The following commit has been merged into the sched/urgent branch of tip:

Commit-ID: ce3614daabea8a2d01c1dd17ae41d1ec5e5ae7db Gitweb: https://git.kernel.org/tip/ce3614daabea8a2d01c1dd17ae41d1ec5e5ae7db Author: Mathieu Desnoyers mathieu.desnoyers@efficios.com AuthorDate: Mon, 06 Jul 2020 16:49:10 -04:00 Committer: Peter Zijlstra peterz@infradead.org CommitterDate: Wed, 08 Jul 2020 11:38:50 +02:00

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.c... --- kernel/sched/core.c | 2 ++ 1 file changed, 2 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 950ac45..e15543c 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2965,6 +2965,7 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p) * Silence PROVE_RCU. */ raw_spin_lock_irqsave(&p->pi_lock, flags); + rseq_migrate(p); /* * We're setting the CPU for the first time, we don't migrate, * so use __set_task_cpu(). @@ -3029,6 +3030,7 @@ void wake_up_new_task(struct task_struct *p) * as we're not fully set-up yet. */ p->recent_used_cpu = task_cpu(p); + rseq_migrate(p); __set_task_cpu(p, select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0)); #endif rq = __task_rq_lock(p, &rf);