[PATCH 0/2] Update non-RT users of migrate_disable().

[PATCH 0/2] Update non-RT users of migrate_disable().

[Sebastian Andrzej Siewior]

While browsing through code I noticed outdated code/ documentation
regarding migrate_disable() on non-PREEMPT_RT kernels.

Sebastian

[PATCH doc 1/2] Documentation/locking/locktypes: Update migrate_disable() bits.

[Sebastian Andrzej Siewior]

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 the
documentation was not updated.

Update the documentation, remove the claims about migrate_disable()
mapping to preempt_disable() on non-PREEMPT_RT kernels.

Fixes: 74d862b682f51 ("sched: Make migrate_disable/enable() independent of RT")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 Documentation/locking/locktypes.rst | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/Documentation/locking/locktypes.rst b/Documentation/locking/locktypes.rst
index ddada4a537493..4fd7b70fcde19 100644
--- a/Documentation/locking/locktypes.rst
+++ b/Documentation/locking/locktypes.rst
@@ -439,11 +439,9 @@ not allow to acquire p->lock because get_cpu_ptr() implicitly disables
   spin_lock(&p->lock);
   p->count += this_cpu_read(var2);
 
-On a non-PREEMPT_RT kernel migrate_disable() maps to preempt_disable()
-which makes the above code fully equivalent. On a PREEMPT_RT kernel
 migrate_disable() ensures that the task is pinned on the current CPU which
 in turn guarantees that the per-CPU access to var1 and var2 are staying on
-the same CPU.
+the same CPU while the task remains preemptible.
 
 The migrate_disable() substitution is not valid for the following
 scenario::
@@ -456,9 +454,8 @@ The migrate_disable() substitution is not valid for the following
     p = this_cpu_ptr(&var1);
     p->val = func2();
 
-While correct on a non-PREEMPT_RT kernel, this breaks on PREEMPT_RT because
-here migrate_disable() does not protect against reentrancy from a
-preempting task. A correct substitution for this case is::
+This breaks because migrate_disable() does not protect against reentrancy from
+a preempting task. A correct substitution for this case is::
 
   func()
   {
-- 
2.34.0

[PATCH net 2/2] bpf: Make sure bpf_disable_instrumentation() is safe vs preemption.

[Sebastian Andrzej Siewior]

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: 74d862b682f51 ("sched: Make migrate_disable/enable() independent of RT")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/bpf.h    | 16 ++--------------
 include/linux/filter.h |  3 ---
 2 files changed, 2 insertions(+), 17 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index e7a163a3146b6..327a2bec06ca0 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1352,28 +1352,16 @@ 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();
 }
 
diff --git a/include/linux/filter.h b/include/linux/filter.h
index 24b7ed2677afd..534f678ca50fa 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -640,9 +640,6 @@ static __always_inline u32 bpf_prog_run(const struct bpf_prog *prog, const void
  * This uses migrate_disable/enable() explicitly to document that the
  * invocation of a BPF program does not require reentrancy protection
  * against a BPF program which is invoked from a preempting task.
- *
- * For non RT enabled kernels migrate_disable/enable() maps to
- * preempt_disable/enable(), i.e. it disables also preemption.
  */
 static inline u32 bpf_prog_run_pin_on_cpu(const struct bpf_prog *prog,
 					  const void *ctx)
-- 
2.34.0

Re: [PATCH 0/2] Update non-RT users of migrate_disable().

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf.git (master)
by Daniel Borkmann <daniel@iogearbox.net>:

On Sat, 27 Nov 2021 17:31:58 +0100 you wrote:
> While browsing through code I noticed outdated code/ documentation
> regarding migrate_disable() on non-PREEMPT_RT kernels.
> 
> Sebastian

Here is the summary with links:
  - [doc,1/2] Documentation/locking/locktypes: Update migrate_disable() bits.
    https://git.kernel.org/bpf/bpf/c/6a631c0432dc
  - [net,2/2] bpf: Make sure bpf_disable_instrumentation() is safe vs preemption.
    https://git.kernel.org/bpf/bpf/c/79364031c5b4

You are awesome, thank you!
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