You must not, under penalty of complete machine freeze, call schedule() with spinlock locked. You can never be interrupted except by interrupt unless you call schedule() [except with preept patches. Think of a preempt kernel as always running SMP]. You however must always write code for SMP and on UP spinlocks reduce so that they guarantee the same as on SMP taking into account that running on UP guarantees some things itself. > I use them on UP as mutexes.. heavily. You use spinlock if structures you are > protecting are being accessed from interrupt/timer/softirq context. Only _irqsave/_irqrestore variants! Unsing the normal variant to lock against interrupt is a bug under penalty of complete machine freeze. > Mutexes are to be used when structures are accessed *only* from process > context. Yes, semaphores can only be manipulated in process context. ------------------------------------------------------------------------------- Jan 'Bulb' Hudec <bulb@ucw.cz> -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
