[Differential] [Commented On] D1711: Changes to the callout code to restore active semantics and also add a test-framework and test to validate thecallout code (and potentially for use by other tests).

[hselasky (Hans Petter Selasky)]

hselasky added a comment.

Randall: Shooting again:

Thread 1 is executing in "softclock_call_cc()" in the "new_cc = callout_cpu_switch(c, cc, new_cpu)" it has set "c->c_cpu = CPUBLOCK;"
Thread 2 is now executing callout_reset(). As you can see in the implementation detail, it is reading "c_cpu" without CC_LOCK() locked and without checking if it was reading the c_cpu while it was equal to CPUBLOCK !

#define callout_reset(c, on_tick, fn, arg)                              \
    callout_reset_on((c), (on_tick), (fn), (arg), (c)->c_cpu)

This results in migrating the callout to CPUBLOCK, and subsequent calls will hang in callout_lock() like in hirens panic.

--HPS

REVISION DETAIL
  https://reviews.freebsd.org/D1711

To: rrs, gnn, rwatson, lstewart, jhb, kostikbel, sbruno, imp, adrian, hselasky
Cc: julian, hiren, jhb, kostikbel, emaste, delphij, neel, erj, freebsd-net