linux-kernel - Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier

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Message-ID: <15142.1262862297@redhat.com>
Date:	Thu, 07 Jan 2010 11:04:57 +0000
From:	David Howells <dhowells@...hat.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	dhowells@...hat.com, linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>, akpm@...ux-foundation.org,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, laijs@...fujitsu.com,
	dipankar@...ibm.com
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier

Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

> The current implementation simply executes a memory barrier in an IPI
> handler on each active cpu. Going through the hassle of taking run queue
> locks and checking if the thread running on each online CPU belongs to
> the current thread seems more heavyweight than the cost of the IPI
> itself (not measured though).

There's another way to do this:

 (1) For each threads you want to execute a memory barrier, mark in its
     task_struct that you want it to do a memory barrier and set
     TIF_NOTIFY_RESUME.

 (2) Interrupt all CPUs.  The interrupt handler doesn't have to do anything.

 (3) When any of the threads marked in (1) gain CPU time, do_notify_resume()
     will be executed, and the do-memory-barrier flag can be tested and if it
     was set, the flag can be cleared and a memory barrier can be
     interpolated.

The current thread will also pass through stage (3) on its way out, if it's
marked in stage (1).

David
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