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KVM: PPC: Book3S HV: Fix race in starting secondary threads

PATCH David Gibson 1 participant 1 message Sep 3, 2015 1 message not in the archive KVM

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David Gibson

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Quoted 19 lines · Paul Mackerras

From: "Gautham R. Shenoy" <[email protected]>

The current dynamic micro-threading code has a race due to which a secondary thread naps when it is supposed to be running a vcpu. As a side effect of this, on a guest exit, the primary thread in kvmppc_wait_for_nap() finds that this secondary thread hasn't cleared its vcore pointer. This results in "CPU X seems to be stuck!" warnings.

The race is possible since the primary thread on exiting the guests only waits for all the secondaries to clear its vcore pointer. It subsequently expects the secondary threads to enter nap while it unsplits the core. A secondary thread which hasn't yet entered the nap will loop in kvm_no_guest until its vcore pointer and the do_nap flag are unset. Once the core has been unsplit, a new vcpu thread can grab the core and set the do_nap flag *before* setting the vcore pointers of the secondary. As a result, the secondary thread will now enter nap via kvm_unsplit_nap instead of running the guest vcpu.

Quoted 8 lines

Fix this by setting the do_nap flag after setting the vcore pointer in the PACA of the secondary in kvmppc_run_core. Also, ensure that a secondary thread doesn't nap in kvm_unsplit_nap when the vcore pointer in its PACA struct is set.

Fixes b4deba5c41e9
Signed-off-by Gautham R. Shenoy <[email protected]>
Signed-off-by Paul Mackerras <[email protected]>
Reviewed-by David Gibson <[email protected]>
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