Date: Tue, 31 Mar 2020 17:21:06 +0200 From: Koen Vandeputte <koen.vandeputte@...ntric.com> To: musl@...ts.openwall.com, Rich Felker <dalias@...c.org> 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 *PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP* exists? It's the user's responsibility to avoid recursive reading here .. but at least it allows preferred writes. See description in: http://man7.org/linux/man-pages/man3/pthread_rwlockattr_setkind_np.3.html > > Do you have any suggested approaches for making this better? I'll definitely take a look at it. > > Rich
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