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

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Date:   Fri, 8 Feb 2019 09:19:44 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        linux-arch@...r.kernel.org, x86@...nel.org,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH-tip 15/22] locking/rwsem: Merge owner into count on x86-64

On 02/07/2019 03:54 PM, Waiman Long wrote:
> 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

Something like the attached patch.

Cheers,
Longman

View attachment "0023-locking-rwsem-Make-MSbit-of-count-as-guard-bit-to-fa.patch" of type "text/x-patch" (8768 bytes)