CVS commit: src/sys/kern

["Ryota Ozaki" <ozaki-r%netbsd.org@localhost>]

Module Name:    src
Committed By:   ozaki-r
Date:           Fri Feb 16 07:04:51 UTC 2018

Modified Files:
        src/sys/kern: kern_synch.c

Log Message:
Avoid a race condition between an LWP migration and curlwp_bind

curlwp_bind sets the LP_BOUND flag to l_pflags of the current LWP, which
prevents it from migrating to another CPU until curlwp_bindx is called.
Meanwhile, there are several ways that an LWP is migrated to another CPU and in
any cases the scheduler postpones a migration if a target LWP is running.  One
example of LWP migrations is a load balancing; the scheduler periodically
explores CPU-hogging LWPs and schedule them to migrate (see sched_lwp_stats).
At that point the scheduler checks the LP_BOUND flag and if it's set to a LWP,
the scheduler doesn't schedule the LWP.  A scheduled LWP is tried to be migrated
when it is leaving a running CPU, i.e., mi_switch.  And mi_switch does NOT check
the LP_BOUND flag.  So if an LWP is scheduled first and then it sets the
LP_BOUND flag, the LWP can be migrated regardless of the flag.  To avoid this
race condition, we need to check the flag in mi_switch too.

For more details see https://mail-index.netbsd.org/tech-kern/2018/02/13/msg023079.html


To generate a diff of this commit:
cvs rdiff -u -r1.313 -r1.314 src/sys/kern/kern_synch.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.