Messages in this thread | | | Subject | frlock and barrier discussion | From | Stephen Hemminger <> | Date | 29 Jan 2003 17:15:55 -0800 |
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On Tue, 2003-01-28 at 23:06, Richard Henderson wrote: > On Tue, Jan 28, 2003 at 03:42:21PM -0800, Stephen Hemminger wrote: > > +static inline void fr_write_begin(frlock_t *rw) > > +{ > > + preempt_disable(); > > + rw->pre_sequence++; > > + wmb(); > > +} > > + > > +static inline void fr_write_end(frlock_t *rw) > > +{ > > + wmb(); > > + rw->post_sequence++; > > These need to be mb(), not wmb(), if you want the bits in between > to actually happen in between, as with your xtime example. At > present there's nothing stoping xtime from being *read* before > your read from pre_sequence happens.
First, write_begin/end can only be safely used when there is separate writer synchronization such as a spin_lock or semaphore. As far as I know, semaphore or spin_lock guarantees a barrier. So xtime or anything else can not be read before the spin_lock.
Using mb() is more paranoid than necessary.
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