On Sun, Dec 12, 2010 at 10:40 AM, Venkatesh Srinivas < vsrini...@dragonflybsd.org> wrote:
> Hi, > > In the i386 pmap's pmap_zero_page(), there is a fragment... > > sysmaps = &sysmaps_pcpu[PCPU_GET(cpuid)]; > mtx_lock(&sysmaps->lock); > * sched_pin(); > /*map the page we mean to zero at sysmaps->CADDR2*/ > pagezero(sysmaps->CADDR2); > sched_unpin(); > > I don't know this bit of code too well, so I don't know if the sched_pin() > being where it is is okay or not. My first reading says its not okay; if a > thread is moved to another CPU before it is able to pin, it will use the > wrong sysmaps structure. Is this the case? Is it alright that the wrong > sysmap structure is used? > > Oh, Nathaniel Filardo (n...@cs.jhu.edu) first spotted this, not I. > > This isn't a bug. There is nothing about the code that mandates that processor i must always use sysmap entry i. In the unlikely event that the thread migrates from processor X to processor Y before the sched_pin(), the mutex on sysmap entry X will prevent it from being used by processor X until processor Y is done with it. So, it doesn't matter to correctness that the "wrong" sysmap entry is used, and it is extremely unlikely to matter to performance. Alan _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"
