Re: [PATCH] efi/unaccepted: Fix soft lockups caused by parallel memory acceptance

[Kirill A. Shutemov]

On Sun, Oct 15, 2023 at 08:02:16PM +0300, Nikolay Borisov wrote:
> 
> 
> On 14.10.23 г. 23:40 ч., Kirill A. Shutemov wrote:
> > Michael reported soft lockups on a system that has unaccepted memory.
> > This occurs when a user attempts to allocate and accept memory on
> > multiple CPUs simultaneously.
> > 
> > The root cause of the issue is that memory acceptance is serialized with
> > a spinlock, allowing only one CPU to accept memory at a time. The other
> > CPUs spin and wait for their turn, leading to starvation and soft lockup
> > reports.
> > 
> > To address this, the code has been modified to release the spinlock
> > while accepting memory. This allows for parallel memory acceptance on
> > multiple CPUs.
> > 
> > A newly introduced "accepting_list" keeps track of which memory is
> > currently being accepted. This is necessary to prevent parallel
> > acceptance of the same memory block. If a collision occurs, the lock is
> > released and the process is retried.
> > 
> > Such collisions should rarely occur. The main path for memory acceptance
> > is the page allocator, which accepts memory in MAX_ORDER chunks. As long
> > as MAX_ORDER is equal to or larger than the unit_size, collisions will
> > never occur because the caller fully owns the memory block being
> > accepted.
> > 
> > Aside from the page allocator, only memblock and deferered_free_range()
> > accept memory, but this only happens during boot.
> > 
> > The code has been tested with unit_size == 128MiB to trigger collisions
> > and validate the retry codepath.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Reported-by: Michael Roth <michael.roth@xxxxxxx
> > Fixes: 2053bc57f367 ("efi: Add unaccepted memory support")
> > Cc: <stable@xxxxxxxxxx>
> > ---
> >   drivers/firmware/efi/unaccepted_memory.c | 55 ++++++++++++++++++++++--
> >   1 file changed, 51 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/firmware/efi/unaccepted_memory.c b/drivers/firmware/efi/unaccepted_memory.c
> > index 853f7dc3c21d..8af0306c8e5c 100644
> > --- a/drivers/firmware/efi/unaccepted_memory.c
> > +++ b/drivers/firmware/efi/unaccepted_memory.c
> > @@ -5,9 +5,17 @@
> >   #include <linux/spinlock.h>
> >   #include <asm/unaccepted_memory.h>
> > -/* Protects unaccepted memory bitmap */
> > +/* Protects unaccepted memory bitmap and accepting_list */
> >   static DEFINE_SPINLOCK(unaccepted_memory_lock);
> > +struct accept_range {
> > +	struct list_head list;
> > +	unsigned long start;
> > +	unsigned long end;
> > +};
> > +
> > +static LIST_HEAD(accepting_list);
> > +
> >   /*
> >    * accept_memory() -- Consult bitmap and accept the memory if needed.
> >    *
> > @@ -24,6 +32,7 @@ void accept_memory(phys_addr_t start, phys_addr_t end)
> >   {
> >   	struct efi_unaccepted_memory *unaccepted;
> >   	unsigned long range_start, range_end;
> > +	struct accept_range range, *entry;
> >   	unsigned long flags;
> >   	u64 unit_size;
> > @@ -78,20 +87,58 @@ void accept_memory(phys_addr_t start, phys_addr_t end)
> >   	if (end > unaccepted->size * unit_size * BITS_PER_BYTE)
> >   		end = unaccepted->size * unit_size * BITS_PER_BYTE;
> > -	range_start = start / unit_size;
> > -
> > +	range.start = start / unit_size;
> > +	range.end = DIV_ROUND_UP(end, unit_size);
> > +retry:
> >   	spin_lock_irqsave(&unaccepted_memory_lock, flags);
> > +
> > +	/*
> > +	 * Check if anybody works on accepting the same range of the memory.
> > +	 *
> > +	 * The check with unit_size granularity. It is crucial to catch all
> > +	 * accept requests to the same unit_size block, even if they don't
> > +	 * overlap on physical address level.
> > +	 */
> > +	list_for_each_entry(entry, &accepting_list, list) {
> > +		if (entry->end < range.start)
> > +			continue;
> > +		if (entry->start >= range.end)
> > +			continue;
> > +
> > +		/*
> > +		 * Somebody else accepting the range. Or at least part of it.
> > +		 *
> > +		 * Drop the lock and retry until it is complete.
> > +		 */
> > +		spin_unlock_irqrestore(&unaccepted_memory_lock, flags);
> > +		cond_resched();
> > +		goto retry;
> > +	}
> 
> So this works for the cases where we have concurrent acceptance of the same
> range. What about the same range being accepted multiple times, one after
> the other, the current code doesn't prevent this.

That's why we have the bitmap. The bits got cleared there after the first
accept. On the second, attempt for_each_set_bitrange_from() will skip the
range.

> What if you check whether the current range is fully contained within the
> duplicate entry and if it's fully covered simply return ?

If it is fully covered we still need to wait until somebody else finish
the accept, so we cannot "just return".

We can try to return if we saw the range on accepting_list list before,
but it is disappeared, indicating that accept has been completed.

But I don't think this optimization worthwhile. As I mentioned before, the
collision is hardly happens. One more spin and bitmap check would not make
a difference. And it adds complexity.


> 
> > +
> > +	/*
> > +	 * Register that the range is about to be accepted.
> > +	 * Make sure nobody else will accept it.
> > +	 */
> > +	list_add(&range.list, &accepting_list);
> > +
> > +	range_start = range.start;
> >   	for_each_set_bitrange_from(range_start, range_end, unaccepted->bitmap,
> > -				   DIV_ROUND_UP(end, unit_size)) {
> > +				   range.end) {
> >   		unsigned long phys_start, phys_end;
> >   		unsigned long len = range_end - range_start;
> >   		phys_start = range_start * unit_size + unaccepted->phys_base;
> >   		phys_end = range_end * unit_size + unaccepted->phys_base;
> > +		spin_unlock_irqrestore(&unaccepted_memory_lock, flags);
> > +
> >   		arch_accept_memory(phys_start, phys_end);
> > +
> > +		spin_lock_irqsave(&unaccepted_memory_lock, flags);
> >   		bitmap_clear(unaccepted->bitmap, range_start, len);
> >   	}
> > +
> > +	list_del(&range.list);
> >   	spin_unlock_irqrestore(&unaccepted_memory_lock, flags);
> >   }

-- 
  Kiryl Shutsemau / Kirill A. Shutemov