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bpf: Make sure bpf_disable_instrumentation() is safe vs preemption.

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commit 79364031c5b4365ca28ac0fa00acfab5bf465be1 upstream.

The initial implementation of migrate_disable() for mainline was a
wrapper around preempt_disable(). RT kernels substituted this with a
real migrate disable implementation.

Later on mainline gained true migrate disable support, but neither
documentation nor affected code were updated.

Remove stale comments claiming that migrate_disable() is PREEMPT_RT only.

Don't use __this_cpu_inc() in the !PREEMPT_RT path because preemption is
not disabled and the RMW operation can be preempted.

Fixes: 74d862b682 ("sched: Make migrate_disable/enable() independent of RT")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211127163200.10466-3-bigeasy@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Sebastian Andrzej Siewior
2021-11-27 17:32:00 +01:00
committed by Greg Kroah-Hartman
parent 9c983fd7cf
commit e76da2e8a0
2 changed files with 2 additions and 17 deletions
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16
include/linux/bpf.h
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@@ -1321,28 +1321,16 @@ extern struct mutex bpf_stats_enabled_mutex;
* kprobes, tracepoints) to prevent deadlocks on map operations as any of
* these events can happen inside a region which holds a map bucket lock
* and can deadlock on it.
*
* Use the preemption safe inc/dec variants on RT because migrate disable
* is preemptible on RT and preemption in the middle of the RMW operation
* might lead to inconsistent state. Use the raw variants for non RT
* kernels as migrate_disable() maps to preempt_disable() so the slightly
* more expensive save operation can be avoided.
*/
static inline void bpf_disable_instrumentation(void)
{
migrate_disable();
if (IS_ENABLED(CONFIG_PREEMPT_RT))
this_cpu_inc(bpf_prog_active);
else
__this_cpu_inc(bpf_prog_active);
this_cpu_inc(bpf_prog_active);
}
static inline void bpf_enable_instrumentation(void)
{
if (IS_ENABLED(CONFIG_PREEMPT_RT))
this_cpu_dec(bpf_prog_active);
else
__this_cpu_dec(bpf_prog_active);
this_cpu_dec(bpf_prog_active);
migrate_enable();
}