rseq/cleanup: Do not 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 <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Ben Maurer <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Chris Lameter <[email protected]>
Cc: Dave Watson <[email protected]>
Cc: Joel Fernandes <[email protected]>
Cc: Josh Triplett <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Michael Kerrisk <[email protected]>
Cc: Paul E . McKenney <[email protected]>
Cc: Paul Turner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Russell King <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: https://lore.kernel.org/lkml/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: compudj

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);
 	}
 }