Messages in this thread | | | Date | Mon, 18 Jan 2016 20:08:52 +0900 | From | Sergey Senozhatsky <> | Subject | Re: [PATCH v3] zsmalloc: fix migrate_zspage-zs_free race condition |
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On (01/18/16 17:20), Minchan Kim wrote: [..] > > > oh... good find! lost release semantic of unpin_tag()... > > > > Ah, release semantic, good point indeed. OK then we need the v2 approach again, > > with WRITE_ONCE() in record_obj(). Or some kind of record_obj_release() with > > release semantic, which would be a bit more effective, but I guess migration is > > not that critical path to be worth introducing it. > > WRITE_ONCE in record_obj would add more memory operations in obj_malloc > but I don't feel it's too heavy in this phase so, > > How about this? Junil, Could you resend patch if others agree this? > Thanks. > > +/* > + * record_obj updates handle's value to free_obj and it shouldn't > + * invalidate lock bit(ie, HANDLE_PIN_BIT) of handle, otherwise > + * it breaks synchronization using pin_tag(e,g, zs_free) so let's > + * keep the lock bit. > + */ > static void record_obj(unsigned long handle, unsigned long obj) > { > - *(unsigned long *)handle = obj; > + int locked = (*(unsigned long *)handle) & (1<<HANDLE_PIN_BIT); > + unsigned long val = obj | locked; > + > + /* > + * WRITE_ONCE could prevent store tearing like below > + * *(unsigned long *)handle = free_obj > + * *(unsigned long *)handle |= locked; > + */ > + WRITE_ONCE(*(unsigned long *)handle, val); > }
given that memory barriers are also compiler barriers, wouldn't
record_obj() { barrier *(unsigned long *)handle) = new }
suffice?
-ss
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