Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1301473

Re: [RFC PATCH 1/3] getcpu_cache system call: cache CPU number of running thread

Show all headers | View raw

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

[RFC PATCH 1/3] getcpu_cache system call: cache CPU number of running thread Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2016-01-05 08:10 +0100
  Re: [RFC PATCH 1/3] getcpu_cache system call: cache CPU number of  running thread Will Deacon <will.deacon@arm.com> - 2016-01-05 13:10 +0100
    Re: [RFC PATCH 1/3] getcpu_cache system call: cache CPU number of  running thread Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2016-01-05 18:40 +0100
    Re: [RFC PATCH 1/3] getcpu_cache system call: cache CPU number of  running thread Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2016-01-05 18:40 +0100
      Re: [RFC PATCH 1/3] getcpu_cache system call: cache CPU number of  running thread Russell King - ARM Linux <linux@arm.linux.org.uk> - 2016-01-05 18:50 +0100
        Re: [RFC PATCH 1/3] getcpu_cache system call: cache CPU number of  running thread Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2016-01-05 18:50 +0100
        Re: [RFC PATCH 1/3] getcpu_cache system call: cache CPU number of  running thread "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-01-05 22:50 +0100
          Re: [RFC PATCH 1/3] getcpu_cache system call: cache CPU number of  running thread Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2016-01-05 23:40 +0100
            Re: [RFC PATCH 1/3] getcpu_cache system call: cache CPU number of  running thread "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-01-06 00:00 +0100

csiph-web