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| author | Sebastian Andrzej Siewior <bigeasy@linutronix.de> | 2021-11-27 17:31:59 +0100 |
|---|---|---|
| committer | Daniel Borkmann <daniel@iogearbox.net> | 2021-11-30 15:40:31 +0100 |
| commit | 6a631c0432dcccbcf45839016a07c015e335e9ae (patch) | |
| tree | e630bc06ea7b6d06601f58f1add029b5b158a4d1 /Documentation/locking | |
| parent | c0d95d3380ee099d735e08618c0d599e72f6c8b0 (diff) | |
| download | lwn-6a631c0432dcccbcf45839016a07c015e335e9ae.tar.gz lwn-6a631c0432dcccbcf45839016a07c015e335e9ae.zip | |
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/locking')
| -rw-r--r-- | Documentation/locking/locktypes.rst | 9 |
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() { |