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From: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
To: Stephen Hemminger <stephen@networkplumber.org>,
	"dev@dpdk.org" <dev@dpdk.org>
Cc: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>,
	nd <nd@arm.com>, nd <nd@arm.com>
Subject: Re: [dpdk-dev] Reader-Writer lock starvation issues
Date: Fri, 8 Jan 2021 21:27:34 +0000	[thread overview]
Message-ID: <DBAPR08MB5814A214420A76ED2DE0F65A98AE0@DBAPR08MB5814.eurprd08.prod.outlook.com> (raw)
In-Reply-To: <20210108111358.3b88d51c@hermes.local>


> The current version of rte_rwlock doesn't do what it says in the
> documentation.
> " The lock is used to protect data that allows multiple readers in parallel,  but
> only one writer. All readers are blocked until the writer is finished  writing."
> The problem is that the current implementation does not stop a a new reader
> from acquiring the lock while a writer is waiting.
Agree, essentially the arbitration is left to the hardware.

> Writer:
>       repeat until x = __atomic_load(&counter) == 0;
>       __atomic_compare_exchange(&counter, &x, -1);
> Reader:
>       x = __atomic_load(&counter);
>       __atomic_compare_exchange(&counter, &x, x + 1);
> Fixing it likely would require an ABI change to add additional state.
> For more background on reader-writer locks see:
> https://www.cs.rochester.edu/research/synchronization/pseudocode/rw.htm
> l
> The code in DPDK is actually effectively the same as the first example
> "Simple, non-scalable reader-preference lock"
I do not think the DPDK implementation has reader-preference. There is no code to control the arbitration between writers and readers. It is possible that if there are multiple writers the readers might be starved depending on how the hardware does the arbitration.

> It looks like doing the right thing will require increasing the size of the
> rte_rwlock structure and cause an ABI breakage.
> I am running with an alternative which uses ticket locks to do:
>   "Simple, non-scalable writer-preference lock"
Does it provide good scalability?

> My recommendation would be:
>  1. Fix documentation in rte_rwlock.h (and add release note) and put this in
> 20.02 and LTS.
Agree, the document is not clear on the arbitration.

>  2. Add new rte_ticket_rwlock.h which provides the correct semantics.

> Comments?

  reply	other threads:[~2021-01-08 21:27 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-08 19:13 Stephen Hemminger
2021-01-08 21:27 ` Honnappa Nagarahalli [this message]
2021-01-11 11:52   ` Ferruh Yigit
2021-01-11 13:05     ` Honnappa Nagarahalli
2021-01-12  1:04 ` [dpdk-dev] [PATCH] eal/rwlock: add note about writer starvation Stephen Hemminger
2021-01-14 16:55   ` [dpdk-dev] [PATCH v2] " Stephen Hemminger
2021-02-11 22:51     ` Thomas Monjalon
2021-02-12  0:21       ` Honnappa Nagarahalli
2021-05-12 19:10         ` Thomas Monjalon
2021-11-08 10:18           ` Thomas Monjalon

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