On Thu, Apr 30, 2026 at 12:38:16PM +0530, Shrikanth Hegde wrote: > Hi Paul. > > On 4/29/26 11:31 PM, Paul E. McKenney wrote:
[ . . . ] Sorry, missed one... > > ------------------------------------------------------------------------ > > > > commit f8d5aaaf90f8294890802ce8dccbafd9850ac5f9 > > Author: Paul E. McKenney <[email protected]> > > Date: Thu Apr 9 11:16:02 2026 -0700 > > > > srcu: Don't queue workqueue handlers to never-online CPUs > > While an srcu_struct structure is in the midst of switching from CPU-0 > > to all-CPUs state, it can attempt to invoke callbacks for CPUs that > > have never been online. Worse yet, it can attempt in invoke callbacks > > for CPUs that never will be online due to not being present in the > > for CPUs that never will be online due to being present in the > cpu_possible_mask? Exactly. Just because a CPU is in cpu_possible_mask doesn't mean that it will ever actually come online. For example, for single-threaded performance reasons, a given system might choose to bring online only one CPU from each hypertheaded core. In that case, the other CPU in each hyperthreaded core could be in the cpu_possible_mask, but would never come online. Thanx, Paul > > cpu_possible_mask. This can cause hangs on s390, which is not set up > > to > > deal with workqueue handlers being scheduled on such CPUs. This commit > > therefore causes Tree SRCU to refrain from queueing workqueue handlers > > on CPUs that have not yet (and might never) come online. > > Because callbacks are not invoked on CPUs that have not been > > online, it is an error to invoke call_srcu(), synchronize_srcu(), or > > synchronize_srcu_expedited() on a CPU that is not yet fully online. > > However, it turns out to be less code to redirect the callbacks > > from too-early invocations of call_srcu() than to warn about such > > invocations. This commit therefore also redirects callbacks queued on > > not-yet-fully-online CPUs to the boot CPU. > > Reported-by: Vasily Gorbik <[email protected]> > > Signed-off-by: Paul E. McKenney <[email protected]> > > Tested-by: Vasily Gorbik <[email protected]> > > Cc: Tejun Heo <[email protected]>
