[RFC PATCH for 4.18] rseq: cleanup: no need to abort rseq c.s. in child on fork

Considering that we explicitly forbid system calls in rseq critical
sections, it is not valid to issue a fork or clone system call within a
rseq critical section, so rseq_fork() is not required to restart an
active rseq c.s. in the child process.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Thomas Gleixner <redacted>
Cc: Joel Fernandes <redacted>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Watson <redacted>
Cc: Will Deacon <redacted>
Cc: Shuah Khan <redacted>
Cc: Andi Kleen <redacted>
Cc: linux-kselftest@vger.kernel.org
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Chris Lameter <redacted>
Cc: Russell King <redacted>
Cc: Michael Kerrisk <redacted>
Cc: "Paul E . McKenney" <redacted>
Cc: Paul Turner <redacted>
Cc: Boqun Feng <redacted>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Maurer <redacted>
Cc: linux-api@vger.kernel.org
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
---
 include/linux/sched.h | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 87bf02d93a27..c1882643d455 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h

@@ -1831,9 +1831,7 @@ static inline void rseq_migrate(struct task_struct *t)
 
 /*
  * If parent process has a registered restartable sequences area, the
- * child inherits. Only applies when forking a process, not a thread. In
- * case a parent fork() in the middle of a restartable sequence, set the
- * resume notifier to force the child to retry.
+ * child inherits. Only applies when forking a process, not a thread.
  */
 static inline void rseq_fork(struct task_struct *t, unsigned long clone_flags)
 {

@@ -1847,7 +1845,6 @@ static inline void rseq_fork(struct task_struct *t, unsigned long clone_flags)
 		t->rseq_len = current->rseq_len;
 		t->rseq_sig = current->rseq_sig;
 		t->rseq_event_mask = current->rseq_event_mask;
-		rseq_preempt(t);
 	}
 }
 

-- 
2.11.0