Re: [PATCH v7 4/5] sched: Handle set_cpus_allowed_ptr() & sched_setaffinity() race

From: Waiman Long
Date: Thu Sep 08 2022 - 10:54:55 EST

On 9/8/22 07:53, Peter Zijlstra wrote:
On Fri, Sep 02, 2022 at 11:25:55AM -0400, Waiman Long wrote:
Racing is possible between set_cpus_allowed_ptr() and sched_setaffinity()
or between multiple sched_setaffinity() calls from different
CPUs. To resolve these race conditions, we need to update both
user_cpus_ptr and cpus_mask in a single lock critical section instead
of separated ones. This requires moving the user_cpus_ptr update to

The SCA_USER flag will be used to signify that a user_cpus_ptr update
will have to be done. The new user_cpus_ptr will be put into the
a percpu variable pending_user_mask at the beginning of the lock
crtical section. The pending user mask will then be taken up in

Ideally, user_cpus_ptr should only be updated if the sched_setaffinity()
is successful. However, this patch will update user_cpus_ptr when the
first call to __set_cpus_allowed_ptr() is successful. However, if there
is racing between sched_setaffinity() and cpuset update, the subsequent
calls to __set_cpus_allowed_ptr() may fail but the user_cpus_ptr will
still be updated in this corner case. A warning will be printed in this
corner case.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
OK, so obviously this is terrible :/

What's wrong with something like so ?

Thanks for the suggestion. I have no problem adding an affinity_context structure to pass around the functions. Will incorporate your suggestion in the next version of the patch.