Re: [PATCHv2] efi/unaccepted: Fix soft lockups caused by parallel memory acceptance

[Vlastimil Babka]

On 10/16/23 18:31, 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>
> Reviewed-by: Nikolay Borisov <nik.borisov@xxxxxxxx>

Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>

<snip>

> +	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;
>  
> +		/*
> +		 * Keep interrupts disabled until the accept operation is
> +		 * complete in order to prevent deadlocks.
> +		 *
> +		 * Enabling interrupts before calling arch_accept_memory()
> +		 * creates an opportunity for an interrupt handler to request
> +		 * acceptance for the same memory. The handler will continuously
> +		 * spin with interrupts disabled, preventing other task from
> +		 * making progress with the acceptance process.
> +		 */

AFAIU on PREEMPT_RT the spin_lock_irqsave() doesn't disable interrupts, so
this does not leave them disabled. But it also shouldn't be a risk of
deadlock because the interrupt handlers are themselves preemptible. The
latency might be bad as the cpu_relax() retry loop will not cause the task
everyone might be waiting for to be prioritised, but I guess it's not a big
issue as anyone with RT requirements probably won't use unaccepted memory in
the first place, and as you mention hitting the retry loop after boot in a
normal configuration should be pretty much never.

> +		spin_unlock(&unaccepted_memory_lock);
> +
>  		arch_accept_memory(phys_start, phys_end);
> +
> +		spin_lock(&unaccepted_memory_lock);
>  		bitmap_clear(unaccepted->bitmap, range_start, len);
>  	}
> +
> +	list_del(&range.list);
>  	spin_unlock_irqrestore(&unaccepted_memory_lock, flags);
>  }
>