Re: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary
From: Sean Christopherson <seanjc@google.com>
Date: 2021-03-31 21:06:37
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kvm, kvmarm, linux-mips, lkml
On Wed, Mar 31, 2021, Paolo Bonzini wrote:
On 31/03/21 21:47, Sean Christopherson wrote:quoted
Rereading things, a small chunk of the rwsem nastiness can go away. I don't see any reason to use rw_semaphore instead of rwlock_t.Wouldn't it be incorrect to lock a mutex (e.g. inside *another* MMU notifier's invalidate callback) while holding an rwlock_t? That makes sense because anybody that's busy waiting in write_lock potentially cannot be preempted until the other task gets the mutex. This is a potential deadlock.
Yes? I don't think I follow your point though. Nesting a spinlock or rwlock inside a rwlock is ok, so long as the locks are always taken in the same order, i.e. it's never mmu_lock -> mmu_notifier_slots_lock.
I also thought of busy waiting on down_read_trylock if the MMU notifier cannot block, but that would also be invalid for the opposite reason (the down_write task might be asleep, waiting for other readers to release the task, and the down_read_trylock busy loop might not let that task run).quoted
And that's _already_ the worst case since notifications are currently serialized by mmu_lock.But right now notifications are not a single critical section, they're two, aren't they?
Ah, crud, yes. Holding a spinlock across the entire start() ... end() would be bad, especially when the notifier can block since that opens up the possibility of the task sleeping/blocking/yielding while the spinlock is held. Bummer. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel