Re: [syzbot] [kvm-x86?] [kvm?] WARNING: locking bug in kvm_xen_set_evtchn_fast

Re: [syzbot] [kvm-x86?] [kvm?] WARNING: locking bug in kvm_xen_set_evtchn_fast

[David Woodhouse]

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On Sun, 2026-08-09 at 04:34 +0000, syzbot wrote:
> syzbot has tested the proposed patch and the reproducer did not trigger any issue

I found some though. Reworked:

#syz test: git://git.infradead.org/users/dwmw2/linux.git xen-srcu-fixed

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Re: [syzbot] [kvm-x86?] [kvm?] WARNING: locking bug in kvm_xen_set_evtchn_fast

[syzbot]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

failed to checkout kernel repo git://git.infradead.org/users/dwmw2/linux.git/xen-srcu-fixed: failed to run ["git" "-c" "core.hooksPath=/dev/null" "fetch" "--force" "73b08201ab8ba1d188f45210a65f33cb30e5efaf" "xen-srcu-fixed"]: exit status 128


Tested on:

commit:         [unknown 
git tree:       git://git.infradead.org/users/dwmw2/linux.git xen-srcu-fixed
kernel config:  https://syzkaller.appspot.com/x/.config?x=a94030c847137a18
dashboard link: https://syzkaller.appspot.com/bug?extid=919877893c9d28162dc2
compiler:       

Note: no patches were applied.

Re: [PATCH] KVM: x86/xen: Use read_trylock() for GPC locks in hardirq paths

[David Woodhouse]

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On Fri, 2026-08-07 at 17:49 +0530, sai madhu wrote:
> kvm_xen_set_evtchn_fast() is called from hardirq context (xen timer
> callback, kvm_arch_set_irq_inatomic()). On PREEMPT_RT (and under
> lockdep), read_lock_irqsave(&gpc->lock) is invalid there because
> rwlock_t may sleep.
> 
> Switch to read_trylock() and return -EWOULDBLOCK when the lock is
> contended or the cache is invalid. xen_timer_callback() already
> defers to the slow path (timer_pending + KVM_REQ_UNBLOCK +
> kvm_xen_inject_timer_irqs()).
> 
> Also fix __kvm_xen_has_interrupt() the same way for atomic callers.
> 
> Based on David Woodhouse's upstream series (May 2026, patch 3/7).
> Fixes syzbot: https://syzkaller.appspot.com/bug?extid=919877893c9d28162dc2

Thanks... but why? We're literally iterating on this right now.

I'm fairly convinced I want to ditch rwlocks completely and move to RCU:
https://lore.kernel.org/all/20260805195528.3853473-1-dwmw@amazon.co.uk/

Testing found some issues in my conversion which I've since fixed, and
the conversion to SRCU at Sean's request is currently an *incremental*
patch, which I'll eventually squash back into the first.

I'm in the middle of running a weekend-long soak test on this tree:
https://git.infradead.org/?p=users/dwmw2/linux.git;a=shortlog;h=refs/heads/xen-rcu-fixed

Let's see what syzbot thinks of it...

#syz test: git://git.infradead.org/users/dwmw2/linux.git xen-rcu-fixed

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Re: [syzbot] [kvm-x86?] [kvm?] WARNING: locking bug in kvm_xen_set_evtchn_fast

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+919877893c9d28162dc2@syzkaller.appspotmail.com
Tested-by: syzbot+919877893c9d28162dc2@syzkaller.appspotmail.com

Tested on:

commit:         230abd62 KVM: pfncache: Use a dedicated SRCU domain in..
git tree:       git://git.infradead.org/users/dwmw2/linux.git xen-rcu-fixed
console output: https://syzkaller.appspot.com/x/log.txt?x=110522c6580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=9cef045500682f6b
dashboard link: https://syzkaller.appspot.com/bug?extid=919877893c9d28162dc2
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8

Note: no patches were applied.
Note: testing is done by a robot and is best-effort only.