Messages in this thread | | | From | Dexuan Cui <> | Subject | RE: [PATCH 1/1] swiotlb: Track and report io_tlb_used high water mark in debugfs | Date | Wed, 22 Mar 2023 14:10:24 +0000 |
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> From: Michael Kelley (LINUX) <mikelley@microsoft.com> > Sent: Monday, February 27, 2023 8:42 AM > ... > diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c > @@ -76,6 +76,9 @@ struct io_tlb_slot { > static unsigned long default_nslabs = IO_TLB_DEFAULT_SIZE >> IO_TLB_SHIFT; > static unsigned long default_nareas; > > +static atomic_long_t total_used = ATOMIC_LONG_INIT(0); > +static atomic_long_t used_hiwater = ATOMIC_LONG_INIT(0); > ... > @@ -594,6 +597,7 @@ static int swiotlb_do_find_slots(struct device *dev, int > area_index, > unsigned long flags; > unsigned int slot_base; > unsigned int slot_index; > + unsigned long old_hiwater, new_used; > ... > @@ -663,6 +667,14 @@ static int swiotlb_do_find_slots(struct device *dev, int > area_index, > area->index = 0; > area->used += nslots; > spin_unlock_irqrestore(&area->lock, flags); > + > + new_used = atomic_long_add_return(nslots, &total_used); > + old_hiwater = atomic_long_read(&used_hiwater); > + do { > + if (new_used <= old_hiwater) > + break; > + } while (!atomic_long_try_cmpxchg(&used_hiwater, &old_hiwater, > new_used));
Here 'old_hiwater' is not updated in the loop, which looks suspicious to me. Imagine the below scenario:
Initially total_used is 0, used_hiwater is 0.
Thread A: total_used = 10; new_used = 10; old_hiwater = 0; Thread B: total_used = 20; new_used = 20; old_hiwater = 0;
Thread A enters the 'do' loop: used_hiwater is 0; old_hiwater is 0, so used_hiwater = 10, and we exit from the loop.
Thread B enters the 'do' loop: new_used is 20, old_hiwater *always* is 0, used_hiwater is 10; because used_hiwater always doesn't equal old_hiwater, atomic_long_try_cmpxchg() always returns 0, and we get stuck in the loop forever.
I think the line + old_hiwater = atomic_long_read(&used_hiwater); should be moved into the loop to resolve the issue.
> +static int io_tlb_hiwater_set(void *data, u64 val) > +{ > + /* Write of any value resets high water mark to zero */
Maybe it's better if we return -EINVAL when 'val' isn't 0 ?
> + atomic_long_set(&used_hiwater, 0); > + return 0; > +}
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