linux-kernel - Re: [PATCH v2 0/7] swait: Introduce and use swq_has

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-Id: <21d360c5-d98e-896b-ee3d-7fff203f3366@de.ibm.com>
Date:   Tue, 19 Sep 2017 10:25:11 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>, mingo@...nel.org,
        peterz@...radead.org
Cc:     npiggin@...il.com, paulmck@...ux.vnet.ibm.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Cornelia Huck <cohuck@...hat.com>
Subject: Re: [PATCH v2 0/7] swait: Introduce and use swq_has_sleeper()



On 09/15/2017 01:53 PM, Paolo Bonzini wrote:
> On 13/09/2017 22:08, Davidlohr Bueso wrote:
>> The following patches fix and/or justify (in baby steps) some of the
>> callers. The main exception is s390, which I didn't follow how ->valid_wakeup
>> can get hoisted as kvm_vcpu_block does not use that in the wait loop.
> 
> valid_wakeup is just an optimization, so it's not a problem.
> 
> There seems to be always an atomic_or or set_bit before
> kvm_s390_vcpu_wakeup is called (except kvm_s390_idle_wakeup which has no
> store at all and doesn't need any serialization).  So my suggestion is
> to add an smp__mb_after_atomic in kvm_s390_vcpu_wakeup; I'll let the
> s390 guys do it.


I will queue something like this


diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
index a832ad0..44239b5 100644
--- a/arch/s390/kvm/interrupt.c
+++ b/arch/s390/kvm/interrupt.c
@@ -1074,6 +1074,12 @@ void kvm_s390_vcpu_wakeup(struct kvm_vcpu *vcpu)
         * in kvm_vcpu_block without having the waitqueue set (polling)
         */
        vcpu->valid_wakeup = true;
+       /*
+        * This is mostly to document, that the read in swait_active could
+        * be moved before other stores, leading to subtle races.
+        * All current users do not store or use an atomic like update
+        */
+       __smp_mb__after_atomic();
        if (swait_active(&vcpu->wq)) {
                /*
                 * The vcpu gave up the cpu voluntarily, mark it as a good





but I am asking myself if it is "safer" to make this function use swq_has_sleepers
in case we add in a distant future another user to kvm_s390_vcpu_wakeup that does 
use a normal store and everybody has already forgotten this?