• Paolo Bonzini's avatar
    srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context · cdf7abc4
    Paolo Bonzini authored
    Linu Cherian reported a WARN in cleanup_srcu_struct() when shutting
    down a guest running iperf on a VFIO assigned device.  This happens
    because irqfd_wakeup() calls srcu_read_lock(&kvm->irq_srcu) in interrupt
    context, while a worker thread does the same inside kvm_set_irq().  If the
    interrupt happens while the worker thread is executing __srcu_read_lock(),
    updates to the Classic SRCU ->lock_count[] field or the Tree SRCU
    ->srcu_lock_count[] field can be lost.
    
    The docs say you are not supposed to call srcu_read_lock() and
    srcu_read_unlock() from irq context, but KVM interrupt injection happens
    from (host) interrupt context and it would be nice if SRCU supported the
    use case.  KVM is using SRCU here not really for the "sleepable" part,
    but rather due to its IPI-free fast detection of grace periods.  It is
    therefore not desirable to switch back to RCU, which would effectively
    revert commit 719d93cd ("kvm/irqchip: Speed up KVM_SET_GSI_ROUTING",
    2014-01-16).
    
    However, the docs are overly conservative.  You can have an SRCU instance
    only has users in irq context, and you can mix process and irq context
    as long as process context users disable interrupts.  In addition,
    __srcu_read_unlock() actually uses this_cpu_dec() on both Tree SRCU and
    Classic SRCU.  For those two implementations, only srcu_read_lock()
    is unsafe.
    
    When Classic SRCU's __srcu_read_unlock() was changed to use this_cpu_dec(),
    in commit 5a41344a ("srcu: Simplify __srcu_read_unlock() via
    this_cpu_dec()", 2012-11-29), __srcu_read_lock() did two increments.
    Therefore it kept __this_cpu_inc(), with preempt_disable/enable in
    the caller.  Tree SRCU however only does one increment, so on most
    architectures it is more efficient for __srcu_read_lock() to use
    this_cpu_inc(), and any performance differences appear to be down in
    the noise.
    
    Unlike Classic and Tree SRCU, Tiny SRCU does increments and decrements on
    a single variable.  Therefore, as Peter Zijlstra pointed out, Tiny SRCU's
    implementation already supports mixed-context use of srcu_read_lock()
    and srcu_read_unlock(), at least as long as uses of srcu_read_lock()
    and srcu_read_unlock() in each handler are nested and paired properly.
    In other words, it is still illegal to (say) invoke srcu_read_lock()
    in an interrupt handler and to invoke the matching srcu_read_unlock()
    in a softirq handler.  Therefore, the only change required for Tiny SRCU
    is to its comments.
    
    Fixes: 719d93cd ("kvm/irqchip: Speed up KVM_SET_GSI_ROUTING")
    Reported-by: default avatarLinu Cherian <linuc.decode@gmail.com>
    Suggested-by: default avatarLinu Cherian <linuc.decode@gmail.com>
    Cc: kvm@vger.kernel.org
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
    Tested-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    cdf7abc4
srcutree.c 38 KB