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authorSebastian Andrzej Siewior <bigeasy@linutronix.de>2021-11-27 19:31:59 +0300
committerDaniel Borkmann <daniel@iogearbox.net>2021-11-30 17:40:31 +0300
commit6a631c0432dcccbcf45839016a07c015e335e9ae (patch)
treee630bc06ea7b6d06601f58f1add029b5b158a4d1 /Documentation
parentc0d95d3380ee099d735e08618c0d599e72f6c8b0 (diff)
downloadlinux-6a631c0432dcccbcf45839016a07c015e335e9ae.tar.xz
Documentation/locking/locktypes: Update migrate_disable() bits.
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> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Link: https://lore.kernel.org/bpf/20211127163200.10466-2-bigeasy@linutronix.de
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/locking/locktypes.rst9
1 files changed, 3 insertions, 6 deletions
diff --git a/Documentation/locking/locktypes.rst b/Documentation/locking/locktypes.rst
index ddada4a53749..4fd7b70fcde1 100644
--- a/Documentation/locking/locktypes.rst
+++ b/Documentation/locking/locktypes.rst
@@ -439,11 +439,9 @@ preemption. The following substitution works on both kernels::
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 @@ scenario::
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()
{