From: Will Deacon <will.deacon@arm.com>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Paul Turner <pjt@google.com>, Andrew Hunter <ahh@google.com>,
Peter Zijlstra <peterz@infradead.org>,
linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
Andy Lutomirski <luto@amacapital.net>,
Andi Kleen <andi@firstfloor.org>,
Dave Watson <davejwatson@fb.com>, Chris Lameter <cl@linux.com>,
Ingo Molnar <mingo@redhat.com>, Ben Maurer <bmaurer@fb.com>,
Steven Rostedt <rostedt@goodmis.org>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Josh Triplett <josh@joshtriplett.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Andrew Morton <akpm@linux-foundation.org>,
Russell King <linux@arm.linux.org.uk>,
Catalin Marinas <catalin.marinas@arm.com>,
Michael Kerrisk <mtk.manpages@gmail.com>
Subject: Re: [RFC PATCH 1/3] getcpu_cache system call: cache CPU number of running thread
Date: Tue, 5 Jan 2016 12:04:01 +0000 [thread overview]
Message-ID: <20160105120400.GD10705@arm.com> (raw)
In-Reply-To: <1451977320-4886-2-git-send-email-mathieu.desnoyers@efficios.com>
Hi Mathieu,
On Tue, Jan 05, 2016 at 02:01:58AM -0500, Mathieu Desnoyers wrote:
> Expose a new system call allowing threads to register userspace memory
> areas where to store the CPU number on which the calling thread is
> running. Scheduler migration sets the TIF_NOTIFY_RESUME flag on the
> current thread. Upon return to user-space, a notify-resume handler
> updates the current CPU value within each registered user-space memory
> area. User-space can then read the current CPU number directly from
> memory.
What guarantees do you provide if a thread other than the one which
registered the cache tries to access the value? Obviously, there's a
potential data race here with the kernel issuing a parallel update, but
are you intending to have single-copy atomicity semantics (like relaxed
atomics in C11) or is this simply going to give you junk?
I ask because, in the absence of alignment checks on the cache pointer,
we can't guarantee single-copy atomicity on ARM when the kernel writes
the current CPU value.
Cheers,
Will
next prev parent reply other threads:[~2016-01-05 12:04 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-05 7:01 [RFC PATCH 0/3] Implement getcpu_cache system call Mathieu Desnoyers
2016-01-05 7:01 ` [RFC PATCH 1/3] getcpu_cache system call: cache CPU number of running thread Mathieu Desnoyers
2016-01-05 12:04 ` Will Deacon [this message]
2016-01-05 17:31 ` Mathieu Desnoyers
2016-01-05 17:34 ` Mathieu Desnoyers
2016-01-05 17:40 ` Russell King - ARM Linux
2016-01-05 17:49 ` Mathieu Desnoyers
2016-01-05 21:47 ` Paul E. McKenney
2016-01-05 22:34 ` Mathieu Desnoyers
2016-01-05 22:54 ` Paul E. McKenney
2016-01-05 7:01 ` [RFC PATCH 2/3] getcpu_cache: wire up ARM system call Mathieu Desnoyers
2016-01-05 7:02 ` [RFC PATCH 3/3] getcpu_cache: wire up x86 32/64 " Mathieu Desnoyers
2016-01-11 22:38 ` [RFC PATCH 0/3] Implement getcpu_cache " Seymour, Shane M
2016-01-11 23:03 ` Josh Triplett
2016-01-12 0:49 ` Mathieu Desnoyers
2016-01-12 2:45 ` Josh Triplett
2016-01-12 4:27 ` Ben Maurer
2016-01-12 6:40 ` Seymour, Shane M
2016-01-12 13:15 ` Mathieu Desnoyers
2016-01-12 21:02 ` Ben Maurer
2016-01-13 0:22 ` Mathieu Desnoyers
2016-01-13 0:51 ` Josh Triplett
2016-01-14 15:58 ` Mathieu Desnoyers
2016-01-11 23:16 ` Seymour, Shane M
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