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Documentation/locking/locktypes: Update migrate_disable() bits.

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commit 6a631c0432 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 the
documentation was not updated.

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

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/20211127163200.10466-2-bigeasy@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Sebastian Andrzej Siewior 2021-11-27 17:31:59 +01:00 committed by Greg Kroah-Hartman
parent 77d255d28b
commit 3f57215f74
1 changed files with 3 additions and 6 deletions
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9
Documentation/locking/locktypes.rst
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@ -439,11 +439,9 @@ preemption. The following substitution works on both kernels::
spin_lock(&p->lock); spin_lock(&p->lock);
p->count += this_cpu_read(var2); 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 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 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 The migrate_disable() substitution is not valid for the following
scenario:: scenario::
@ -456,9 +454,8 @@ scenario::
p = this_cpu_ptr(&var1); p = this_cpu_ptr(&var1);
p->val = func2(); p->val = func2();
While correct on a non-PREEMPT_RT kernel, this breaks on PREEMPT_RT because This breaks because migrate_disable() does not protect against reentrancy from
here migrate_disable() does not protect against reentrancy from a a preempting task. A correct substitution for this case is::
preempting task. A correct substitution for this case is::
func() func()
{ {