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- Reassign the thread queue lock to newtd prior to switching. Assigning

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after the switch leads to a race where the outgoing thread still owns
   the local queue lock while another cpu may switch it in.  This race
   is only possible on machines where cpu_switch can take significantly
   longer on different cpus which in practice means HTT machines with
   unfair thread scheduling algorithms.

Found by:	kris (of course)
Approved by:	re
This commit is contained in:
Jeff Roberson 2007-10-02 01:30:18 +00:00
parent 89d4e7b7da
commit 59c6813475
1 changed files with 6 additions and 4 deletions
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10
sys/kern/sched_ule.c
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@ -1892,6 +1892,7 @@ sched_switch(struct thread *td, struct thread *newtd, int flags)
if (PMC_PROC_IS_USING_PMCS(td->td_proc))
PMC_SWITCH_CONTEXT(td, PMC_FN_CSW_OUT);
#endif
TDQ_LOCKPTR(tdq)->mtx_lock = (uintptr_t)newtd;
cpu_switch(td, newtd, mtx);
/*
* We may return from cpu_switch on a different cpu. However,
@ -1900,7 +1901,6 @@ sched_switch(struct thread *td, struct thread *newtd, int flags)
*/
cpuid = PCPU_GET(cpuid);
tdq = TDQ_CPU(cpuid);
TDQ_LOCKPTR(tdq)->mtx_lock = (uintptr_t)td;
#ifdef HWPMC_HOOKS
if (PMC_PROC_IS_USING_PMCS(td->td_proc))
PMC_SWITCH_CONTEXT(td, PMC_FN_CSW_IN);
@ -2607,6 +2607,7 @@ sched_idletd(void *dummy)
void
sched_throw(struct thread *td)
{
struct thread *newtd;
struct tdq *tdq;
tdq = TDQ_SELF();
@ -2619,9 +2620,11 @@ sched_throw(struct thread *td)
tdq_load_rem(tdq, td->td_sched);
}
KASSERT(curthread->td_md.md_spinlock_count == 1, ("invalid count"));
newtd = choosethread();
TDQ_LOCKPTR(tdq)->mtx_lock = (uintptr_t)newtd;
PCPU_SET(switchtime, cpu_ticks());
PCPU_SET(switchticks, ticks);
cpu_throw(td, choosethread()); /* doesn't return */
cpu_throw(td, newtd); /* doesn't return */
}
/*
@ -2646,8 +2649,7 @@ sched_fork_exit(struct thread *td)
td->td_lock = TDQ_LOCKPTR(tdq);
MPASS(td->td_lock == TDQ_LOCKPTR(tdq));
td->td_oncpu = cpuid;
TDQ_LOCKPTR(tdq)->mtx_lock = (uintptr_t)td;
THREAD_LOCK_ASSERT(td, MA_OWNED | MA_NOTRECURSED);
TDQ_LOCK_ASSERT(tdq, MA_OWNED | MA_NOTRECURSED);
}
static SYSCTL_NODE(_kern, OID_AUTO, sched, CTLFLAG_RW, 0,