Re: [PATCH 04/30] firmware: google: Convert regular spinlock into trylock on panic path
From: Evan Green <hidden>
Date: 2022-05-03 18:04:24
Also in:
kexec, linux-alpha, linux-edac, linux-hyperv, linux-leds, linux-pm, linux-remoteproc, linux-s390, linux-tegra, linux-um, linuxppc-dev, lkml, netdev, rcu, sparclinux, xen-devel
On Wed, Apr 27, 2022 at 3:51 PM Guilherme G. Piccoli [off-list ref] wrote:
quoted hunk ↗ jump to hunk
Currently the gsmi driver registers a panic notifier as well as reboot and die notifiers. The callbacks registered are called in atomic and very limited context - for instance, panic disables preemption, local IRQs and all other CPUs that aren't running the current panic function. With that said, taking a spinlock in this scenario is a dangerous invitation for a deadlock scenario. So, we fix that in this commit by changing the regular spinlock with a trylock, which is a safer approach. Fixes: 74c5b31c6618 ("driver: Google EFI SMI") Cc: Ard Biesheuvel <ardb@kernel.org> Cc: David Gow <redacted> Cc: Evan Green <redacted> Cc: Julius Werner <jwerner@chromium.org> Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com> --- drivers/firmware/google/gsmi.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)diff --git a/drivers/firmware/google/gsmi.c b/drivers/firmware/google/gsmi.c index adaa492c3d2d..b01ed02e4a87 100644 --- a/drivers/firmware/google/gsmi.c +++ b/drivers/firmware/google/gsmi.c@@ -629,7 +629,10 @@ static int gsmi_shutdown_reason(int reason) if (saved_reason & (1 << reason)) return 0; - spin_lock_irqsave(&gsmi_dev.lock, flags); + if (!spin_trylock_irqsave(&gsmi_dev.lock, flags)) { + rc = -EBUSY; + goto out; + }
gsmi_shutdown_reason() is a common function called in other scenarios as well, like reboot and thermal trip, where it may still make sense to wait to acquire a spinlock. Maybe we should add a parameter to gsmi_shutdown_reason() so that you can get your change on panic, but we don't convert other callbacks into try-fail scenarios causing us to miss logs. Though thinking more about it, is this really a Good Change (TM)? The spinlock itself already disables interrupts, meaning the only case where this change makes a difference is if the panic happens from within the function that grabbed the spinlock (in which case the callback is also likely to panic), or in an NMI that panics within that window. The downside of this change is that if one core was politely working through an event with the lock held, and another core panics, we now might lose the panic log, even though it probably would have gone through fine assuming the other core has a chance to continue. -Evan