Re: [PATCH-tip 15/22] locking/rwsem: Merge owner into count on x86-64

[Waiman Long <longman@redhat.com>]

On 02/07/2019 03:08 PM, Peter Zijlstra wrote:
> On Thu, Feb 07, 2019 at 02:07:19PM -0500, Waiman Long wrote:
>> On 32-bit architectures, there aren't enough bits to hold both.
>> 64-bit architectures, however, can have enough bits to do that. For
>> x86-64, the physical address can use up to 52 bits. That is 4PB of
>> memory. That leaves 12 bits available for other use. The task structure
>> pointer is also aligned to the L1 cache size. That means another 6 bits
>> (64 bytes cacheline) will be available. Reserving 2 bits for status
>> flags, we will have 16 bits for the reader count.  That can supports
>> up to (64k-1) readers.
> 64k readers sounds like a number that is fairly 'easy' to reach, esp. on
> 64bit. These are preemptible locks after all, all we need to do is get
> 64k tasks nested on enough CPUs.
>
> I'm sure there's some willing Java proglet around that spawns more than
> 64k threads just because it can. Run it on a big enough machine (ISTR
> there's a number of >1k CPU systems out there) and voila.

Yes, that can be a problem.

One possible solution is to check if the count goes negative. If so,
fail the read lock and make the readers wait in the wait queue until the
count is in positive territory. That effectively reduces the reader
count to 15 bits, but it will avoid the overflow situation. I will try
to add that support into the next version.

Cheers,
Longman