Messages in this thread | | | Date | Mon, 10 Oct 2022 17:38:47 +0000 | From | Sean Christopherson <> | Subject | Re: [PATCH] kvm: x86: keep srcu writer side operation mutually exclusive |
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On Sun, Oct 09, 2022, Hao Peng wrote: > On Sat, Oct 8, 2022 at 1:12 AM Sean Christopherson <seanjc@google.com> wrote: > > > > On Sat, Oct 08, 2022, Hao Peng wrote: > > > From: Peng Hao <flyingpeng@tencent.com> > > > > > > Synchronization operations on the writer side of SRCU should be > > > invoked within the mutex. > > > > Why? Synchronizing SRCU is necessary only to ensure that all previous readers go > > away before the old filter is freed. There's no need to serialize synchronization > > between writers. The mutex ensures each writer operates on the "new" filter that's > > set by the previous writer, i.e. there's no danger of a double-free. And the next > > writer will wait for readers to _its_ "new" filter. > > > Array srcu_lock_count/srcu_unlock_count[] in srcu_data, which is used > alternately to determine > which readers need to wait to get out of the critical area. If two > synchronize_srcu are initiated concurrently, > there may be a problem with the judgment of gp. But if it is confirmed > that there will be no writer concurrency, > it is not necessary to ensure that synchronize_srcu is executed within > the scope of the mutex lock.
I don't see anything in the RCU documentation or code that suggests that callers need to serialize synchronization calls. E.g. the "tree" SRCU implementation uses a dedicated mutex to serialize grace period work
struct mutex srcu_gp_mutex; /* Serialize GP work. */
static void srcu_advance_state(struct srcu_struct *ssp) { int idx;
mutex_lock(&ssp->srcu_gp_mutex);
<magic> }
and its state machine explicitly accounts for "Someone else" starting a grace period
if (idx != SRCU_STATE_IDLE) { mutex_unlock(&ssp->srcu_gp_mutex); return; /* Someone else started the grace period. */ }
and srcu_gp_end() also guards against creating more than 2 grace periods.
/* Prevent more than one additional grace period. */ mutex_lock(&ssp->srcu_cb_mutex);
And if this is a subtle requirement, there is a lot of broken kernel code, e.g. mmu_notifier, other KVM code, srcu_notifier_chain_unregister(), etc...
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