Messages in this thread | | | Subject | Re: [PATCH v3] zsmalloc: fix migrate_zspage-zs_free race condition | From | Vlastimil Babka <> | Date | Mon, 18 Jan 2016 13:18:31 +0100 |
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On 01/18/2016 09:20 AM, Minchan Kim wrote: > On Mon, Jan 18, 2016 at 08:54:07AM +0100, Vlastimil Babka wrote: >> On 18.1.2016 8:39, Sergey Senozhatsky wrote: >>> On (01/18/16 16:11), Minchan Kim wrote: >>> [..] >>>>> so, even if clear_bit_unlock/test_and_set_bit_lock do smp_mb or >>>>> barrier(), there is no corresponding barrier from record_obj()->WRITE_ONCE(). >>>>> so I don't think WRITE_ONCE() will help the compiler, or am I missing >>>>> something? >>>> >>>> We need two things >>>> 2. memory barrier. >>>> >>>> As compiler barrier, WRITE_ONCE works to prevent store tearing here >>>> by compiler. >>>> However, if we omit unpin_tag here, we lose memory barrier(e,g, smp_mb) >>>> so another CPU could see stale data caused CPU memory reordering. >>> >>> 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
A simple WRITE_ONCE would just add a compiler barrier. What you suggests below does indeed add more operations, which are actually needed just in the migration. What I suggested is the v2 approach of adding the PIN bit before calling record_obj, *and* simply doing a WRITE_ONCE in record_obj() to make sure the PIN bit is indeed applied *before* writing to the handle, and not as two separate writes.
> but I don't feel it's too heavy in this phase so,
I'm afraid it's dangerous for the usage of record_obj() in zs_malloc() where the handle is freshly allocated by alloc_handle(). Are we sure the bit is not set?
The code in alloc_handle() is: return (unsigned long)kmem_cache_alloc(pool->handle_cachep, pool->flags & ~__GFP_HIGHMEM);
There's no explicit __GFP_ZERO, so the handles are not guaranteed to be allocated empty? And expecting all zpool users to include __GFP_ZERO in flags would be too subtle and error prone.
> 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); > } > > > >> >> Thanks, >> Vlastimil >> >>> >>> -ss >>> >>
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