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srcu: Prevent sdp->srcu_gp_seq_needed counter wrap

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If a given CPU never happens to ever start an SRCU grace period, the
grace-period sequence counter might wrap.  If this CPU were to decide to
finally start a grace period, the state of its sdp->srcu_gp_seq_needed
might make it appear that it has already requested this grace period,
which would prevent starting the grace period.  If no other CPU ever started
a grace period again, this would look like a grace-period hang.  Even
if some other CPU took pity and started the needed grace period, the
leaf rcu_node structure's ->srcu_data_have_cbs field won't have record
of the fact that this CPU has a callback pending, which would look like
a very localized grace-period hang.

This might seem very unlikely, but SRCU grace periods can take less than
a microsecond on small systems, which means that overflow can happen
in much less than an hour on a 32-bit embedded system.  And embedded
systems are especially likely to have long-term idle CPUs.  Therefore,
it makes sense to prevent this scenario from happening.

This commit therefore scans each srcu_data structure occasionally,
with frequency controlled by the srcutree.counter_wrap_check kernel
boot parameter.  This parameter can be set to something like 255
in order to exercise the counter-wrap-prevention code.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney
2017-05-03 15:35:32 -07:00
parent 71c40fd0b5
commit c350c00829
2 changed files with 27 additions and 0 deletions
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18
kernel/rcu/srcutree.c
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@@ -45,6 +45,10 @@
static ulong exp_holdoff = DEFAULT_SRCU_EXP_HOLDOFF;
module_param(exp_holdoff, ulong, 0444);
/* Overflow-check frequency. N bits roughly says every 2**N grace periods. */
static ulong counter_wrap_check = (ULONG_MAX >> 2);
module_param(counter_wrap_check, ulong, 0444);
static void srcu_invoke_callbacks(struct work_struct *work);
static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay);
@@ -496,10 +500,13 @@ static void srcu_gp_end(struct srcu_struct *sp)
{
unsigned long cbdelay;
bool cbs;
int cpu;
unsigned long flags;
unsigned long gpseq;
int idx;
int idxnext;
unsigned long mask;
struct srcu_data *sdp;
struct srcu_node *snp;
/* Prevent more than one additional grace period. */
@@ -538,6 +545,17 @@ static void srcu_gp_end(struct srcu_struct *sp)
smp_mb(); /* GP end before CB invocation. */
srcu_schedule_cbs_snp(sp, snp, mask, cbdelay);
}
/* Occasionally prevent srcu_data counter wrap. */
if (!(gpseq & counter_wrap_check))
for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
sdp = per_cpu_ptr(sp->sda, cpu);
spin_lock_irqsave(&sdp->lock, flags);
if (ULONG_CMP_GE(gpseq,
sdp->srcu_gp_seq_needed + 100))
sdp->srcu_gp_seq_needed = gpseq;
spin_unlock_irqrestore(&sdp->lock, flags);
}
}
/* Callback initiation done, allow grace periods after next. */