Re: [PATCH] fix race in AF_UNIX

[David Miller]

From: Miklos Szeredi <miklos@xxxxxxxxxx>
Date: Wed, 06 Jun 2007 07:26:52 +0200

> > Holding a global mutex over recvmsg() calls under AF_UNIX is pretty
> > much a non-starter, this will kill performance for multi-threaded
> > apps.
> 
> That's an rwsem held for read.  It's held for write in unix_gc() only
> for a short duration, and unix_gc() should only rarely be called.  So
> I don't think there's any performance problem here.

It pulls a non-local cacheline into the local thread, that's extremely
expensive on SMP.

If everyone starts grabbing this thing during recvmsg() it's going to
become a really hot lock and kill performance, even if it's a read
side lock being taken.

That's why I said we need to investigate solutions involving
u->readlock, that already has to be taken and is local to the socket.
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