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Date: Tue, 31 Mar 2020 17:21:06 +0200
From: Koen Vandeputte <>
To:, Rich Felker <>
Subject: Re: Simple question regarding read-write locks precedence

On 31.03.20 17:09, Rich Felker wrote:
> On Tue, Mar 31, 2020 at 05:05:27PM +0200, Koen Vandeputte wrote:
>> Hi All,
>> I've written a user app which make use of reader-writer locks.
>> Topology is pretty simple:
>> - 1 writer
>> - 4 readers
>> Writes only occur once in a while.
>> Readers are heavy users of the lock.
>> The default behavior in musl is Reader precedence.
>> In my usecase, it means that a writer never aquires the lock causing
>> writer starvation.
>> Debugging nicely shows that readers also "jump over" the waiting
>> writer as there is always at least 1 reader present in the critical
>> section at any time.
>> Going through the source code shows that there is no support for
>> specifying lock attributes which give writers precedence over
>> readers.
>> Is there an update scheduled to add the required attribute types
>> which allow writer precedence to avoid starvation?
> The POSIX model of allowing recursive read locks fundamentally doesn't
> admit writer preference -- there's no way to distinguish the case of
> new reader vs an additional recursive lock by an existing reader
> without O(n) space. If you disallow the latter (recursive locks while
> a writer is waiting) you get deadlocks all over the place in intended
> usage model.

Hi Rich,

Thanks for the very fast reply.

I've red about the trivial deadlocks, but isn't this the reason why 

It's the user's responsibility to avoid recursive reading here .. but at 
least it allows preferred writes.

See description in:

> Do you have any suggested approaches for making this better?
I'll definitely take a look at it.
> Rich

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