On Wed, Mar 28, 2007 at 01:11:45AM +0400, Oleg Nesterov wrote: > On 03/27, Venki Pallipadi wrote: > > > > for (;;) { > > - base = timer->base; > > + tvec_base_t *prelock_base = timer->base; > > + base = timer_get_base(timer); > > if (likely(base != NULL)) { > > spin_lock_irqsave(&base->lock, *flags); > > - if (likely(base == timer->base)) > > + if (likely(prelock_base == timer->base)) > > return base; > > I don't think this is correct, at least in theory. > > Suppose that > > tvec_base_t *prelock_base = timer->base; > base = timer_get_base(timer); > > are re-ordered (the second LOAD happens after the first one), and the timer > changes its base in between. Now, we lock the old base, and return it because > "prelock_base == timer->base" == true. >
Great catch. Yes. this is a theoritical possibility, even though most compilers would load base only once and use it for prelock_base and 'and' it for base. Atleast that is what I see on i386/gcc. Incremental patch below eliminates this race. Index: new/kernel/timer.c =================================================================== --- new.orig/kernel/timer.c 2007-03-26 15:19:35.000000000 -0800 +++ new/kernel/timer.c 2007-03-27 13:00:33.000000000 -0800 @@ -96,9 +96,9 @@ return tbase_get_deferrable(timer->base); } -static inline struct tvec_t_base_s *timer_get_base(struct timer_list *timer) +static inline struct tvec_t_base_s *tbase_get_base(struct tvec_t_base_s *base) { - return ((struct tvec_t_base_s *)((unsigned long)(timer->base) & + return ((struct tvec_t_base_s *)((unsigned long)base & ~TBASE_DEFERRABLE_FLAG)); } @@ -368,7 +368,7 @@ for (;;) { tvec_base_t *prelock_base = timer->base; - base = timer_get_base(timer); + base = tbase_get_base(prelock_base); if (likely(base != NULL)) { spin_lock_irqsave(&base->lock, *flags); if (likely(prelock_base == timer->base)) @@ -592,7 +592,7 @@ * don't have to detach them individually. */ list_for_each_entry_safe(timer, tmp, &tv_list, entry) { - BUG_ON(timer_get_base(timer) != base); + BUG_ON(tbase_get_base(timer->base) != base); internal_add_timer(base, timer); } - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/