Messages in this thread | | | Date | Mon, 18 Jan 2016 17:20:00 +0900 | From | Minchan Kim <> | Subject | Re: [PATCH v3] zsmalloc: fix migrate_zspage-zs_free race condition |
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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 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); }
> > Thanks, > Vlastimil > > > > > -ss > > >
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