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    powerpc, membarrier: Skip memory barrier in switch_mm() · 3ccfebed
    Mathieu Desnoyers 提交于
    Allow PowerPC to skip the full memory barrier in switch_mm(), and
    only issue the barrier when scheduling into a task belonging to a
    process that has registered to use expedited private.
    
    Threads targeting the same VM but which belong to different thread
    groups is a tricky case. It has a few consequences:
    
    It turns out that we cannot rely on get_nr_threads(p) to count the
    number of threads using a VM. We can use
    (atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1)
    instead to skip the synchronize_sched() for cases where the VM only has
    a single user, and that user only has a single thread.
    
    It also turns out that we cannot use for_each_thread() to set
    thread flags in all threads using a VM, as it only iterates on the
    thread group.
    
    Therefore, test the membarrier state variable directly rather than
    relying on thread flags. This means
    membarrier_register_private_expedited() needs to set the
    MEMBARRIER_STATE_PRIVATE_EXPEDITED flag, issue synchronize_sched(), and
    only then set MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY which allows
    private expedited membarrier commands to succeed.
    membarrier_arch_switch_mm() now tests for the
    MEMBARRIER_STATE_PRIVATE_EXPEDITED flag.
    Signed-off-by: NMathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Acked-by: NThomas Gleixner <tglx@linutronix.de>
    Acked-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Alan Stern <stern@rowland.harvard.edu>
    Cc: Alexander Viro <viro@zeniv.linux.org.uk>
    Cc: Andrea Parri <parri.andrea@gmail.com>
    Cc: Andrew Hunter <ahh@google.com>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Avi Kivity <avi@scylladb.com>
    Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    Cc: Boqun Feng <boqun.feng@gmail.com>
    Cc: Dave Watson <davejwatson@fb.com>
    Cc: David Sehr <sehr@google.com>
    Cc: Greg Hackmann <ghackmann@google.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Maged Michael <maged.michael@gmail.com>
    Cc: Michael Ellerman <mpe@ellerman.id.au>
    Cc: Nicholas Piggin <npiggin@gmail.com>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Paul Mackerras <paulus@samba.org>
    Cc: Russell King <linux@armlinux.org.uk>
    Cc: Will Deacon <will.deacon@arm.com>
    Cc: linux-api@vger.kernel.org
    Cc: linux-arch@vger.kernel.org
    Cc: linuxppc-dev@lists.ozlabs.org
    Link: http://lkml.kernel.org/r/20180129202020.8515-3-mathieu.desnoyers@efficios.comSigned-off-by: NIngo Molnar <mingo@kernel.org>
    3ccfebed
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