Paolo Bonzini 1123a60416 srcu: Allow use of Classic SRCU from both process and interrupt context ... 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 719d93cd5f ("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 5a41344a3d ("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. Cc: stable@vger.kernel.org Fixes: 719d93cd5f ("kvm/irqchip: Speed up KVM_SET_GSI_ROUTING") Reported-by: Linu Cherian <linuc.decode@gmail.com> Suggested-by: Linu Cherian <linuc.decode@gmail.com> Cc: kvm@vger.kernel.org Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>		2017-06-08 08:25:19 -07:00
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Makefile	rcu: Separately compile large rcu_segcblist functions	2017-05-02 07:21:02 -07:00
rcu.h	srcu: Merge ->srcu_state into ->srcu_gp_seq	2017-04-18 11:38:22 -07:00
rcu_segcblist.c	rcu: Separately compile large rcu_segcblist functions	2017-05-02 07:21:02 -07:00
rcu_segcblist.h	rcu: Open-code the rcu_cblist_n_lazy_cbs() function	2017-05-02 09:22:48 -07:00
rcuperf.c	sched/headers: Prepare for new header dependencies before moving code to <uapi/linux/sched/types.h>	2017-03-02 08:42:27 +01:00
rcutorture.c	srcu: Make rcutorture writer stalls print SRCU GP state	2017-04-26 11:23:28 -07:00
srcu.c	srcu: Allow use of Classic SRCU from both process and interrupt context	2017-06-08 08:25:19 -07:00
srcutiny.c	srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context	2017-06-08 08:24:26 -07:00
srcutree.c	srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context	2017-06-08 08:24:26 -07:00
sync.c	locking, rcu, cgroup: Avoid synchronize_sched() in __cgroup_procs_write()	2016-08-18 15:36:59 +02:00
tiny.c	rcu: Semicolon inside RCU_TRACE() for Tiny RCU	2017-04-18 11:38:17 -07:00
tiny_plugin.h	srcu: Allow SRCU to access rcu_scheduler_active	2017-04-18 11:38:18 -07:00
tree.c	Merge branch 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip	2017-05-10 10:30:46 -07:00
tree.h	srcu: Debloat the <linux/rcu_segcblist.h> header	2017-05-02 06:29:22 -07:00
tree_exp.h	srcu: Improve rcu_seq grace-period-counter abstraction	2017-04-18 11:38:21 -07:00
tree_plugin.h	rcu: Open-code the rcu_cblist_n_lazy_cbs() function	2017-05-02 09:22:48 -07:00
tree_trace.c	rcu: Open-code the rcu_cblist_n_lazy_cbs() function	2017-05-02 09:22:48 -07:00
update.c	rcu: Make non-preemptive schedule be Tasks RCU quiescent state	2017-04-21 05:59:27 -07:00