Messages in this thread | | | Date | Sat, 22 Feb 2014 13:48:02 -0500 | From | Peter Hurley <> | Subject | Re: [PATCH 4/9] firewire: don't use PREPARE_DELAYED_WORK |
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On 02/22/2014 01:43 PM, James Bottomley wrote: > > On Fri, 2014-02-21 at 18:01 -0500, Peter Hurley wrote: >> On 02/21/2014 11:57 AM, Tejun Heo wrote: >>> Yo, >>> >>> On Fri, Feb 21, 2014 at 11:53:46AM -0500, Peter Hurley wrote: >>>> Ok, I can do that. But AFAIK it'll have to be an smp_rmb(); there is >>>> no mb__after unlock. >>> >>> We do have smp_mb__after_unlock_lock(). >>> >>>> [ After thinking about it some, I don't think preventing speculative >>>> writes before clearing PENDING if useful or necessary, so that's >>>> why I'm suggesting only the rmb. ] >>> >>> But smp_mb__after_unlock_lock() would be cheaper on most popular >>> archs, I think. >> >> smp_mb__after_unlock_lock() is only for ordering memory operations >> between two spin-locked sections on either the same lock or by >> the same task/cpu. Like: >> >> i = 1 >> spin_unlock(lock1) >> spin_lock(lock2) >> smp_mb__after_unlock_lock() >> j = 1 >> >> This guarantees that the store to j happens after the store to i. >> Without it, a cpu can >> >> spin_lock(lock2) >> j = 1 >> i = 1 >> spin_unlock(lock1) > > No the CPU cannot. If the CPU were allowed to reorder locking > sequences, we'd get speculation induced ABBA deadlocks. The rules are > quite simple: loads and stores cannot speculate out of critical > sections.
If you look carefully, you'll notice that the stores have not been moved from their respective critical sections; simply that the two critical sections overlap because they use different locks.
Regards, Peter Hurley
PS - Your reply address is unroutable.
> All architectures have barriers in place to prevent this ... > I know from personal experience because the barriers on PARISC were > originally too weak and we did get some speculation out of the critical > sections, which was very nasty to debug. > > Stuff may speculate into critical sections from non-critical but never > out of them and critical section boundaries may not reorder to cause an > overlap.
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