sched: Fix unreliable rseq cpu_id for new tasks
While integrating rseq into glibc and replacing glibc's sched_getcpu implementation with rseq, glibc's tests discovered an issue with incorrect __rseq_abi.cpu_id field value right after the first time a newly created process issues sched_setaffinity. For the records, it triggers after building glibc and running tests, and then issuing: for x in {1..2000} ; do posix/tst-affinity-static & done and shows up as: error: Unexpected CPU 2, expected 0 error: Unexpected CPU 2, expected 0 error: Unexpected CPU 2, expected 0 error: Unexpected CPU 2, expected 0 error: Unexpected CPU 138, expected 0 error: Unexpected CPU 138, expected 0 error: Unexpected CPU 138, expected 0 error: Unexpected CPU 138, expected 0 This is caused by the scheduler invoking __set_task_cpu() directly from sched_fork() and wake_up_new_task(), thus bypassing rseq_migrate() which is done by set_task_cpu(). Add the missing rseq_migrate() to both functions. The only other direct use of __set_task_cpu() is done by init_idle(), which does not involve a user-space task. Based on my testing with the glibc test-case, just adding rseq_migrate() to wake_up_new_task() is sufficient to fix the observed issue. Also add it to sched_fork() to keep things consistent. The reason why this never triggered so far with the rseq/basic_test selftest is unclear. The current use of sched_getcpu(3) does not typically require it to be always accurate. However, use of the __rseq_abi.cpu_id field within rseq critical sections requires it to be accurate. If it is not accurate, it can cause corruption in the per-cpu data targeted by rseq critical sections in user-space. Reported-By: Florian Weimer <fweimer@redhat.com> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Tested-By: Florian Weimer <fweimer@redhat.com> Cc: stable@vger.kernel.org # v4.18+ Link: https://lkml.kernel.org/r/20200707201505.2632-1-mathieu.desnoyers@efficios.com
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