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u64_stats: Disable preemption on 32bit UP+SMP PREEMPT_RT during updates.
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On PREEMPT_RT the seqcount_t for synchronisation is required on 32bit
architectures even on UP because the softirq (and the threaded IRQ handler) can
be preempted.

With the seqcount_t for synchronisation, a reader with higher priority can
preempt the writer and then spin endlessly in read_seqcount_begin() while the
writer can't make progress.

To avoid such a lock up on PREEMPT_RT the writer must disable preemption during
the update. There is no need to disable interrupts because no writer is using
this API in hard-IRQ context on PREEMPT_RT.

Disable preemption on 32bit-RT within the u64_stats write section.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
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Sebastian Andrzej Siewior authored and David S. Miller committed Dec 13, 2021
1 parent d147dd7 commit 3c11854
Showing 1 changed file with 28 additions and 14 deletions.
42 changes: 28 additions & 14 deletions include/linux/u64_stats_sync.h
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Original file line number Diff line number Diff line change
Expand Up @@ -66,7 +66,7 @@
#include <linux/seqlock.h>

struct u64_stats_sync {
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
seqcount_t seq;
#endif
};
Expand Down Expand Up @@ -125,7 +125,7 @@ static inline void u64_stats_inc(u64_stats_t *p)
}
#endif

#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
#define u64_stats_init(syncp) seqcount_init(&(syncp)->seq)
#else
static inline void u64_stats_init(struct u64_stats_sync *syncp)
Expand All @@ -135,15 +135,19 @@ static inline void u64_stats_init(struct u64_stats_sync *syncp)

static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
if (IS_ENABLED(CONFIG_PREEMPT_RT))
preempt_disable();
write_seqcount_begin(&syncp->seq);
#endif
}

static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
write_seqcount_end(&syncp->seq);
if (IS_ENABLED(CONFIG_PREEMPT_RT))
preempt_enable();
#endif
}

Expand All @@ -152,8 +156,11 @@ u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
{
unsigned long flags = 0;

#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
local_irq_save(flags);
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
if (IS_ENABLED(CONFIG_PREEMPT_RT))
preempt_disable();
else
local_irq_save(flags);
write_seqcount_begin(&syncp->seq);
#endif
return flags;
Expand All @@ -163,15 +170,18 @@ static inline void
u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
unsigned long flags)
{
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
write_seqcount_end(&syncp->seq);
local_irq_restore(flags);
if (IS_ENABLED(CONFIG_PREEMPT_RT))
preempt_enable();
else
local_irq_restore(flags);
#endif
}

static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
return read_seqcount_begin(&syncp->seq);
#else
return 0;
Expand All @@ -180,7 +190,7 @@ static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *

static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
#if BITS_PER_LONG == 32 && (!defined(CONFIG_SMP) && !defined(CONFIG_PREEMPT_RT))
preempt_disable();
#endif
return __u64_stats_fetch_begin(syncp);
Expand All @@ -189,7 +199,7 @@ static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *sy
static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
unsigned int start)
{
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
return read_seqcount_retry(&syncp->seq, start);
#else
return false;
Expand All @@ -199,7 +209,7 @@ static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
unsigned int start)
{
#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
#if BITS_PER_LONG == 32 && (!defined(CONFIG_SMP) && !defined(CONFIG_PREEMPT_RT))
preempt_enable();
#endif
return __u64_stats_fetch_retry(syncp, start);
Expand All @@ -213,7 +223,9 @@ static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
*/
static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
#if BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT_RT)
preempt_disable();
#elif BITS_PER_LONG == 32 && !defined(CONFIG_SMP)
local_irq_disable();
#endif
return __u64_stats_fetch_begin(syncp);
Expand All @@ -222,7 +234,9 @@ static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync
static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
unsigned int start)
{
#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
#if BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT_RT)
preempt_enable();
#elif BITS_PER_LONG == 32 && !defined(CONFIG_SMP)
local_irq_enable();
#endif
return __u64_stats_fetch_retry(syncp, start);
Expand Down

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