Messages in this thread | | | Date | Sun, 19 Nov 2006 21:47:26 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync |
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On Mon, Nov 20, 2006 at 01:28:47AM +0300, Oleg Nesterov wrote: > On 11/19, Paul E. McKenney wrote: > > > > On Mon, Nov 20, 2006 at 12:17:31AM +0300, Oleg Nesterov wrote: > > > > > > It will wait for xxx_read_unlock() on reader's side. And for this reason > > > this idx in fact is not exactly wrong :) > > > > I am not seeing this. > > > > Let's assume sp->completed starts out zero. > > > > o CPU 0 starts executing xxx_read_lock(), but is interrupted > > (or whatever) just before the atomic_inc(). Upon return, > > it will increment sp->ctr[0]. > > Right. > > > o CPU 1 executes synchronize_xxx() to completion, which it > > can because CPU 0 has not yet incremented the counter. > > It waited on sp->ctr[0], and incremented sp->completed to 1. > > > > o CPU 0 returns from interrupt and completes xxx_read_lock(), > > but has incremented sp->ctr[0]. > > > > o CPU 0 continues into its critical section, picking up a > > pointer to an xxx-protected data structure (or, in Jens's > > case starting an xxx-protected I/O). > > > > o CPU 1 executes another synchronize_xxx(). This completes > > immediately because it is waiting for sp->ctr[1] to go > > to zero, but CPU 0 incremented sp->ctr[0]. (Right?) > > Right! > > > o CPU 1 continues, either freeing a data structure while > > CPU 0 is still referencing it, or, in Jens's case, completing > > an I/O barrier while there is still outstanding I/O. > > > > Or am I missing something? > > No, it is me. > > Alan, Paul, thanks a lot for your patience!
No problem -- now we just need to work out if Jens's optimization is safe, either in his situation or in general. And I need to chase down any remaining bugs in the patch...
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