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
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