On Mon, Dec 8, 2008 at 9:17 PM, John Baldwin <[EMAIL PROTECTED]> wrote: > On Sunday 07 December 2008 02:00:30 pm Marius Nünnerich wrote: >> See subject. >> Interesting commit: >> http://svn.freebsd.org/viewvc/base?view=revision&revision=77059 > > Lost wakeups. If you have code like so that doesn't use any locks: > > int flag; > > void > foo(void) > { > > flag = 1; > wakeup(&flag); > } > > void > bar(void) > { > > if (flag == 0) > tsleep(&foo, ..., 0); > } > > Then one CPU may run the 'foo' routine to completion after another CPU has > seen 'flag == 0' but before it has put the thread to sleep in tsleep(). Even > on UP systems with preemption you can still get this race if you get > preempted by an interrupt (which runs foo()) in between the 'flag == 0' test > and calling tsleep(). Using an interlock avoid this: > > struct mtx lock; > int flag; > > void > foo(void) > { > > mtx_lock(&lock); > flag = 1; > mtx_unlock(&lock); > wakeup(&flag); > } > > void > bar(void) > { > > mtx_lock(&lock); > if (flag == 0) > mtx_sleep(&foo, &lock, ..., 0); > mtx_unlock(&lock); > } > > In this case 'lock' closes the SMP/preemption races. > > -- > John Baldwin >
Thank you for the explanation, John! _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "[EMAIL PROTECTED]"
