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Re: [PATCH] ipc: perform conditional locking while changing the port seq

From: Marin Ramesa
Subject: Re: [PATCH] ipc: perform conditional locking while changing the port sequence number
Date: Sat, 07 Sep 2013 10:17:29 +0200

On 07.09.2013 08:58:14, Samuel Thibault wrote:
> Marin Ramesa, le Sat 07 Sep 2013 08:00:47 +0200, a écrit :
> > * ipc/ipc_port.c (ipc_port_set_seqno) [MACH_SLOCKS]: Conditional
> > locking.
> What is the rationale?  Does it really bring an noticeable
> improvement? The locking is already conditional inside 
> ipc_port_lock_mqueue, from the simple_*lock* macros themselves.  That 
> is way more readable than having ifdefs inside the main source code.

That code in ipc_port_set_seqno() is simply not functional (except the 
change in sequence number) in the case when MACH_SLOCKS is not defined. 
We have a local variable mqueue that is set by the lock, but if you 
look at the definition of imq_unlock() and then simple_unlock() members 
of mqueue just change state and they don't have any effect on any other 
variable, plus it's a local variable - I would understand the code if 
mqueue is more global, but it's not. And in the end mqueue never get's 
really used in the ipc_port_set_seqno().

I tried to go around this by using MACH_SLOCKS and just change the 
sequence number otherwise. But if you think it's a bad idea maybe 
there's another way.

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