| Message ID | 20210722175157.1367122-4-valentin.schneider@arm.com |
|---|---|
| State | New |
| Headers | show |
| Series | [1/3] sched/preempt: Introduce preempt_{enable, disable}_bh() | expand |
On 22/07/21 19:32, Mark Brown wrote: > On Thu, Jul 22, 2021 at 06:51:57PM +0100, Valentin Schneider wrote: >> Running v5.13-rt1 on my arm64 Juno board triggers: > > Acked-by: Mark Brown <broonie@kernel.org> > Thanks! >> [ 11.337654] WARNING: CPU: 4 PID: 1 at arch/arm64/kernel/fpsimd.c:296 task_fpsimd_load (arch/arm64/kernel/fpsimd.c:296 (discriminator 1)) >> [ 11.337692] Modules linked in: >> [ 11.337705] CPU: 4 PID: 1 Comm: init Not tainted 5.13.0-rt1 #52 >> [ 11.337719] Hardware name: ARM Juno development board (r0) (DT) >> [ 11.337727] pstate: 40000005 (nZcv daif -PAN -UAO -TCO BTYPE=--) >> [ 11.337740] pc : task_fpsimd_load (arch/arm64/kernel/fpsimd.c:296 (discriminator 1)) >> [ 11.337755] lr : task_fpsimd_load (arch/arm64/kernel/fpsimd.c:296 (discriminator 3)) >> [ 11.337769] sp : ffff800012f4bdd0 >> [ 11.337775] x29: ffff800012f4bdd0 x28: ffff000800160000 x27: 0000000000000000 >> [ 11.337797] x26: 0000000000000000 x25: 0000000000000000 x24: ffff0008001606f0 >> [ 11.337816] x23: ffff000800160000 x22: ffff000800160700 x21: ffff000800160000 > > For readability it's probably better to not include the entire register > contents here - the backtrace ends up about as large as the actual > changelog, a good chunk of which is just the register contents. Noted.
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c index 9bf86cd7b605..0c66ea0dd97e 100644 --- a/arch/arm64/kernel/fpsimd.c +++ b/arch/arm64/kernel/fpsimd.c @@ -177,10 +177,12 @@ static void __get_cpu_fpsimd_context(void) * * The double-underscore version must only be called if you know the task * can't be preempted. + * + * Disabling preemption prevents nesting via fpsimd_thread_switch(). */ static void get_cpu_fpsimd_context(void) { - local_bh_disable(); + preempt_disable_bh(); __get_cpu_fpsimd_context(); } @@ -201,7 +203,7 @@ static void __put_cpu_fpsimd_context(void) static void put_cpu_fpsimd_context(void) { __put_cpu_fpsimd_context(); - local_bh_enable(); + preempt_enable_bh(); } static bool have_cpu_fpsimd_context(void)
Running v5.13-rt1 on my arm64 Juno board triggers: [ 11.337654] WARNING: CPU: 4 PID: 1 at arch/arm64/kernel/fpsimd.c:296 task_fpsimd_load (arch/arm64/kernel/fpsimd.c:296 (discriminator 1)) [ 11.337692] Modules linked in: [ 11.337705] CPU: 4 PID: 1 Comm: init Not tainted 5.13.0-rt1 #52 [ 11.337719] Hardware name: ARM Juno development board (r0) (DT) [ 11.337727] pstate: 40000005 (nZcv daif -PAN -UAO -TCO BTYPE=--) [ 11.337740] pc : task_fpsimd_load (arch/arm64/kernel/fpsimd.c:296 (discriminator 1)) [ 11.337755] lr : task_fpsimd_load (arch/arm64/kernel/fpsimd.c:296 (discriminator 3)) [ 11.337769] sp : ffff800012f4bdd0 [ 11.337775] x29: ffff800012f4bdd0 x28: ffff000800160000 x27: 0000000000000000 [ 11.337797] x26: 0000000000000000 x25: 0000000000000000 x24: ffff0008001606f0 [ 11.337816] x23: ffff000800160000 x22: ffff000800160700 x21: ffff000800160000 [ 11.337837] x20: ffff800012f4beb0 x19: 0000000000000008 x18: 000000000000c9a0 [ 11.337857] x17: 00000000ae3495d5 x16: 000000000000c9a0 x15: ffff80001240e128 [ 11.337878] x14: ffff8000124b0128 x13: 000000000000000a x12: ffff80001205e5f0 [ 11.337898] x11: 0000000000000000 x10: ffff800011a37d28 x9 : 0000000000000050 [ 11.337917] x8 : ffff000800160000 x7 : 0000000000000002 x6 : 0000000000000000 [ 11.337937] x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000000 [ 11.337956] x2 : 0000000000000008 x1 : 0000000000000000 x0 : 0000000000000000 [ 11.337975] Call trace: [ 11.337980] task_fpsimd_load (arch/arm64/kernel/fpsimd.c:296 (discriminator 1)) [ 11.337996] fpsimd_restore_current_state (arch/arm64/kernel/fpsimd.c:1186) [ 11.338012] do_notify_resume (./arch/arm64/include/asm/daifflags.h:28 arch/arm64/kernel/signal.c:947) [ 11.338032] work_pending (arch/arm64/kernel/entry.S:839) [ 11.338045] irq event stamp: 1228377 [ 11.338051] hardirqs last enabled at (1228375): _raw_spin_unlock_irqrestore (./include/linux/spinlock_api_smp.h:160 kernel/locking/spinlock.c:194) [ 11.338076] hardirqs last disabled at (1228377): el1_dbg (arch/arm64/kernel/entry-common.c:144 arch/arm64/kernel/entry-common.c:234) [ 11.338098] softirqs last enabled at (1227024): __local_bh_enable_ip (./arch/arm64/include/asm/irqflags.h:85 kernel/softirq.c:262) [ 11.338121] softirqs last disabled at (1228376): fpsimd_restore_current_state (./include/linux/bottom_half.h:19 arch/arm64/kernel/fpsimd.c:183 arch/arm64/kernel/fpsimd.c:1182) This is caused by local_bh_disable() not disabling preemption under CONFIG_PREEMPT_RT, which fails have_cpu_fpsimd_context(). The per-CPU access safety proved by the CONFIG_PREEMPT_RT version of local_bh_disable() is not sufficient here, as we end up with both preemption and IRQs enabled when running do_notify_resume(). This means we can hit: el0_sync `\ do_notify_resume() `\ fpsimd_restore_current_state() ~~> el1_irq `\ preempt_schedule_irq() `\ fpsimd_thread_switch() IOW we *really* need to disable preemption here, even on CONFIG_PREEMPT_RT. The preempt_{enable, disable}_bh() helpers exist to handle this exact case, so use them here. This is spiritually close to: cba08c5dc6dc ("x86/fpu: Make kernel FPU protection RT friendly") Signed-off-by: Valentin Schneider <valentin.schneider@arm.com> --- arch/arm64/kernel/fpsimd.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)