Message ID | 0000014194309205-d0af4d2d-a046-4189-b4f0-ffea37374988-000000@email.amazonses.com |
---|---|
State | Not Applicable, archived |
Delegated to: | David Miller |
Headers | show |
* Christoph Lameter <cl@linux.com> wrote: > SNMP stats are not protected by preemption but by bh handling. Most forms of bh exclusion work via the preemption count though, and softirq contexts themselves are generally not preemptible [to other CPUs] either. So the warnings should, in most cases, not trigger. > Since protection is provided outside of preemption raw_cpu_ops > need to be used to avoid false positives. Could you quote the warning that pops up? It might be better to annotate the specific safe usages than to potentially hide bugs: > --- linux.orig/include/net/snmp.h 2013-10-07 09:16:07.595206864 -0500 > +++ linux/include/net/snmp.h 2013-10-07 09:16:07.591206909 -0500 > @@ -126,7 +126,7 @@ struct linux_xfrm_mib { > extern __typeof__(type) __percpu *name[SNMP_ARRAY_SZ] > > #define SNMP_INC_STATS_BH(mib, field) \ > - __this_cpu_inc(mib[0]->mibs[field]) > + raw_cpu_inc(mib[0]->mibs[field]) > > #define SNMP_INC_STATS_USER(mib, field) \ > this_cpu_inc(mib[0]->mibs[field]) > @@ -141,7 +141,7 @@ struct linux_xfrm_mib { > this_cpu_dec(mib[0]->mibs[field]) > > #define SNMP_ADD_STATS_BH(mib, field, addend) \ > - __this_cpu_add(mib[0]->mibs[field], addend) > + raw_cpu_add(mib[0]->mibs[field], addend) > > #define SNMP_ADD_STATS_USER(mib, field, addend) \ > this_cpu_add(mib[0]->mibs[field], addend) Doing this will hide any true bugs if any of these primitives is used in an unsafe context. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Tue, Oct 08, 2013 at 09:21:14AM +0200, Ingo Molnar wrote: > > * Christoph Lameter <cl@linux.com> wrote: > > > SNMP stats are not protected by preemption but by bh handling. > > Most forms of bh exclusion work via the preemption count though, and > softirq contexts themselves are generally not preemptible [to other CPUs] > either. > > So the warnings should, in most cases, not trigger. Right, so softirqs run either in the irq tail at which point preempt_count += SOFTIRQ_OFFSET and thus preemption is disabled, or it runs in ksoftirqd which has strict cpu affinity which also disables the warning, and it also increments preempt_count with SOFTIRQ_OFFSET to exclude the softirq from interrupts while its running, also disabling the warning. So it should very much not trigger.. if it does you want to know about it. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
* Peter Zijlstra <peterz@infradead.org> wrote: > On Tue, Oct 08, 2013 at 09:21:14AM +0200, Ingo Molnar wrote: > > > > * Christoph Lameter <cl@linux.com> wrote: > > > > > SNMP stats are not protected by preemption but by bh handling. > > > > Most forms of bh exclusion work via the preemption count though, and > > softirq contexts themselves are generally not preemptible [to other CPUs] > > either. > > > > So the warnings should, in most cases, not trigger. > > Right, so softirqs run either in the irq tail at which point > preempt_count += SOFTIRQ_OFFSET and thus preemption is disabled, or it > runs in ksoftirqd which has strict cpu affinity which also disables the > warning, and it also increments preempt_count with SOFTIRQ_OFFSET to > exclude the softirq from interrupts while its running, also disabling > the warning. A third context would be syscall-level code that runs with local_bh_disable()/enable() - but that too ought to have the preempt count elevated. > So it should very much not trigger.. if it does you want to know about > it. Yes. If nothing else then for the education value. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Index: linux/include/net/snmp.h =================================================================== --- linux.orig/include/net/snmp.h 2013-10-07 09:16:07.595206864 -0500 +++ linux/include/net/snmp.h 2013-10-07 09:16:07.591206909 -0500 @@ -126,7 +126,7 @@ struct linux_xfrm_mib { extern __typeof__(type) __percpu *name[SNMP_ARRAY_SZ] #define SNMP_INC_STATS_BH(mib, field) \ - __this_cpu_inc(mib[0]->mibs[field]) + raw_cpu_inc(mib[0]->mibs[field]) #define SNMP_INC_STATS_USER(mib, field) \ this_cpu_inc(mib[0]->mibs[field]) @@ -141,7 +141,7 @@ struct linux_xfrm_mib { this_cpu_dec(mib[0]->mibs[field]) #define SNMP_ADD_STATS_BH(mib, field, addend) \ - __this_cpu_add(mib[0]->mibs[field], addend) + raw_cpu_add(mib[0]->mibs[field], addend) #define SNMP_ADD_STATS_USER(mib, field, addend) \ this_cpu_add(mib[0]->mibs[field], addend)
SNMP stats are not protected by preemption but by bh handling. Since protection is provided outside of preemption raw_cpu_ops need to be used to avoid false positives. Cc: Eric Dumazet <edumazet@google.com> CC: netdev@vger.kernel.org Signed-off-by: Christoph Lameter <cl@linux.com> -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html